CHAPTER VII
Influence of Volcanic Rocks on the Scenery of the Land—Effects of Denudation.
As considerable popular misapprehension exists respecting the part which volcanism has played in the evolution of the existing topography of the earth's surface, and as the British Isles, from their varied geological structure, offer special facilities for the discussion of this subject, it may not be out of place to devote a final section of the present Introduction to a consideration of the real topographical influence of volcanic action.
With modern, and especially with active, volcanoes we need not here concern ourselves. Their topographical forms are well known, and give rise to no difficulty. The lofty cones of the Vesuvian type, with their widespread lavas and ashes, their vast craters and their abundant parasitic volcanoes; the crowded, but generally diminutive, cones and domes of the puy type, so well displayed in Auvergne, the Eifel and the Bay of Naples; and the vast lava deserts of the plateaux, so characteristically developed in Iceland and Western America, illustrate the various ways in which volcanic energy directly changes the contours of a terrestrial surface.
But the circumstances are altered when we deal with the topographical influence of long extinct volcanoes. Other agencies then come into play, and some caution may be needed in the effort to disentangle the elements of the complicated problem, and to assign to each contributing cause its own proper effect.
Reference has already been made to the continuous denudation of volcanic hills from the time that they are first erupted. But the comparative rapidity of the waste and the remarkable topographical changes which it involves can hardly be adequately realized without the inspection of an actual example. A visit to the back of Monte Somma, already alluded to, will teach the observer, far more vividly than books can do, how a volcanic cone is affected by daily meteoric changes. The sides of such a cone may remain tolerably uniform slopes so long as they are always being renewed by deposits from fresh eruptions. But when the volcanic activity ceases, and the declivities undergo no such reparation, they are rapidly channelled by the descent of rain-water, until the furrows grow by degrees wide and deep ravines, with only narrow and continually-diminishing crests between them. If unchecked by any fresh discharge of volcanic material, the degradation will at last have removed the whole cone.
It is thus obvious that purely volcanic topography, that is, the terrestrial scenery due directly to the eruption of materials from within the earth, can never become in a geological sense very old. It can only endure so long as it is continually renewed by fresh eruptions, or where it is carried down by subsidence under water and is there buried under a cover of protecting sediments. When, therefore, we meet with volcanic rocks of ancient date exposed at the surface, we may be quite certain that their present contours are not those of the original volcano, but have been brought about by the processes of denudation.
It is true that, in the general erosion of the surface of the land, volcanic rocks of ancient date sometimes rise into wonderfully craggy heights, including, perhaps, cones and deep crater-like hollows, which to popular imagination betoken contours left by now extinguished volcanic fires. Examples of such scenery are familiar in various parts of Britain; but the resemblance to recent volcanic topography is deceptive. There are, indeed, a few hills wherein the progress of denudation seems not as yet to have entirely removed the lavas and tuffs that gathered round the original vents. Some of the tuff-cones of eastern Fife, for example, present cases of this kind. Again, the great granophyre domes and cones of the Tertiary volcanic series of the Inner Hebrides, though they have undoubtedly been extensively denuded, may possibly retain contours that do not greatly differ from those which these protruded bosses originally assumed under the mass of rock which has been removed from them. Nevertheless, putting such doubtful exceptions aside, we may confidently affirm that hills composed of ancient volcanic material give no clue to the forms of the original volcanoes.
It can hardly be too often repeated that the fundamental law in the universal decay and sculpture of the land is that the waste is proportioned to the resistance offered to it: the softer rocks are worn down with comparative rapidity, while the harder varieties are left projecting above them. As a general rule, volcanic rocks are more durable than those among which they are interstratified, and hence project above them, but this is not always the case. No universal rule can, indeed, be laid down with regard to the relative durability of any rocks. While, therefore, topographic contours afford a valuable indication of the nature and disposition of the rocks below the surface, they cannot be relied upon as in all circumstances an infallible guide in this respect. No better proof can be offered of the caution that is needed in tracing such contours back to their origin than is furnished by the old volcanic rocks of Britain. These eruptive masses, consisting usually of durable materials and ranging through a vast cycle of geological time, usually rise into prominent features and thus support the general law. But they include also many easily eroded members, which, instead of forming eminences, are worn into hollows. They include, in short, every type of scenery, from featureless plains and rolling lowlands to craggy and spiry mountains.
The first point, then, which is established in an investigation of the topographical influence of old volcanic rocks is that their prevailing prominence arises from relative durability amidst universal degradation. When we proceed further to inquire why they vary so much from each other in different places, and how their complicated details of feature have been elaborated, we soon learn that such local peculiarities have arisen mainly from variations in the internal structure and grouping of the rocks themselves.
Here again the general law of sculpture comes into play. The local features have depended upon the comparative resistance offered to the sculpturing agents by the different portions of a volcanic series. Each distinct variety of rock possesses its own characteristic internal structure. The lines along which atmospheric disintegration will most effectually carry on its carving work are thus already traced in the very substance and architecture of the rock itself. Each rock consequently yields in its own way to the processes of disintegration, and thus contributes its own distinctive share to topographical feature.
Among the massive rocks abundant examples of such special types of weathering may be cited, from the acid and basic series, and from superficial lavas as well as from intrusive bosses and sills. Acid bosses, such as those of granite, granophyre and quartz-porphyry, tend to weather into blocks and finally into sand, and as this tendency is somewhat uniformly distributed through the rocks, they are apt to assume rounded, dome-shaped or conical forms which, at a distance, may seem to have smooth declivities, but on examination are generally found to be covered with a slowly-descending sheet of disintegrated blocks and debris ([Fig. 346]). When less prone to decay, and especially where traversed by a strongly-defined system of vertical joints, they may shoot up into tower-like heights, with prominent spires and obelisks. Basic bosses, when their materials decay somewhat rapidly, give rise to analogous topographical forms, though the more fertile soils which they produce generally lead to their being clothed with vegetation. Where they consist of an obdurate rock, much jointed and fissured, like the gabbro of the Inner Hebrides, they form exceedingly rugged mountains, terminating upward in serrated crests and groups of aiguilles (Figs. [331], [333]).
Acid lavas that have been superficially erupted weather into irregularly craggy hills, like the flanks of Snowdon. Those of intermediate composition, where they have accumulated in thick masses, are apt to weather into conical forms, as may be seen among the Cheviot, Pentland and Garleton Hills (Figs. [109], [110], [133]); but where they have been poured out in successive thin sheets they have built up undulating plateaux with terraced sides, as among the Ayrshire and Campsie Fells and the hills of Lorne (Figs. [99], [107]). Basic lavas have issued in comparatively thin sheets, frequently columnar or slaggy, forming flat-topped hills and terraced escarpments, such as are typically developed among the Tertiary basalt-plateaux of the Inner Hebrides and the Faroe Islands (Figs. [11], [265], [283], [284], [286]).
One of the most frequent causes of local peculiarities of topography among old volcanic rocks is the intercalation of very distinct varieties of material in the same volcanic series. Where, for instance, lavas and tuffs alternate, great inequalities of surface may be produced. The tuffs, being generally more friable, decay faster and give rise to hollows, while the lavas, being more durable, project in bold ridges or rise into mural escarpments ([Fig. 265]). Again, where dykes weather more readily than the rocks which they traverse, they originate deep narrow clefts, while where they weather more slowly than the rocks around them, they project as dark ribs. Thus in Skye some dykes which rise through the obdurate gabbro are marked by chasms which reach up even to the highest crests of the mountains ([Fig. 333]), while of those which run in the pale crumbling granophyre, some stand up as black walls that can be followed with the eye across the ridges even from a long distance.
Many further illustrations of these principles might be cited here from the old volcanic districts of Britain. But they will present themselves successively in later chapters. For my present purpose it is enough to show that the scenery of these districts is not directly due to volcanic action, but is the immediate result of denudation acting upon volcanic rocks, modified and directed by their geological structure.
It may, however, be useful, in concluding the discussion of this subject, to cite some typical volcanic regions in the British Isles as illustrations of the relations between geology and topography, which, besides impressing the main lesson here enforced, may serve also to show some of the striking contrasts which geology reveals between the present and former conditions of the surface of the globe. Among these contrasts none are more singular than those offered by tracts where volcanic action has once been rife, and where the picture of ancient geography presented in the rocks differs so widely from the scenery of the same places to-day as to appeal vividly to the imagination.
The first district to which I may refer where ancient volcanic rocks are well developed is that of Devonshire. The story of the Devonian volcanoes will be told in some detail in later chapters, when it will be shown that the eruptions were again and again renewed during a long course of ages. Yet, abundant as the intercalated lavas and tuffs are, they can hardly be said to have had any marked effect on the scenery, though here and there a harder or larger mass of diabase rises into a prominent knoll or isolated hill. When the amount of volcanic material in this region is considered, we may feel some surprise at the trifling influence which it has exerted in the general denudation of the surface.
To one who wanders over the rich champaign of southern Devonshire, and surveys from some higher prominence the undulating tree-crowned ridges that slope down into orchard-filled hollows, and the green uplands that sweep in successive waves of verdure to the distant blue tors of Dartmoor, the scene appears as a type of all that is most peaceful, varied and fertile in English landscape. In the trim luxuriance that meets the eye on every side, the hand of man is apparent, though from many a point of vantage no sound may be heard for a time to show that he himself is anywhere near us. Yet ever and anon from the deep lanes, hidden out of sight under their canopy of foliage, there will come the creak of the groaning waggon and the crack of the waggoner's whip, as evidence that there are roads and human traffic through this bosky silent country.
Amid so much quiet beauty, where every feature seems to be eloquent of long generations of undisturbed repose, it must surely stir the imagination to be told that underneath these orchards, meadows and woodlands lie the mouldering remnants of once active and long-lived volcanoes. Yet we have only to descend into one of the deep lanes to find the crumbling lavas and ashes of the old eruptions. The landscape has, in truth, been carved out of these volcanic rocks, and their decomposition has furnished the rich loam that nourishes so luxuriant a vegetation.
Not less impressive is the contrast presented between the present and former condition of the broad pastoral uplands of the south of Scotland. Nowhere in the British Islands can the feeling of mere loneliness be more perfectly experienced than among these elevated tracts of bare moorland. They have nothing of the grandeur of outline peculiar to mountain tracts. Sometimes, for miles around one of their conspicuous summits, we may see no projecting knob or pinnacle. The rocks have been gently rounded off into broad featureless hills, which sink into winding valleys, each with its thread of streamlet and its farms along the bottom, and its scattered remnants of birch-wood or alder-copse along its slopes and dingles. Across miles of heathy pasture and moorland, on the summits of this great tableland, we may perchance see no sign of man or his handiwork, though the bleating of the sheep and the far-off barking of the collie tell that we are here within the quiet domain of the south-country shepherd.
In this pastoral territory, also, though they hardly affect the scenery, volcanic rocks come to the surface where the foldings of the earth's crust have brought up the oldest formations. Their appearance extends over so wide an area as to show that a large part of these uplands lies on a deeply-buried volcanic floor. A whole series of submarine volcanoes, extending over an area of many hundreds of square miles, and still in great part overlain with the accumulated sands and silts of the sea-bottom, now hardened into stone, underlies these quiet hills and lonely valleys.
A contrast of another type meets us in the broad midland valley of Scotland. Around the city of Edinburgh, for instance, the landscape is diversified by many hills and crags which show where harder rocks project from amidst the sediments of the Carboniferous system. On some of these crags the forts of the early races, the towers of Celt and Saxon, and the feudal castles of the middle ages were successively planted, and round their base clustered for protection the cots of the peasants and the earliest homesteads of the future city. Beneath these crags many of the most notable events in the stormy annals of the country were transacted. Under their shadow, and not without inspiration from their local form and colour, literature, art and science have arisen and flourished. Nowhere, in short, within the compass of the British Isles has the political and intellectual progress of the people been more plainly affected by the environment than in this central district of Scotland.
When now we inquire into the origin and history of the topography which has so influenced the population around it, we find that its prominences are relics of ancient volcanoes. The feudal towers are based on sills and dykes and necks. The fields and gardens, monuments and roadways, overlie sheets of lava or beds of volcanic ashes. Not only is every conspicuous eminence immediately around of volcanic origin, but even the ranges of blue hills that close in the distant view to south and north and east and west are mainly built up of lavas and tuffs. The eruptions of which these heights are memorials belong to a vast range of geological ages, the latest of them having passed away long before the advent of man. But they have left their traces deeply engraven in the rocky framework of the landscape. While human history, stormy or peaceful, has been slowly evolving itself during the progress of the centuries in these fertile lowlands, the crags and heights have remained as memorials of an earlier history when Central Scotland continued for many ages to be the theatre of vigorous volcanic activity.
As a final illustration of the influence of volcanic rocks in scenery, and of the contrast between their origin and their present condition, I may cite the more prominent groups of hills in the Inner Hebrides. In the singularly varied landscapes of that region three distinct types of topography attract the eye of the traveller. These are best combined and most fully developed in the island of Skye. Throughout the northern half of that picturesque island, the ground rises into a rolling tableland, deeply penetrated by arms of the sea, into which it slopes in green declivities, while along its outer borders it plunges in ranges of precipice into the Atlantic. Everywhere, alike on the cliffs and the inland slopes, long parallel lines of rock-terrace meet the eye. These mount one above another from the shores up to the flat tops of the highest hills, presenting level or gently-inclined bars of dark crag that rise above slopes of debris, green sward and bracken. It is these parallel, sharply-defined bars of rock, with their intervening strips of verdure, that give its distinctive character to the scenery of northern Skye. On hillside after hillside and in valley after valley, they reappear with the same almost artificial monotony. And far beyond the limits of Skye they are repeated in one island after another, all down the chain of the Inner Hebrides.
In striking contrast to this scenery, and abruptly bounding it on the south, rise the Red Hills of Skye—a singular group of connected cones. Alike in form and in colour, these hills stand apart from everything around them. The verdure of the northern terraced tableland here entirely disappears. The slopes are sheets of angular debris,—huge blocks of naked stone and trails of sand, amidst which hardly any vegetation finds a footing. The decay of the rock gives it a pale yellowish-grey hue, which after rain deepens into russet, so that in favourable lights these strange cones gleam with a warm glow as if they, in some special way, could catch and reflect the radiance of the sky.
Immediately to the west of these pale smooth-sloped cones, the dark mass of the Cuillin Hills completes the interruption of the northern tableland. In almost every topographical feature these hills present a contrast to the other two kinds of scenery. Their forms are more rugged than those of any other hill-group in Britain ([Fig. 331]). Every declivity among them is an irregular pile of crags, every crest is notched like a saw, every peak is sharpened into a pinnacle. Instead of being buried under vast sheets of their own debris, these hills show everywhere their naked rock, which seems to brave the elements as few other rocks can do. Unlike the pale Red Hills, they are dark, almost black in tone, though when canopied with cloud they assume a hue of deepest violet.
Each of these three distinct types of topography owes its existence to the way in which a special kind of volcanic rock yields to the influences of denudation. The terraced tableland of the north is built up of hundreds of sheets of basaltic lava, each of the long level ledges of brown rock marking the outcrop of one or more of these once molten streams. The black rugged mass of the Cuillin Hills consists of a vast protruded body of eruptive material, which, in the form of endless sills and bosses of gabbro and dolerite, has invaded the basalt-plateau, and has now been revealed by the gradual removal of the portion of that plateau which it upraised. The pale cones and domes of the Red Hills mark the place of one of the last protrusions in the volcanic history of Britain—that of large masses of an acid magma, which broke through the basalt-plateau and also disrupted the earlier gabbro.
In no part of North-Western Europe has volcanic activity left more varied and abundant records of its operations than in these three contiguous tracts of Skye. It is interesting therefore to note the striking contrast between the former and the present landscapes of the region. The lavas of the basaltic tableland crumble into a rich loam, that in the mild moist climate of the Hebrides supports a greener verdure than any of the other rocks around will yield. The uplands have accordingly become pasture-grounds for herds of sheep and cattle. The strips of lowland along the valleys and in the recesses of the coast-line furnish the chief tracts of arable land in the island, and are thus the main centres of the crofter population. The bays and creeks of the much-indented shores form natural harbours, which in former days attracted the Norse sea-rovers, and supplied them with sites for their settlements. Norse names still linger on headland and inlet, but the spirit of adventure has passed away, and a few poor fishing-boats, here and there drawn up on the beach, are usually the only token that the islanders make any attempt to gather the harvest of the sea.
The mountain groups which so abruptly bound the basalt-plateau on the south, and present in their topographical features such distinctive scenery, comprise a region too lofty, too rugged and too barren for human occupation. The black Cuillins and the pale Red Hills are solitudes left to the few wild creatures that have not yet been exterminated. The corries are the home of the red deer. The gabbro cliffs are haunts of the eagle and the raven. Where patches of soil have gathered in the crannies of the gabbro, alpine plants find their home. In the chasms left by the decay of the dykes between the vertical walls of their fissures, the winter snows linger into summer, and conceal with their thick drifts the mouldering surface of the once molten rock beneath them. On every side and at every turn a mute appeal is made to the imagination by the strange contrasts between the quiet restfulness of to-day, when the sculpture-tools of nature are each busily carving the features of the landscape, and the tumult of the time when the rocks, now so silent, were erupted.
The general discussion of the subject of Volcanism in this Introduction will, I hope, have prepared the reader who has no special geological training for entering upon the more detailed descriptions in the rest of this treatise. As already stated, the chronological order of arrangement will be followed. Beginning with the records of the earliest ages, we shall follow the story of volcanic action down to the end of the latest eruptions.
Each great geological system will be taken as a whole, representing a long period of time, and its volcanic evolution will be traced from the beginning of the period to the close. Some variety of treatment is necessarily entailed by the wide range in the nature and amount of the evidence for the volcanic history of different ages. But where practicable, an outline will first be given of what can be gathered respecting the physical geography of each geological period in Britain. In the description which will then follow of the volcanic phenomena, an account of the general characters of the erupted rocks will precede the more detailed narrative of the history of the volcanic eruptions in the several regions where they took place. References to the published literature of each formation will be given in the first part of each section, or will be introduced in subsequent pages, as may be found most convenient.
BOOK II
VOLCANIC ACTION IN PRE-CAMBRIAN TIME
CHAPTER VIII
PRE-CAMBRIAN VOLCANOES
The Beginnings of Geological History—Difficulties in fixing on a generally-applicable Terminology—i. The Lewisian (Archæan) Gneiss; ii. The Dalradian or Younger Schists of Scotland; iii. The Gneisses and Schists of Anglesey; iv. The Uriconian Volcanoes; v. The Malvern Volcano; vi. The Charnwood Forest Volcano.
The early geological history of this globe, like the early history of mankind, must be drawn from records at once scanty and hardly decipherable. Exposed to the long series of revolutions which the surface of the planet has undergone, these records, never perhaps complete at the first, have been in large measure obliterated. Even where they still exist, their meaning is often so doubtful that, in trying to interpret it, we find little solid footing, and feel ourselves to be groping, as it were, in the dimness of mythological legend, rather than working in the light of trustworthy and intelligible chronicles. These primeval records have been more particularly the objects of sedulous study during the last twenty years all over Europe and in North America. A certain amount of progress in their decipherment has been made. But the problems they still present for solution are numerous and obscure. Fortunately, with many of these problems the subject of the present treatise is not immediately connected. We need only concern ourselves with those which are related to the history of primeval volcanic activity.
To the earliest and least definite division of the geological annals various names have been applied. Some writers, believing that this period preceded the first appearance of plants or animals upon the globe, have named it Azoic—the lifeless age of geological history. But the absence of any hitherto detected trace of organic existence among the oldest known rocks cannot be held to prove that these rocks were formed before the advent of living things on the surface of the earth. The chance discovery of a single fossil, which might at any moment be made, would show the name "Azoic" to be a misnomer. Other geologists, believing that, as a matter of fact, organic structures of low types do actually occur in them, have called these old rocks "Eozoic," to denote that they were deposited during the dawn of life upon our planet. But the supposed organisms have not been everywhere accepted as evidence of former life. By many able observers they are regarded as mere mineral aggregates. Another term, "Archæan," has been proposed for the primeval ages of geological history, which are recorded in rocks that carry us as far as may ever be possible towards the beginnings of that history.
In choosing some general term to include the oldest known parts of the earth's crust, geologists are apt unconsciously to assume that the rocks thus classed together represent a definite section of geological time, comparable, for instance, to that denoted by one of the Palæozoic systems. Yet it is obvious that, under one of these general terms of convenient classification, a most multifarious series of rocks may be included, representing not one but possibly many, and widely separated, periods of geological history.
In many countries the oldest sedimentary accumulations, whether fossiliferous or not, are underlain by a series of crystalline rocks, which consist in great part of coarse massive gneisses and other schists. All over the world these rocks present a singular sameness of structure and composition. What might be found below them no man can say. They are in each country the oldest rocks of which anything is yet known, and whatsoever may be our theory of their origin, we must, at least for the present, start from them as the fundamental platform of the terrestrial crust.
But though crystalline rocks of this persistent character are widely distributed, both in the Old World and in the New, they in themselves furnish no means of determining their precise geological age. No method has yet been devised whereby the oldest gneiss of one country can be shown to be the true stratigraphical equivalent of the oldest gneiss of another. Palæontology is here of no avail, and Petrology has not yet provided us with such a genetic scheme as will enable us to make use of minerals and rock-structures, as we do of fossils, in the determination of geological horizons. All that can be positively affirmed regarding the stratigraphical relations of the rocks in question is that they are vastly more ancient than the oldest sedimentary and fossiliferous formations in each country where they are found. The "Lewisian" gneiss of the north-west of Scotland, the "Urgneiss" of Central Europe, and the "Laurentian" gneiss of Canada occupy similar stratigraphical positions, and present a close resemblance in lithological characters. We may conveniently class them under one common name to denote this general relationship. But we have, as yet, no means of determining how far they belong to one continuous period of geological history. They may really be of vastly different degrees of antiquity.
From the very nature of the case, any name by which we may choose to designate such ancient rocks cannot possess the precise stratigraphical value of the terms applied to the fossiliferous formations. Yet the convenience of possessing such a general descriptive epithet is obvious.
Until much more knowledge of the subject has been gained, any terminology which may be proposed must be regarded as more or less provisional. The comprehensive term "pre-Cambrian" may be usefully adopted as a general designation for all rocks older than the base of the Cambrian system, irrespective of their nature and origin. Already it is well known that under this term a vast series of rocks, igneous and sedimentary, is included. In some regions several successive formations, or systems of formations, may be recognized in this series. But until some method has been devised for determining the stratigraphical relations of these formations in different regions, it would seem safest not to attempt to introduce general names for universal adoption, but to let the sequence of rocks in each distinct geological province be expressed by a local terminology. This caution is more especially desirable in the case of sedimentary deposits. We may surmise as to the equivalence of the rocks called Huronian, Torridonian and Longmyndian, but whilst so much is mere conjecture, it is certainly injudicious to transfer the local names of one province to the rocks of another.
The only relaxation of this general precaution which I think may at present be made is the adoption of a common name for the oldest type of gneisses. The term "Archæan" has been applied to these rocks, and if it is used simply to express a common petrographical type, occupying the lowest horizon in the stratigraphical series of a country, it has obvious advantages. But I would still retain the local names as subordinate terms to mark the local characteristics of the Archæan rocks of each province. Thus the "Laurentian" rocks of Canada and the "Lewisian" rocks of Scotland are widely-separated representatives of the peculiar stratigraphical series which is known as Archæan.
The pre-Cambrian rocks of Britain include several distinct systems or groups. How far those of even one part of this comparatively limited region are the proper equivalents of those of another and distant part is a problem still unsolved. Hence each distinct area, with its own type of rocks, will here be treated by itself. The following rock-types will be described: I. The Lewisian (Archæan) Gneiss; II. The Younger (Dalradian) Schists of Scotland; III. The Gneisses and Schists of Anglesey; IV. The Uriconian Group; V. The Malvern Group; VI. The Charnwood Forest Group (see [Map I.]).
i. THE LEWISIAN (ARCHAÆN) GNEISS
The British Isles are singularly fortunate in possessing an admirable development of pre-Cambrian rocks. These ancient masses rise up in various parts of the islands, but the region where they are most extensively displayed, and where their stratigraphical position and sequence are most clearly shown, lies in the north-west of Scotland.[50] In that territory they form the whole chain of the Outer Hebrides, and likewise extend as an irregular selvage along the western margin of the counties of Sutherland and Ross. The lowest known platform of the fossiliferous formations has there been discovered and has been traced for a distance of more than 100 miles. From this definite horizon, the high antiquity of all that lies below it is impressively demonstrated. The accompanying diagram ([Fig. 35]) will explain the general relations of the various geological formations of the region.
[50] These rocks have been the subject of much discussion, but geologists are now agreed as to their succession and structure. A full summary of the literature of the controversy regarding them will be found in the Quarterly Journal of the Geological Society, vol. xliv. (1888), p. 378.
In certain dark shales (b) which occupy a well-defined and readily-traceable position among the rocks of Sutherland and Ross, numerous specimens of the trilobite genus Olenellus, together with other fossils, have been found. By common consent among geologists, the zone of rock in which this genus appears is taken as the lowest stage of the Cambrian system. In Britain it marks the oldest known group of fossiliferous strata—the platform on which the whole of the Palæozoic systems rest.
Fig. 35.—Diagram illustrating the stratigraphical relations of the pre-Cambrian and Cambrian rocks of the North-west Highlands of Scotland.
c, Durness Limestones, with Upper Cambrian and perhaps Lower Silurian fossils, 1500 feet, top nowhere seen. b, Serpulite grit and "fucoid" shales, 70 to 80 feet, containing the Olenellus-zone. a, Quartzite, with abundant annelid tubes, about 600 feet. II. Red Sandstones and Conglomerates, sometimes 8000 feet or more (Torridonian). I. Gneiss with dykes, etc. (Lewisian).
From the definite geological epoch indicated by this platform, we can go backward into pre-Cambrian time, and realize in some measure how prodigious must be the antiquity of the successive groups of rock which emerge from beneath the base of the Palæozoic systems. Nowhere is this antiquity more impressively proclaimed than in the north-west of Scotland. From below the Olenellus-zone with its underlying sheets of quartzite (a), a thick group of dull red sandstones and conglomerates (II.) rises into a series of detached conical or pyramidal mountains, which form one of the most characteristic features in the scenery of that region. As this detrital formation is well developed around Loch Torridon, it has been termed Torridonian. It attains a thickness of at least 8000 or 10,000 feet, and is traceable all the way from the extreme northern headlands of Sutherland to the southern cliffs of the island of Rum.
In judging of the chronological significance of the geological structure of the north-west of Scotland, we are first impressed by the stratigraphical break between the base of the Cambrian system and the Torridonian deposits below. This break is so complete that here and there the thick intervening mass of sandstones and conglomerates has been nearly or wholly removed by denudation before the lowest Cambrian strata were laid down. Such a discordance marks the passage of a protracted interval of time.
Again, when the composition of the Torridonian rocks is considered, further striking evidence is obtained of the lapse of long periods. The sandstones, conglomerates and shales of this pre-Cambrian system present no evidence of cataclysmal action. On the contrary, they bear testimony that they were accumulated much in the same way and at the same rate as the subsequent Palæozoic systems. In that primeval period, as now, sand and silt were spread out under lakes and seas, were ripple-marked by the agitation of the water, and were gradually buried under other layers of similar sediment. The accumulation of 10,000 feet of such gradually-assorted detritus must have demanded a long series of ages. Here, then, in the internal structure of the Torridonian rocks, there is proof that in passing across them, from their summit to their base, we make another vast stride backward into the early past of geological history.
But when attention is directed to the relations of the Torridonian strata to the rocks beneath them, a still more striking proof of an enormously protracted period of time is obtained. Between the two series of formations lies one of the most marked stratigraphical breaks in the geological structure of the British Isles. There is absolutely nothing in common between them, save that the conglomerates and sandstones have been largely made out of the waste of the underlying gneiss. The denudation of the crystalline rocks before the deposition of any of the Torridonian sediments must have been prolonged and gigantic. The more, indeed, we study the gneiss, the more do we feel impressed by the evidence for the lapse of a vast interval of time, here unrecorded in rock, between the last terrestrial movements indicated by the gneiss and the earliest of the Torridonian sediments.
In this manner, reasoning backward from the horizon of the Olenellus-zone, we are enabled to form some conception of the vastness of the antiquity of the fundamental rocks of the North-west Highlands. The nature and origin of these rocks acquire a special interest from a consideration of their age. They contain the chronicles of the very beginnings of geological history, in so far as this history is contained in the crust of the earth. No part of the geological record is so obscure as this earliest chapter, but we need not here enter further into its difficulties than may be necessary for the purpose of understanding what light it can be made to throw on the earliest manifestations of volcanic action.
Under the term Lewisian Gneiss ([I. in Fig. 35]) a series of rocks is comprised which differ from each other in composition, structure and age, though most of them possess such crystalline and generally foliated characters as may be conveniently included under the designation of gneiss. The complexity of these ancient crystalline masses was not recognized at the time when Murchison called them the "Fundamental" or "Lewisian" gneiss. It is only since the Geological Survey began to study and map them in full detail that their true nature and history have begun to be understood.[51]
[51] See the Report of this Survey work by Messrs. Peach, Horne, Gunn, Clough, Cadell and Hinxman, Quart. Journ. Geol. Soc. vol. xliv. (1888), pp. 378-441; and Annual Reports of Director-General of the Geological Survey in the Report of The Science and Art Department for 1894, p. 279, and 1895, p. 17 of reprint. The general area of the gneiss is shown in [Map I.]
The researches of the Survey have shown the so-called Lewisian gneiss to comprise the following five groups of rock: 1. A group of various more or less banded and foliated rocks which form together the oldest and chief part of the gneiss (Fundamental complex); 2. Highly basic dykes cutting the first group; 3. Dykes and sills of dolerite, epidiorite and hornblende-schist; 4. A few dykes of peculiar composition; 5. Gneissose granite and pegmatite.
The first of these groups, forming the main body of the gneiss, has been critically studied on the mainland from Cape Wrath to Skye. But its development in the Outer Hebrides has not yet been worked out, although the name "Lewisian" was actually taken from that chain of islands. So far as at present known, however, the gneiss of the Hebrides repeats the essential characters of that of the mainland.
Mr. Teall, as the result of a careful investigation in the field and with the microscope, has ascertained that on the mainland between Skye and Cape Wrath the rocks of the "fundamental complex" are essentially composed of olivine, hypersthene, augite (including diallage), hornblende, biotite, plagioclase, orthoclase, microcline and quartz. He has further observed that these minerals are associated together in the same manner as in peridotites, gabbros, diorites and granites. Treating the rocks in accordance with their composition and partly with their structure, but excluding theoretical considerations, he has arranged them in the following five subdivisions:—
1. Rocks composed of ferro-magnesian minerals, without felspar or quartz—Pyroxenites, Hornblendites.
2. Rocks in which pyroxenes are the dominating ferro-magnesian constituents, felspar always being present, sometimes quartz: A, Without quartz, Hypersthene-augite-rocks (pyroxene granulites; rocks of the Baltimore-gabbro type) and augite-rocks (gabbros); B, With quartz, Augite-gneiss.
3. Rocks in which hornblende is the prevalent ferro-magnesian constituent: A, Without quartz, or containing it only in small quantity; rocks basic in composition: (a) massive or only slightly foliated (Amphibolites, as epidote-amphibolite, zoisite-amphibolite, garnet-amphibolite); (b) foliated (Hornblende-schist). B, With quartz; rocks intermediate or acid in composition: (a) with compact hornblende and a granular structure (Hornblende-gneiss proper); (b) with hornblende occurring in fibrous or other aggregates; (c) with compact hornblende and a more or less granulitic structure (Granulitic hornblende-gneiss).
4. Rocks in which biotite is the predominant ferro-magnesian constituent; felspar and quartz both present: (a) Biotite occurring as independent plates or in aggregates of two or three large individuals (Biotite-gneiss); (b) Biotite occurring in aggregates of numerous small individuals (rare type); (c) Biotite occurring as independent plates in a granulitic structure.
5. Rocks in which muscovite and biotite are present, together with felspar and quartz—Muscovite-biotite-gneiss. These, though not forming a well-defined natural group, are placed together for purposes of description. They are all foliated, some having the aspect of mica-schists, others being typical augen-gneisses, or light grey gneisses with abundant oligoclase and inclusions of microlitic epidote.
The rocks of each of these types are usually restricted to relatively small areas, and they succeed each other with much irregularity all the way from Skye to Cape Wrath. Their chemical and mineralogical composition proves them to have decided affinities with the plutonic igneous masses of the earth's crust.
The only exceptions to this prevalent igneous type occur in the districts of Gairloch and Loch Carron, where the gneiss appears to be associated with a group of mica-schists, graphitic-schists, quartzites and siliceous granulites, limestones, dolomites, chlorite-schists and other schists. That these are altered sedimentary formations can hardly be doubted. What their precise relations to the fundamental complex of the gneiss may be has not yet been satisfactorily determined. They are certainly far older than the Torridon sandstone which covers them unconformably. Possibly they may represent a sedimentary formation still more ancient than the gneiss.
Save these obscure relics of a pre-Torridonian system of strata, the gneiss never presents any structure which suggests the alteration of clastic constituents. Everywhere its mineral composition points to a connection with the subterranean intrusions of different igneous magmas, while the manner in which its different rock-groups are associated together, and the internal structure of some of them, still further link it with phenomena which will be described in succeeding chapters as parts of the records of volcanic action.
An interesting feature of the fundamental complex, as bearing on the origin of the gneiss, is to be found in the occurrence of bosses and bands which are either non-foliated or foliated only in a slight degree. These comparatively structureless portions present much of the character of bosses or sills of true eruptive rocks. They occur in various parts of Sutherland and Ross. Their external margins are not well defined, and they pass insensibly into the ordinary gneiss, the dark basic massive rocks shading off into coarse basic gneisses, and the pegmatites of quartz and felspar which traverse them merging into bands of grey quartzose gneiss.
So far, therefore, as present knowledge goes, the main body or fundamental complex of the Lewisian gneiss in the North-west Highlands of Scotland consists of what may have been originally a mass of various eruptive rocks. It has subsequently undergone a succession of deformations from enormous stresses within the terrestrial crust, which have been investigated with great care by the Geological Survey. But it presents structures which, in spite of the abundant proofs of great mechanical deformation, are yet, I venture to think, original, or at least belong to the time of igneous protrusion before deformation took place. The alternation of rocks of different petrographical constitution suggests a succession of extravasations of eruptive materials, though it may not be always possible now to determine the order in which these followed each other. In the feebly foliated or massive bands and bosses there is a parallel arrangement of their constituent minerals or of fine and coarse crystalline layers which recalls sometimes very strikingly the flow-structure of rhyolites and other lavas. This resemblance was strongly insisted on by Poulett Scrope, who believed that the laminar structure of such rocks as gneiss and mica-schist was best explained by the supposition of the flow of a granitic magma under great pressure within the earth's crust.[52]
[52] Volcanoes, pp. 140, 283, 299.
The conviction that these parallel structures do, in some cases, really represent traces of movements in the original unconsolidated igneous masses, not yet wholly effaced by later mechanical stresses, has been greatly strengthened in my mind by a recent study of the structures of various eruptive bosses, especially those of gabbro in the Tertiary volcanic series of the Inner Hebrides. The banded structure, the separation of the constituent minerals into distinct layers or zones, the alternation of markedly basic with more acid layers, and the puckering and plication of those bands, can be seen as perfectly among the Tertiary gabbro bosses of Skye as in the Lewisian gneiss (see Figs. [336] and [337]). It cannot be contended that such structures in the gabbro are due to any subsequent terrestrial disturbance and consequent deformation. They must be accepted as part of the original structure of the molten magma.[53] It seems to me, therefore, highly probable that the parallel banding in the uncrushed cores of the Lewisian gneiss reveals to us some of the movements of the original magma at the time of its extrusion and before it underwent those great mechanical stresses which have so largely contributed to the production of many of its most characteristic structures.
[53] See A. Geikie and J. J. H. Teall, Quart. Journ. Geol. Soc. vol. 1. (1894), p. 645.
While the material of the oldest gneiss presents many affinities to plutonic rocks of much younger date, a wide region of mere speculation opens out when we try to picture the conditions under which this material was accumulated. Some geologists have boldly advanced the doctrine that the Archæan gneisses represent the earliest crust that consolidated upon the surface of the globe. But these rocks offer no points of resemblance to the ordinary aspect of superficial volcanic ejections. On the contrary, the coarsely-crystalline condition even of those portions of the gneiss which seem most nearly to represent original structure, the absence of anything like scoriæ or fragmental bands of any kind, and the resemblances which may be traced between parts of the gneiss and intrusive bosses of igneous rock compel us to seek the nearest analogies to the original gneiss in deep-seated masses of eruptive material. It is difficult to conceive that any rocks approaching in character to the gabbros, picrites, granulites and other coarsely-crystalline portions of the old gneiss could have consolidated at or near the surface.
When the larger area of gneiss forming the chain of the Outer Hebrides is studied, we may obtain additional information regarding the probable origin and the earliest structures of the fundamental complex of the Lewisian gneiss. In particular, we may look for some unfoliated cores of a more acid character, and perhaps for evidence which will show that both acid and basic materials were successively protruded. We may even entertain a faint hope that some trace may be discovered of superficial or truly volcanic products connected with the bosses which recall those of later date and obviously eruptive nature. But up to the present time no indication of any such superficial accompaniments has been detected. If any portions of the old gneiss represent the deeper parts of columns of molten rock that flowed out at the surface as lava, with discharges of fragmentary materials, all this superincumbent material, at least in the regions which have been studied in detail, had disappeared entirely before the deposition of the very oldest part of the Torridonian rocks, unless some trace of it may remain among the pebbles of the Torridonian conglomerates, to which reference will be immediately made.
So far, then, as the evidence now available allows a conclusion to be drawn, the Lewisian gneiss reveals to us a primeval group of eruptive rocks presenting a strong resemblance to some which in later formations are connected, as underground continuations, with bedded lavas and tuffs that were erupted at the surface; and although no proof has yet been obtained of true volcanic ejections associated with the fundamental complex, the rocks seem to be most readily understood if we regard them as having consolidated from igneous fusion at some depth, and we may plausibly infer that they may have been actually connected with the discharge of volcanic materials at the surface. The graphite-schists, mica-schists, and limestones of the Gairloch and Loch Carron may thus be surviving fragments of the stratified crust into which these deep-seated masses were intruded, and through which any volcanic eruptions that were connected with them had to make their way.
The limited areas occupied by the several varieties of rock in the fundamental complex suggests the successive protrusion of different magmas, or of different portions from one gradually changing magma. Mr. Teall has ascertained that whenever in this series of rocks the relative ages of two petrographical types can be clearly ascertained, the more basic is older than the more acid.
But besides all the complexity arising from original diversity of area, structure and composition among the successive intrusions, a further intricacy has been produced by the subsequent terrestrial disturbances, which on a gigantic scale affected the north-west of Europe after the formation of the fundamental complex of the old gneiss, but long before the Torridonian period. By a series of terrestrial stresses that came as precursors of those which in later geological times worked such great changes among the rocks of the Scottish Highlands, the original bosses and sheets of the gneiss were compressed, plicated, fractured and rolled out, acquiring in this process a crumpled, foliated structure. Whether or not these disturbances were accompanied by any manifestations of superficial volcanic action has not yet been determined. But we know that they were followed by a succession of dyke-eruptions, to which, for extent and variety, there is no parallel in the geological structure of Britain, save in the remarkable assemblage of dykes belonging to the Tertiary volcanic period[54] ([Fig. 36]).
[54] Quart. Journ. Geol. Soc. vol. xliv. (1888), p. 389 et seq.
Click on image to view larger.
Walker & Boutall sc.
Fig. 36.—Map of a portion of the Lewisian gneiss of Ross-shire.
Taken from Sheet 107 of the Geological Survey of Scotland on the scale of one inch to a mile. The white ground (A) marks the general body of the Lewisian gneiss. This is traversed by dykes of dolerite (B), which are cut by later dykes of highly basic material (peridotite, picrite, etc., P). The gneiss and its system of dykes is overlain unconformably by the nearly horizontal Torridon Sandstone (t), which is injected by sheets of oligoclase-porphyry (F).
For the production of these dykes a series of fissures was first opened through the fundamental complex of the gneiss, having a general trend from E.S.E. to W.N.W., running in parallel lines for many miles, and so close together in some places that fifteen or twenty of them occurred within a horizontal space of one mile. The fissures were probably not all formed at the same time; at all events, the molten materials that rose in them exhibit distinct evidence of a succession of upwellings from the igneous magma below.
Considered simply from the petrographical point of view, the materials that have filled the fissures have been arranged by Mr. Teall in the following groups: 1. Ultrabasic dykes, sometimes massive (peridotites), sometimes foliated (talcose schists containing carbonates and sometimes gedrite); 2. Basic dykes which where massive take the forms of dolerite and epidiorite, and where foliated appear as hornblende-schist, the same dyke often presenting the three conditions of dolerite, epidiorite and hornblende-schist; 3. Dykes of peculiar composition, comprising microcline-mica rocks and biotite-diorite with macro-poikilitic plagioclase; 4. Granites and gneissose granites (biotite-granite with microcline); 5. Pegmatites (microcline-quartz rocks with a variable amount of oligoclase or albite).[55]
[55] Annual Report of Geological Survey for 1895, p. 18 of reprint.
Distinct evidence of a succession of eruptions can be made out among these rocks. By far the largest proportion of the dykes consists of basic materials. The oldest and most abundant of them are of plagioclase-augite rocks, which, where uncrushed, differ in no essential feature of structure or composition from the dolerites and basalts of more modern periods, though they have been plentifully changed into epidiorite and hornblende-schist.[56] They present, too, most of the broad features that characterize the dykes of later times—the central more coarsely-crystalline portion, the marginal band of finer grain, passing occasionally into what was probably a basic glass, and the transverse jointing. They belong to more than one period of emission, for they cross each other. They vary in width up to nearly 200 feet, and sometimes run with singular persistence completely across the whole breadth of the strip of gneiss in the west of Sutherland and Ross. Dozens of dykes have been followed by the Geological Survey for distances of ten or twelve miles.
[56] See Mr. Teall, Quart. Journ. Geol. Soc. vol. xli. (1885), p. 133.
Later in time, and much less abundant, are certain highly basic dykes—peridotites with schistose modifications—which cut across the dolerites in a more nearly east-and-west direction. There are likewise occasional dykes of peculiar composition, which, as above stated, have been distinguished by Mr. Teall as microcline-mica rocks and biotite-diorite.
Last of all comes a group of thoroughly acid rocks—varieties of granite and pegmatite—which form intrusive sheets and dykes. The granites contain biotite with microcline, and are sometimes gneissose. The pegmatites are microcline-quartz rocks with a variable amount of oligoclase or albite. These dykes coincide in direction with the basalts and dolerites, but they are apt to run together into belts of granite and pegmatite, sometimes 1500 feet broad.
Up to the present time no evidence has been found of any superficial outpouring of material in connection with this remarkable series of dykes in the Lewisian gneiss. That they may have been concomitant with true volcanic eruptions may be plausibly inferred from the close analogy which, in spite of their antiquity and the metamorphism they have undergone, they still present to the system of dykes that forms a part of the great Tertiary volcanic series of Antrim and the Inner Hebrides. The close-set fissures running in a W.N.W direction, the abundant uprise into these fissures of basic igneous rocks, followed by a later and more feeble extravasation of acid material, are features which in a singular manner anticipate the volcanic phenomena of Tertiary time.
There can be no question as to the high antiquity of these dykes. They were already in place before the advent of those extraordinary vertical lines of shearing which have so greatly affected both the gneiss and the dykes; and these movements, in turn, had long been accomplished before the Torridon Sandstone was laid down, for the dykes, with their abundant deformation, run up to and pass beneath the sandstone which buries them and all the rocks with which they are associated. Though later than the original fundamental complex, the dykes have become so integral and essential a part of the gneiss as it now exists that they must be unhesitatingly grouped with it.
With so wide an extension of the subterranean relics of volcanic energy, it is surely not too much to hope that somewhere there may have been preserved, and may still be discovered, proofs that these eruptive rocks opened a connection with the surface, and that we may thus recognize vestiges of the superficial products of actual Archæan volcanoes. Among the pebbles in the conglomerates of the Torridon Sandstone there occur, indeed, fragments of felsites which possess great interest from the perfection with which they retain some of the characteristic features of younger lavas. Mr. Teall has described their minute structure. They are dark, purplish, compact rocks, consisting of a spherulitic micropegmatitic, micropoikilitic or microcrystalline groundmass, in which are imbedded porphyritic crystals or crystal-groups of felspar, often oligoclase. These spherulitic rocks occasionally show traces of perlitic structure. They bear a striking resemblance to some of the Uriconian felsites of Shropshire, pebbles from which occur in the Longmynd rocks.[57] These fragments suggest the existence of volcanic materials at the surface when the Torridon Sandstone was deposited. Possibly they may represent some vanished Lewisian lavas. But the time between the uprise of the dykes and the formation of the Torridonian series was vast enough for the advent of many successive volcanic episodes. The pebbles may therefore be the relics of eruptions that took place long after the period of the dykes.
[57] Annual Report of Geological Survey for 1895, p. 21 of reprint.
Among the Torridonian strata no undoubted trace of any contemporaneous volcanic eruptions has been met with.[58] The only relics of volcanic rocks in this enormous accumulation of sediments are the pebbles just referred to, which may be referable to a time long anterior to the very oldest parts of the Torridonian series.
[58] The supposed tuff referred to in Quart. Journ. Geol. Soc. vol. xlviii. (1892), p. 168, is probably not of truly volcanic origin.
That Archæan time witnessed volcanic eruptions on a considerable scale, and with great variety of petrographical material, has recently been shown in detail by Mr. Otto Nordenskjöld from a study of the rocks of Småland in Sweden. He has described a series of acid outbursts, including masses of rhyolite and dacite, together with agglomerates and tuffs, likewise basic eruptions, with dioritic rocks, augite-porphyrite and breccia. He refers these rocks to the same age as most of the Scandinavian gneisses, and remarks that though they have undergone much mechanical deformation and metamorphism, they have yet here and there retained some of their distinctive volcanic structures, such as the spherulitic.[59] When the large area of Lewisian gneiss forming the chain of the Outer Hebrides is investigated it may possibly supply examples of a similar series of ancient volcanic masses.
[59] "Über Archæische Ergussgesteine aus Småland," Sveriges Geol. Undersökn, No. 135 (1894).
ii. THE DALRADIAN OR YOUNGER SCHISTS OF SCOTLAND
We now come to one of the great gaps in the geological record. The Lewisian gneiss affords us glimpses of probable volcanic activity at the very beginning of geological history. An enormous lapse of time, apparently unrepresented in Britain by any geological record, must be marked by the unconformability between the gneiss and the Torridon Sandstone. Another prodigious interval is undoubtedly shown by the Torridonian series. Neither this thick accumulation of sediment nor the Cambrian formations, which to a depth of some 2000 feet overlie the Torridon Sandstone, have yielded any evidence of true superficial eruptions, though they are traversed by numerous dykes, sills and bosses. The age of these intrusive masses cannot be precisely fixed; a large proportion of them is certainly older than the great terrestrial displacements and concurrent metamorphism of the North-West Highlands.
While from the Lewisian gneiss upward to the highest visible Cambrian platform in Sutherland, no vestige of contemporaneous volcanic rocks is to be seen, the continuity of the geological record is abruptly broken at the top of the Durness Limestone. By a series of the most stupendous dislocations, the rocks of the terrestrial crust have there been displaced to such a degree that portions have been thrust westward for a horizontal distance of sometimes as much as ten miles, while they have been so crushed and sheared as to have often lost entirely their original structures, and to have passed into the crystalline and foliated condition of schists. Portions of the floor of Lewisian gneiss, and large masses of the Torridon Sandstone, which had been buried under the Cambrian sediments, have been torn up and driven over the Durness Limestone and quartzite.
Though much care has been bestowed by the officers of the Geological Survey on the investigation of the complicated mass of material which, pushed over the Cambrian strata, forms the mountainous ground that lies to the east of a line drawn from Loch Eribol, in the north of Sutherland, to the south-east of Skye, some uncertainty still exists as to the age and history of the rocks of that region. For the purposes of this work, therefore, the rest of the country eastwards to the line of the Great Glen—that remarkable valley which cuts Scotland in two—may be left out of account.
To the east of the Great Glen the Scottish Highlands display a vast succession of crystalline schists, the true stratigraphical relations of which to the Lewisian gneiss have still to be determined, but which, taken as a whole, no one now seriously doubts must be greatly younger than that ancient rock. Murchison first suggested that the quartzites and limestones found in this newer series are the equivalents of those of the North-West. This identification may yet be shown to be correct, but must be regarded as still unproved. Traces of fossils (annelid-pipes) have been found in some of the quartzites, but they afford little or no help in determining the horizons of the rocks. In Donegal, where similar quartzites, limestones and schists are well developed, obscure indications of organic remains (corals and graptolites) have likewise been detected, but they also fail to supply any satisfactory basis for stratigraphical comparison.
Essentially the schists of the Scottish Highlands east of the Great Glen consist of altered sedimentary rocks. Besides quartzites and limestones, there occur thick masses of clay-slate and other slates and schists, with bands of graphitic schist, greywacke, pebbly grit, quartzite, boulder-beds and conglomerates. Among rocks that have been so disturbed and foliated it is necessarily difficult to determine the true order of succession. In the Central Highlands, however, a certain definite sequence has been found to continue as far as the ground has yet been mapped. Were the rocks always severely contorted, broken and placed at high angles, this sequence might be deceptive, and leave still uncertain the original order of deposition of the whole series. But over many square miles the angles of inclination are low, and the successive bands may be traced from hill to hill, across strath and glen, forming escarpments along the slopes and outliers on the summits, precisely as gently-undulating beds of sandstone and limestone may be seen to do in the dales of Yorkshire. It is difficult to resist the belief, though it may, perhaps, be premature to conclude, that this obvious and persistent order of succession really marks the original sequence of deposition. In Donegal also a definite arrangement of the rock-groups has been ascertained which, when followed across the country, gives the key to its geological structure.[60]
[60] Geol. Survey Memoirs: Geology of N.W. Donegal, 1891.
In the order of succession which has been recognized during the progress of the Geological Survey through the Central and Southern Highlands, it is hard in many places to determine whether the sequence that can be recognized is in an upward or downward direction. Two bands of limestone, which appear to retain their relative positions across Scotland for a distance of some 230 miles, may afford a solution of this difficulty, and if, as is probable, they are to be identified with the similar limestones of Donegal, Mayo and Galway, their assistance will thus be available across a tract of more than 400 miles. What is regarded as the lower zone of limestone is particularly well seen about Loch Tay; what is believed to be the upper is typically displayed in the heart of Perthshire, about Blair-Athol.
From under the Loch Tay Limestone a great thickness of mica-schists, "green schists," schistose grits and conglomerates, slates and greywackes, emerges up to the border of the Highlands. Above that calcareous band thick masses of mica-schist and sericite-schist are succeeded by a well-marked zone of quartzite, which forms the mountains of Ben-y-Glo and Schihallion, and stretches south-westward across Argyllshire into Islay and Jura. The second or Blair-Athol Limestone lies next to this quartzite. If the limestones are identical with those of Donegal, Mayo and Galway, the quartzites may doubtless be also regarded as continued in those of the same Irish counties, where they form some of the most conspicuous features in the scenery, since they rise into such conspicuous mountains as Erigal, Slieve League, Nephin, and the twelve Bins of Connemara.
The age of this vast system of altered rocks has still to be determined. It is possible that they may include some parts of the Torridonian series, or even here and there a wedge of the Lewisian gneiss driven into position by gigantic disruptions, like those of the North-West Highlands. But there can be no doubt that the schists, quartzites and limestones form an assemblage of metamorphosed sedimentary strata which differs much in variety of petrographical character, as well as in thickness, from the Torridonian sandstone, and which has not been identified as the equivalent of any known Palæozoic system or group of formations in Britain. It may conceivably embrace the Cambrian series of the North-West Highlands, and also the sedimentary deposits that succeeded the Durness Limestone, of which no recognizable vestige remains in Sutherland or Ross.
That the metamorphic rocks east of the line of the Great Glen are at least older than the Arenig formation of the Lower Silurian system may be inferred from an interesting discovery recently made by the officers of the Geological Survey. A narrow strip of rocks has been found which, from their remarkable petrographical characters, their order of sequence and their scanty fossil contents (Radiolaria), are with some confidence identified with a peculiar assemblage of rocks on the Arenig horizon of the Silurian system in the Southern Uplands of Scotland, to which fuller reference will be made in [Chapter xii.] This strip or wedge of probably Lower Silurian strata intervenes between the Highland schists and the Old Red Sandstone in Kincardineshire, Forfarshire and Dumbartonshire. It has been recognized also, occupying a similar position, in Tyrone in Ireland. The schists in some places retain their foliated character up to the abrupt line of junction with the presumably Lower Silurian strata, while in other districts, as at Aberfoyle, they have been so little affected that it is hardly possible to draw a line between the Highland rocks and those of this border-zone, which indeed are there perhaps more metamorphosed than the Highland grits to the north of them. The metamorphism of the schists may have been mainly effected before the final disturbances that wedged in this strip of Silurian strata along the Highland border, though some amount of crushing and schist-making seems to have accompanied these disturbances. No trace of any similar strip of Palæozoic rocks has ever been detected among the folds of the schists further into the Highlands. But some of the Highland rocks in the region of Loch Awe lose their metamorphosed character, and pass into sedimentary strata which, so far as petrographical characters are concerned, might well be Palæozoic.
Until some clue is found to the age of the Younger or Eastern schists, quartzites and limestones of the Highlands, it is desirable to have some short convenient adjective to distinguish them. As a provisional term for them I have proposed the term "Dalradian," from Dalriada, the name of the old Celtic kingdom of the north of Ireland and south-west of Scotland.[61]
[61] Presidential Address to Geological Society, 1891, p. 39.
The special feature for which this Dalradian series is cited in the present volume is the evidence it furnishes of powerful and extensive volcanic action. In a series of rocks so greatly dislocated, crumpled and metamorphosed, we cannot look for the usual clear proofs of contemporaneous eruptions. Nevertheless all over the Scottish Highlands, from the far coast of Aberdeenshire to the Mull of Cantyre, and across the west of Ireland from the headlands of Donegal into Galway, there occurs abundant evidence of the existence of rocks which, though now forming an integral part of the schists, can be paralleled with masses of undoubtedly volcanic origin.
Fig. 37.—Section showing the position of Sills in the mica-schist series between Loch Tay and Amulree.
a, Mica-schist; b, b, Sills.
Intercalated in the vast pile of altered sediments lie numerous sheets of epidiorite and hornblende-schist, which were erupted as molten materials, not improbably as varieties of diabase-lava. Most of these sheets are doubtless intrusive "sills," for they can be observed to break across from one horizon to another. But some of them may possibly be contemporaneous lava-streams. A sheet may sometimes be followed for many miles, occupying the same stratigraphical platform. Thus a band of sills may be traced from the coast of Banffshire to near Ben Ledi, a distance of more than 100 miles. Among the hornblendic sills of this band some occur on a number of horizons between the group of Ben Voirlich grits and the Ben-y-Glo quartzite. One of the most marked of these is a sheet, sometimes 200 feet thick, which underlies the Loch Tay Limestone. Another interesting group in the same great band has been mapped by the Geological Survey on the hills between Loch Tay and Amulree, some of them being traceable for several miles among the mica-schists with which they alternate ([Fig. 37]).
In Argyllshire also, between Loch Tarbert and Loch Awe, and along the eastern coasts of the islands of Islay and Jura, an abundant series of sheets of epidiorite, amphibolite and hornblende-schist runs with the prevalent strike of the schists, grits and limestones of that region. Similar rocks reappear in a like position in Donegal, where, as in Scotland, the frequency of the occurrence of these eruptive rocks on the horizons of the limestones is worthy of remark. The persistence, number and aggregate thickness of the sills in this great band mark it out as the most extensive series of intrusive sheets in the British Isles.
In addition to the sills there occur also bosses of similar material, which in their form and their obvious relation to the sheets recall the structure of volcanic necks. They consist of hornblendic rocks, like the sills, but are usually tolerably massive, and show much less trace of superinduced foliation.
Besides the obviously eruptive masses there is another abundant group of rocks which, I believe, furnishes important evidence as to contemporaneous volcanic action during the accumulation of the Dalradian series. Throughout the Central and South-Western Highlands certain zones of "green schist" have long occupied the attention of the officers of the Geological Survey. They occur more especially on two horizons between the Loch Tay Limestone and a much lower series of grits and fine conglomerates, which run through the Trossachs and form the craggy ridges of Ben Ledi, Ben Voirlich and other mountains near the Highland border. In the lower group of "green schists," thick hornblendic sills begin to make their appearance, increasing in number upwards. The upper group of "green schists" lies between two bands of garnetiferous mica-schist, above the higher of which comes the Loch Tay Limestone. The peculiar greenish tint and corresponding mineral constituents of these schists, however, are likewise found diffused through higher parts of the series.
So much do the "green schists" vary in structure and composition that no single definition of them is always applicable. At one extreme are dull green chlorite-schists, passing into a "potstone," which, like that of Trondhjem, can be cut into blocks for architectural purposes.[62] At the other extreme lie grits and quartzites, with a slight admixture of the same greenish-coloured constituent. Between these limits almost every stage may be met with, the proportion of chlorite or hornblende and of granular or pebbly quartz varying continually, not only vertically, but even in the extension of the same bed. The quartz-pebbles are sometimes opalescent, and occasionally larger than peas. An average specimen from one of the zones of "green schists" is found, on closer examination, to be a thoroughly schistose rock, composed of a matrix of granular quartz, through which acicular hornblende and biotite crystals, or actinolite and chlorite, are ranged along the planes of foliation.
[62] From such a rock, which crosses the upper part of Loch Fyne, the Duke of Argyll's residence at Inveraray has been built.
That these rocks are essentially of detrital origin admits of no doubt. They differ, however, from the other sedimentary members of the Dalradian series in the persistence and abundance of the magnesian silicates diffused through them. The idea which they suggested to my mind some years ago was that the green colouring-matter represents fine basic volcanic dust, which was showered out during the accumulation of ordinary quartzose, argillaceous and calcareous sediments, and that, under the influence of the metamorphism which has so greatly affected all the rocks of the region, the original pyroxenes and felspars suffered the usual conversion into hornblendes, chlorites and micas. This view has occurred also to my colleagues on the Survey, and is now generally adopted by them.
Not only are these "green schists" traceable all through the Central and South-Western Highlands, rocks of similar character, and not improbably on the same horizons, reappear in the north-west of Ireland, and run thence south-westward as far as the Dalradian rocks extend. If we are justified in regarding them as metamorphosed tuffs and ashy sediments, they mark a widespread and long-continued volcanic period during the time when the later half of the Dalradian series was deposited.
Besides the extensive development of basic sills which, though probably in great part later than the "green schists," may belong to the same prolonged period of subterranean activity, numerous acid protrusions are to be observed in the Dalradian series of Scotland and Ireland. That these masses were erupted at several widely-separated intervals is well shown by their relation to the schists among which they occur. Some of the great bosses and sills of granite were undoubtedly injected before the metamorphism of the schists was completed, for they have shared in the foliation of the region. Others have certainly appeared after the metamorphism was complete, for they show no trace of having suffered from its effects. Thus some of the vast tracts of newer granite in the Grampian chain, which cover many square miles of ground, must be among the newest rocks of that area. They have recently been found by Mr. G. Barrow, of the Geological Survey, to send veins into the belt of probably Lower Silurian strata which flanks the Highland schists. They are thus later than the Arenig period. Not impossibly they may be referable to the great granite intrusions which formed so striking a feature in the history of the Lower Old Red Sandstone.
iii. THE GNEISSES AND SCHISTS OF ANGLESEY
In the island of Anglesey an interesting series of schists and quartzites presents many points of resemblance to the Dalradian or younger schists of the Highlands. At present the geologist possesses no means of determining whether these Welsh rocks are the equivalents of the Scottish in stratigraphical position, but their remarkable similarity justifies a brief allusion to them in this place. Much controversy has arisen regarding the geology of Anglesey, but into this dispute it is not necessary for my present purpose to enter.[63] I will content myself with expressing what seems to me, after several traverses, to be the geological structure of the ground.
[63] The literature of Anglesey geology is now somewhat voluminous, but I may refer to the following as the chief authorities. The island is mapped in Sheet 78 of the Geological Survey of England and Wales, and its structure is illustrated in Horizontal Sections, Sheet 40. A full account of its various formations and of their relations to each other is given in vol. iii. of the Memoirs of the Geological Survey, "The Geology of North Wales," by Sir A. C. Ramsay, 2nd edit. 1881. The subject has been discussed by Professor Hughes, Quart. Journ. Geol. Soc. vols. xxxiv. (1878) p. 137, xxxv. (1879) p. 682, xxxvi. (1880) p. 237, xxxviii. (1882) p. 16; Brit. Assoc. Rep. (1881) pp. 643, 644; Proc. Camb. Phil. Soc. vol. iii. pp. 67, 89, 341; by Professor Bonney, Quart. Journ. Geol. Soc. vol. xxxv. (1879) pp. 300, 321; Geol. Mag. (1880) p. 125; by Dr. H. Hicks, Quart. Journ. Geol. Soc. vols. xxxiv. (1878) p. 147, xxxv. (1879) p. 295; Geol. Mag. (1879) pp. 433, 528 (1893) p. 548; by Dr. C. Callaway, Quart. Journ. Geol. Soc. vols. xxxvii. (1881) p. 210, xl. (1884) p. 567; and by the Rev. J. F. Blake, Quart. Journ. Geol. Soc. vol. xliv. (1888) p. 463. Further references to the work of these observers in Anglesey are given in [Chapter xiii. p. 220] et seq. The Pre-Cambrian areas of Anglesey are shown in [Map II.]
There are two groups of rocks in Anglesey to which a pre-Cambrian age may with probability be assigned. In the heart of the island lies a core of gneiss which, if petrographical characters may be taken as a guide, must certainly be looked upon as Archæan. In visiting that district with my colleague Mr. Teall I was much astonished to find there so striking a counterpart to portions of the Lewisian gneiss of the north-west of Sutherland and Ross. The very external features of the ground recall the peculiar hummocky surface which so persistently characterizes the areas of this rock throughout the north-west of Scotland. If the geologist could be suddenly transported from the rounded rocky knolls of Sutherland, Ross-shire or the Hebrides to those in the middle of Anglesey, south of Llanerchymedd, he would hardly be aware of the change, save in the greater verdure of the hollows, which has resulted from a more advanced state of decomposition of the rocks at the surface, as well as from a better climate and agriculture.
When we examine these rocky hummocks in detail we find them to consist of coarse gneisses, the foliation of which has a prevalent dip to N.N.W. Some portions abound in dark hornblende and garnets, others are rich in brown mica, the folia being coarsely crystalline and rudely banded, as in the more massive gneisses of Sutherland. Abundant veins of coarse pegmatite may here and there be seen, with pinkish and white felspars and milky quartz. Occasionally the gneiss is traversed by bands of a dark greenish-grey rock, which remind one of the dykes of the north-west of Scotland. There are other rocks, some of them probably intrusive and of later date, to be seen in the same area; but they require more detailed study than they have yet received.
The relation of this core of gneiss and its associated rocks to the second group of pre-Cambrian rocks has not hitherto been satisfactorily ascertained. The core may conceivably be an eruptive boss in that group, and may have acquired its foliation during the movements that produced the foliation of the surrounding schists. But it seems more probable that the gneiss is much older than these schists, though it would undoubtedly participate in the effects of the mechanical movements which gave rise to their deformation, cleavage and foliation.
The second group of rocks occupies a large area in the west and in the centre and south of Anglesey. The schists of which it consists are obviously in the main a clastic series. One of their most conspicuous members is quartzite, which, besides occurring sporadically all over the island, forms the prominent mass of Holyhead Mountain. There are likewise flaggy chloritic schists, green and purple phyllites or slates, and bands of grit, while parts of the so-called "grey gneiss" consist of pebbly sandstones that have acquired a crystalline structure. That some order of sequence among these various strata may yet be worked out is not impossible, but the task will be one of no ordinary difficulty, for the plications and fractures are numerous, and much of the surface of the ground is obscured by the spread of Palæozoic formations and superficial deposits.
These Anglesey schists are so obviously an altered sedimentary series that it is not surprising that they should have been regarded as metamorphosed Cambrian strata. All that can be positively affirmed regarding their age is that they are not only older than the lowest fossiliferous rocks around them—that is, than Arenig or even Tremadoc strata—but that they had already acquired their present metamorphic character before these strata were laid down unconformably upon them. There is no actual proof that they include no altered Cambrian rocks. But when we consider their distinctly crystalline structure, and the absence of such a structure from any portion of the Cambrian areas of the mainland; when, moreover, we reflect that the metamorphism which has affected them is of the regional type, and can hardly have been restricted to merely the limited area of Anglesey; we must agree with those observers who, in spite of the absence of positive proof of their true geological horizon, have regarded these rocks as of much higher antiquity than the Cambrian strata of the neighbourhood. No one familiar with the Dalradian rocks of Scotland and Ireland can fail to be struck with the close resemblance which these younger Anglesey schists bear to them, down even into the minutest details. Petrographically they are precisely the counterparts of the quartzites and schists of Perthshire and Donegal, and a further connection may be established of a palæontological kind. The upper part of the Holyhead quartzite was found by Mr. B. N. Peach and myself in the autumn of the year 1890 to be at one place crowded with annelid-pipes, and I subsequently found the same to be the case with some of the flaggy quartzites near the South Stack.
Fig. 38.—Sketch of crushed basic igneous rock among the schists, E. side of Porth-tywyn-mawr, E. side of Holyhead Straits.
For the purpose of the inquiry which forms the theme of this work, the feature of greatest interest about these younger schists of Anglesey is the association of igneous rocks with them. They include bands of dark basic material, the less crushed parts of which resemble the diabases of later formations, while the sheared portions pass into epidiorites and true hornblende-schists. As in other regions where eruptive rocks have been crushed down and changed into the schistose modification, it is frequently possible to see groups of uncrushed cores round which, under severe mechanical stresses, the rock has undergone this conversion. Lines of movement through the body of the rock may be detected by bands of schist, the gradation from the solid core to the hornblende-schist being quite gradual. The accompanying figure ([Fig. 38]) represents a portion of one of these crushed basic igneous rocks on the east side of Holyhead Straits.
As in the Dalradian series of the Highlands, many, perhaps most, of these igneous bands are probably intrusive sills, but others may be intercalated contemporaneous sheets. They occur across the whole breadth of the island from the Menai Strait to the shores of Holyhead.
Besides these undoubtedly igneous rocks, the green chloritic slates of Anglesey deserve notice. They are well-bedded strata, consisting of alternations of foliated fine grit or sandstone, with layers more largely made up of schistose chlorite. The gritty bands sometimes contain pebbles of blue quartz, and evidently represent original layers of sandy sediment, but with an admixture of chloritic material. The manner in which this green chloritic constituent is diffused through the whole succession of strata, and likewise aggregated into bands with comparatively little quartzose sediment, reminds one of the "green schists" of the Central Highlands and Donegal, and suggests a similar explanation. Taken in connection with the associated basic igneous rocks, these chloritic schists seem to me to represent a thick group of volcanic tuffs and interstratified sandy and clayey layers. If this inference is well founded, and if we are justified in grouping these Anglesey rocks with the Dalradian schists of Scotland and Ireland, a striking picture is presented to the mind of the wide extent and persistent activity of the volcanoes of that primeval period in Britain.[64]
[64] Mr. E. Greenly, late of the Geological Survey of Scotland, has recently established himself on the Menai Strait for the purpose of working out in detail the geological structure of this interesting and complicated region. We may therefore hope that some of the still unsolved problems presented by the rocks of Anglesey will before long be satisfactorily explained.
iv. THE URICONIAN VOLCANOES
Along the eastern borders of Wales a ridge of ancient rocks, much broken by faults and presenting several striking unconformabilities, has long been classic ground in geology from the descriptions and illustrations of Murchison's Silurian System.[65] The main outlines of the structure of that district, first admirably worked out by this great pioneer, were delineated on the maps and sections of the Geological Survey, wherein it was shown that in the Longmynd an enormously thick group of stratified rocks, which, though unfossiliferous, were referred to the Cambrian system, rose in the very heart of the country; that to the east of these rocks lay strata of Caradoc or Bala age; that by a great hiatus in the stratigraphy the Upper Silurian series transgressively wrapped round everything below it; that yet again the Coal-measures crept over all these various Palæozoic formations, followed once more unconformably by Permian and Triassic deposits.[66] Besides all this evidence of extraordinary and repeated terrestrial movement, it was found that the region was traversed by some of the most powerful dislocations in this country, while to complete the picture of disturbance, many protrusions of igneous rocks were recognized.
[65] See especially chap. xix. vol. i. p. 225.
[66] The area is embraced in Sheet 61 of the Geological Survey, and is illustrated by Nos. 33 and 36 of the sheets of Horizontal Sections. In the early editions of the Survey maps the "felspathic traps" and the "greenstones" of the Wrekin district were distinguished by separate colours, but unfortunately this useful and so far correct discrimination was given up in subsequent editions, where all the acid and basic rocks are merged into one.
In a territory so complicated, though it had been sedulously and skilfully explored, there could hardly fail to remain features of structure which had escaped the notice of the first observers. In particular, the igneous rocks had been dealt with only in a general way, and they consequently offered a favourable field for more detailed study; while by a more searching examination of some of the rocks for fossils, important corrections of the earlier work might yet be made.
A notable step towards a revision of the received opinions regarding the igneous rocks of this region was taken by Mr. Allport, who showed that the so-called "greenstone" included masses of devitrified spherulitic pitchstones and perlites, together with indurated volcanic breccias, agglomerates and ashes.[67] Subsequently Professor Bonney described more fully the petrographical characters of the Wrekin igneous rocks, confirming and extending the observations of Mr. Allport.[68]
[67] Quart. Journ. Geol. Soc. vol. xxxiii. (1877) p. 449.
[68] Op. cit. vol. xxxv. (1879) p. 662; vol. xxxviii. (1882) p. 124.
But the correction of the prevalent error as to the geological age of these rocks was due to Dr. Callaway, who, after spending much time and labour in ascertaining, by a careful search for fossils, the position of the superincumbent rocks (wherein he discovered Cambrian organisms), and in a detailed investigation of the structure and relationships of the igneous masses themselves, was led to regard them as part of an ancient pre-Cambrian ridge; and he proposed for the volcanic group the name of Uriconian, from the name of the former Roman town which stood not far to the west of them.[69] He has shown how essentially volcanic this ancient series of rocks is, how seldom they present any clearly-marked evidence of stratification, and how small is the proportion of sedimentary material associated with them.[70]
[69] Quart. Journ. Geol. Soc. vols. xxx. (1874) p. 196, xxxiv. (1878) p. 754, xxxv. (1879) p. 643, xlii. (1886) p. 481. For a criticism of Dr. Callaway's views as to the order of succession among the rocks of this district, see Prof. Blake, op. cit. vol. xlvi. (1890) p. 386, and Dr. Callaway's reply, vol. xlvii. (1891) p. 109.
[70] Op. cit. vol. xlvii. (1891) p. 123.
Subsequently Professor Lapworth, by his discovery of the Olenellus-fauna, marking the lowest known fossiliferous Cambrian zone in the Wrekin district, and his recognition of Cambrian fossils under the Coal-measures of Warwickshire, supplied valuable evidence for the discussion of the geological position of the older rocks of the Midlands. He has mapped in minute detail the rocks of the Wrekin, and has exhausted all the evidence that is at present obtainable on the subject. But unfortunately the publication of his researches is still delayed.[71]
[71] Geol. Mag. (1882) p. 563, (1886) p. 319, (1887) p. 78, (1888) p. 484; and a joint paper with Mr. W. W. Watts on the Geology of South Shropshire, Proc. Geol. Assoc. vol. xiii. (1894) pp. 302, 335.
It is now recognized that the core of the ancient ridge, extending from near Wellington through the Wrekin, Caer Caradoc and other hills, until it sinks beneath the Upper Silurian formations, is formed of igneous rocks that consist partly of lavas, partly of volcanic breccias and fine tuffs. The lavas are thoroughly acid rocks of the felsitic or rhyolitic type. One of them, about 100 feet thick, which forms a prominent feature on the flanks and crest of Caer Caradoc, shows abundant finely-banded flow-structure, often curved or on end, while its bottom and upper parts are strongly amygdaloidal, the cavities being occasionally pulled out in the direction of flow and lined with quartz or chalcedony. Some of the detached areas of eruptive rocks show the beautiful spherulitic and perlitic structures first noticed in this region by Mr. Allport. More recently the structures of these acid rocks have been described by Mr. F. Rutley.[72]
[72] Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 540. Mr. Rutley more particularly describes those of Caradoc Hill.
The breccias and tuffs appear to consist mainly of felsitic material. In the coarser varieties, fragments of finely-banded felsite may be noticed, while the finer kinds pass into a kind of hornstone (hälleflinta), which in hand-specimens could hardly be distinguished from close-grained felsite. In some places, these pyroclastic rocks are well stratified, but elsewhere no satisfactory bedding can be recognized in them. Various other rocks, which are probably intrusive, occur in the ridge. At either end of the Wrekin there is a mass of pink microgranite, while at Caer Caradoc numerous sheets of "greenstone," intercalated in the fine tuffs, sweep across the hill. Mr. Rutley has published an account of these basic rocks, which he classes as "melaphyres," or altered forms of basalt or andesite.[73] That at least some of them are intrusive is manifest by the way in which they ramify through the surrounding strata. But others are so strongly amygdaloidal and slaggy that they may possibly be true interbedded lavas, though there may be some hesitation in admitting that such basic outflows could be erupted in the midst of thoroughly acid ejections.[74] Leaving these doubtful flows out of account, we have here a group of undoubted volcanic rocks represented by acid lavas and pyroclastic materials, by intrusive bosses of acid rocks, and by younger basic sills. The general lithological characters of these masses and the sequence of their appearance thus strongly resemble those of subsequent Palæozoic volcanic episodes.
[73] Op. cit. p. 534.
[74] This difficulty, however, need not be in itself insuperable, as is evident from the remarkable alternation of basic and acid lavas and tuffs in the Cambrian volcanic group of St. David's and in the Old Red Sandstone series of the Pentland Hills.
The geological age of this volcanic group is a question of much interest and importance in regard to the history of volcanism in this country. An inferior limit to the antiquity of the group can at once be fixed by the fact that, as originally pointed out by Dr. Callaway, the quartzite which overlies the volcanic rocks passes under a limestone containing Cambrian fossils in which Professor Lapworth has since recognized Olenellus, Paradoxides and other Lower Cambrian forms. The eruptions, therefore, must be at least as old as the earlier part of the Cambrian period. But it is affirmed that the quartzite rests with a complete unconformability on the volcanic rocks. If this be so, then the epoch of eruption must be shifted much farther back.
Fig. 39.—Section across the Uriconian series of Caer Caradoc.
S3, Upper Silurian; S2, Bala group; S1, Arenig group; C, Cambrian; L, Longmyndian; u, Uriconian; f f, faults.
The evidence adduced in favour of this great break appears to me to be threefold. In the first place, the quartzite contains fragments of the volcanic rocks. I do not think much stress can be laid on this fact. When I visited the ground, what struck me most in the composition of the quartzite was its singularly pure quartzose character, and the comparative scarcity of felsite-pebbles in it. Any deposit laid down conformably upon the top of the breccias and tuffs might obviously contain some of these materials, while, if laid down unconformably, it might reasonably be expected to be full of them. In the second place, this quartzite is alleged to pass transgressively across the edges of successive sheets of the volcanic group, and thus to have a quite discordant dip and strike. I failed to find satisfactory evidence of this unconformability in the northern part of the district. But in the Caer Caradoc area the quartzite does appear to steal across the outcrops of the older rocks, which plunge at nearly right angles in an opposite direction. In the third place, the felsitic volcanic group is believed by Professor Lapworth to pass upwards into the Longmynd rocks. Obviously, if this group lies at the very bottom of the vast Longmynd series, the discordance between it and the quartzite must be enormous, and the date of the volcanic eruptions must be placed vastly farther back in geological antiquity. Though the evidence does not seem to me to amount to clear proof, I am disposed, in the meantime, to accept it as affording the most probable solution of the difficulties presented by the structure of the ground.
The sequence of the rocks around Caer Caradoc is partly concealed by surface accumulations, but if these could be cleared away the structure of the ground would be, according to Messrs. Lapworth and Watts, as shown in [Fig. 39].[75]
[75] Proc. Geol. Assoc. vol. xiii. (1894), pp. 314, 315.
If, then, this volcanic group underlies the whole of the Longmynd series, and if, as it now appears, that series is older than the Olenellus-zone of the Lower Cambrian rocks, we can hardly include the volcanic rocks of the Wrekin and Caer Caradoc in the Cambrian system. They must belong to a still older geological formation, and I think we cannot do better than adopt for them Dr. Callaway's name, Uriconian.
There are still, however, many problems to be solved before the geological history of that region is completely understood. The rocks of the Longmynd must be more fully worked out. It is improbable that strata which look so likely to yield fossils should for ever prove barren. The lower half at least may be hopefully searched, although the upper massive reddish sandstones and conglomerates offer less prospect of success. On the west side of the Longmynd, above Pontesbury, there occurs a small area of volcanic rocks like those of the Wrekin district, including a well-marked nodular felsite and fine tuffs. These rocks have been regarded by Dr. Callaway as another axis of the Uriconian series. It is very difficult, however, by any combination of geological structures, to bring up a portion of the very bottom of the Longmynd series and place it apparently at the top. This is a feat which a detailed study of the region, and the detection of unconformabilities in the Longmynd, may possibly accomplish. In the meantime, however, I would venture to suggest whether it is not more probable that we have here a detached area of much younger volcanic rocks, like those which, in various districts, may be included in the Cambrian system, and which will be referred to in some detail in subsequent pages.
v. THE MALVERN VOLCANO
Regarding the age and origin of the oldest rocks of the Malvern Hills some controversy has arisen, and no general agreement has yet been reached.[76] It is clear that the core of crystalline rocks which is overlain unconformably by the Hollybush Sandstone must be older than the Upper Cambrian rocks. There is no good evidence of any stratigraphical break in the Cambrian system of England or Wales, and it may be reasonably inferred that the break seen at the base of the Hollybush Sandstones indicates that the rocks underneath that horizon are pre-Cambrian. Some portions of these certainly very ancient rocks are gneisses or schists; others have been described as "felsites," and have been regarded as passing into schists, and as the original material from which portions of the foliated series of the range have been produced by mechanical deformation. Not improbably the whole series of rocks is of igneous origin, but has been subsequently rendered more or less schistose.
[76] There is no room here for a full bibliography of the geological literature devoted to this locality. In the monograph by J. Phillips in vol. ii. part i. of the Memoirs of the Geological Survey, a list of writings is given up to the time of its publication in 1848. Since that year many additional papers have appeared. I may especially refer to H. B. Holl, Quart. Journ. Geol. Soc. xxi. (1865) p. 72; J. H. Timins, op. cit. xxii. (1867); Mr. F. Rutley, op. cit. xliii. (1887) p. 481; Dr. Callaway, op. cit. xliii. (1887) p. 525, xlv. (1889) p. 475, xlix. (1893) p. 398; Prof. Green, op. cit. li. (1895) p. 1; Mr. H. D. Acland, Geol. Mag. 1894, p. 48.
There is one area where the rocks have escaped metamorphism, and where they present some of the well-known features of ancient volcanic materials. This tract was first indicated by Dr. H. B. Holl as one occupied by "altered primordial rocks and post-primordial trap." Its evidently igneous materials have been examined and described by different observers, among whom Dr. Callaway has contributed some detailed papers on the subject. More recently Professor Green, who had the advantage of sections exposed in the excavations for the construction of a reservoir for supplying water to Great Malvern, came to the conclusion that the rocks consist mainly of felsites, having many of the characters of rhyolites. With these are associated felsitic tuffs, while bands of dolerite, probably intrusive, form likewise part of the series. So far as the somewhat meagre evidence allows an opinion to be formed, there appears to be an alternation of felsites, lavas and tuffs placed in a more or less vertical position, striking in a northerly direction, and traversed by several sheets of intrusive dolerite.
No junction has been found between these unfoliated volcanic rocks and the schists that form the core of the range. Judging merely from their present relative condition, one would naturally infer that the volcanic rocks must be the younger of the two groups. But, as Professor Green has pointed out, it is conceivable that the latter may have locally escaped crushing, and yet be of the same age as the felsites and epidiorites of the neighbouring Raggedstone Hill, which have been in part considerably affected by mechanical movements.[77]
[77] Op. cit. p. 7. The metamorphism of the igneous rocks of the Malvern Hills into schists has been especially investigated by Dr. Callaway.
For our present inquiry it is perhaps sufficient to take note that in the heart of the Malvern Hills there lies a remnant of a volcanic district, probably of pre-Cambrian age, the rocks of which had been raised up into a vertical position so as to form islets or reefs in the sea in which the Upper Cambrian strata (Hollybush Sandstone and Upper Lingula shales) were deposited. Until some more precise evidence is obtained as to the geological age of these rocks it may be convenient to place them provisionally with the volcanic Uriconian series.
vi. THE CHARNWOOD FOREST VOLCANO
In the heart of England the great Triassic plain is diversified by the uprise through it of the peaks and crests of an old Triassic land-surface, which are embraced in the district known as Charnwood Forest. These scattered eminences consist of materials not only immensely older than the Trias, but once doubtless buried under thousands of feet of Palæozoic strata. They had been laid bare by denudation and carved into picturesque crags and pinnacles before the New Red Sandstone was deposited around and above them.
To these vestiges of an early Mesozoic land, still half buried among Triassic strata, a peculiar interest attaches from the obviously high antiquity of their rocks and their uprise in the very centre of the island. Various opinions have been expressed as to the age of their component rocks. When they were mapped by the Geological Survey they were recognized to be as old as any group of rocks then known, and they were accordingly placed in the Cambrian system. More recent research has suggested that they may be still more ancient, and may be regarded as pre-Cambrian.
The rocks of Charnwood Forest have been the subject of an exhaustive research by the Rev. E. Hill and Professor Bonney, to whom most of our knowledge regarding them is due. These observers first pointed out the truly volcanic nature of the coarse clastic rocks of the district. They have traced their relations in the field, and have likewise described their structure and composition as shown by the microscope. Subsequently the district has been re-mapped on the scale of six inches to a mile by Mr. Fox Strangways for the Geological Survey, while Mr. W. W. Watts, another member of the Survey, has studied the petrography of the ground, and has traced the boundaries of the several rock-groups so far as these can be determined. Confirming generally the stratigraphical arrangement sketched by Messrs. Hill and Bonney, Mr. Watts has proposed the following classification of the rocks:—[78]
| 7. | Groby and Swithland slates. |  | The Brand series. |
| 6. | Hanging Rocks conglomerate and Bradgate quartzite. | ||
| 5. | Woodhouse beds (ashy grits). |  | The Maplewell series (volcanic tuffs and agglomerates). |
| 4. | Slate-agglomerate of Roecliffe. | ||
| 3. | Hornstone beds of Beacon Hill. | ||
| 2. | Felsitic agglomerate of Benscliffe. | ||
| 1. | Quartzose, felspathic and felsitic grits. | The Blackbrook series. | |
[78] Annual Report of Director-General of the Geological Survey, in the Report of Science and Art Department for 1895.
Under any computation or measurement, the total thickness of detrital material in this series of formations must amount to several thousand feet. The chief interest centres in the middle series, which consists largely of fragmental volcanic rocks, with intercalations of slate and grit. As was first shown by Mr. Hill and Professor Bonney, these volcanic materials vary from exceedingly coarse agglomerates to fine, ashy or felspathic slates. In most cases distinct bedding can be recognized in them, but more particularly in the fine-grained material. Yet even among the massive agglomerates a tendency may be seen towards an orientation of the blocks with their long axes parallel. That this arrangement is not entirely due to the effects of cleavage may be inferred from the many exceptions to it, which would hardly have occurred had such powerful cleavage affected the whole district, as would be needed to rearrange the large blocks in the agglomerates. Besides, the coarser parts often intercalate with fine felspathic grits, which distinctly mark the stratification of the whole.
The remarkably coarse breccia of Benscliffe is mainly made up of blocks of quartz-porphyry, felsite or rhyolite, with slate fragments. The Roecliffe agglomerate, another extraordinarily coarse rock, consists of slate fragments imbedded in an andesitic matrix, some of the blocks of slate being six feet long. The finer tuffs have been ascertained to consist of felsitic or andesitic detritus, sometimes forming exceedingly compact flinty rocks or hornstones.
In this thick accumulation of detrital rocks we are presented with a series of alternations of coarser and finer pyroclastic material, interstratified among green, grey and purple slates and grits, which probably represent the non-volcanic sediments of the time of eruption. The succession of strata bears witness to a long series of eruptions of varying intensity, but culminating at two distinct periods in the discharge of huge blocks of rock (Benscliffe and Roecliffe agglomerates).
After some search I have been unable to detect a single vesicular fragment among the stones in the breccias and tuffs, and Messrs. Hill and Bonney were not more successful. Not a trace of anything in the least degree scoriaceous is anywhere to be found. The paste in which the blocks lie consists of such fine material as would result from the trituration of felsite and slate. It contains many broken crystals of felspar, with grains of clear quartz. A gradation can be traced from the coarser into the finer bands of volcanic and non-volcanic material, fine slates being also interleaved with highly-felspathic partings of grit.
Having looked with some care for a trace of a true volcanic neck in the district, I have not seen anything that could be unhesitatingly so designated. Even in the north-western part of the district, where the breccias are coarsest, and there is least trace of ordinary sediments, some signs of bedding can usually be detected in the position of the imbedded stones and the partings of finer tuff. Both the coarser and finer detritus suggest the kind of material discharged from vents before the uprise of any lava. The entire absence of scoriaceous fragments is noteworthy, and the abundance of slate blocks rather points to the early eruptions of a volcanic focus. Possibly, while the chief centre of eruption lay towards the north-west, numerous vents may have been opened all over the district, discharging abundant showers of dust and stones, but seldom or never culminating in the actual outpouring of lava.
No indubitable lava-sheet has, in my judgment, been yet recognized in Charnwood Forest. Various opinions have been expressed as to some of the more compact close-grained rocks, and even the verdicts of the same observers have varied from time to time, the rocks once considered as felsites being afterwards regarded as tuffs, and subsequently placed with the felsites or andesites after all. It is not necessary for my present purpose to enter into these questions, which are rather of local interest. I will only say that, in my opinion, the rocks of Sharpley, Peldar, and Bardon Hill are massive rocks, as they have finally been classed by Messrs. Hill and Bonney. But I cannot look upon them as lavas, at least I have seen no evidence to lead me to believe that they were ever erupted at the surface. I have fully considered the arguments of Mr. Hill and Professor Bonney on this point.[79] There can, I think, be no doubt of the close association of these felsitic rocks and the breccias, but the structure of the rocks in the field seems to me to be decidedly in favour of the view expressed above. The microscope affords no assistance in the question.[80] The doubtful rocks seem to me rather to be intrusive masses which have been protruded into the volcanic sedimentary series among which they rise. They are acid, fine-grained, porphyritic rocks, which would formerly have been included under the general name of felsites or quartz-porphyries. Their coarse porphyritic parts rapidly pass into close-grained felsitic material. Many of the blocks in the breccias are precisely like parts of these rocks. It might hence be asserted that these fragmental deposits are later than the eruptive bosses. At least it is obvious that rocks of the same type as those of Sharpley, Peldar, and Bardon Hill must have been disrupted to produce the coarse breccias.
[79] Quart. Journ. Geol. Soc. xlvii. (1891), pp. 80-88.
[80] See Messrs. Hill and Bonney, op. cit. xxxiv. (1878), p. 211.
Later eruptive rocks, consisting of masses of syenite and granite, with still younger dykes of dolerite, andesite, diorite and felsite, have successively made their appearance, and add to the diversity of the igneous phenomena of this district.
The question of the age of this isolated volcanic series is one of much interest, but of great perplexity. Though a resemblance may be admitted to exist between some of the slates and parts of the Cambrian system of North Wales, the difference between the Charnwood rocks and the undoubted Cambrian series of Warwickshire, only thirteen miles to the south-west, is such as to indicate that the former are probably older than the latter. While the Charnwood rocks have been intensely cleaved and crushed, those of Warwickshire have undergone no such change. The argillaceous strata in the one region have been converted into slates, in the other they remain mere shales. Though cleavage is sometimes irregularly developed, its rapid disappearance in so short a distance as the interval between Charnwood Forest and Nuneaton seems most explicable if we suppose that the rocks at the more easterly locality were cleaved before those towards the west were deposited. If this inference be well grounded the pre-Cambrian age of the Charnwood volcanoes would be established. But the argument is not conclusive. No fossils of any kind have yet been found in any of the old rocks of Charnwood.[81] Merely lithological resemblances or differences are all that can be used as a guide to the geological age of these masses. Mr. Watts has suggested that possibly the quartzite of Bradgate (No. 6 of the Charnwood groups) may be the equivalent of the quartzite which in Shropshire and Warwickshire forms the base of the sedimentary Cambrian formations. If that correlation could be established, the volcanic series below the quartzite in Charnwood might be regarded as representing the Uriconian volcanic series of Shropshire.
[81] Since this page was in type, Professor Lapworth has found a worm-burrow low down in the Brand Series, and one or two additional examples have since been obtained by Mr. J. Rhodes of the Geological Survey. These are the first undoubted organisms from the Charnwood Forest rocks. Mr. Watts, Geol. Mag. 1896, p. 487.
BOOK III
THE CAMBRIAN VOLCANOES
CHAPTER IX
CHARACTERISTICS OF THE CAMBRIAN SYSTEM IN BRITAIN
The Physical Geography of the Cambrian Period—The Pioneers of Palæozoic Geology in Britain—Work of the Geological Survey in Wales—Subdivisions of the Cambrian System in Britain.
In leaving the investigation of the pre-Cambrian rocks and entering upon that of the Palæozoic systems, that is, the great series of sedimentary formations which include the earliest records of organized life upon the surface of the globe, the geologist feels much as the historian when, quitting the domain of legend and tradition, he can tread firmly in the region of documentary evidence. From the bottom of the Cambrian system upward through the long series of geological formations, the chronicle, though often sadly incomplete, is usually clear and legible. As we follow the lowest fossiliferous strata across a territory, we recognize that they bear witness to the same processes of denudation and deposition which have been going on uninterruptedly on the face of the globe ever since. The beds of conglomerate represent the gravels and shingles of old coast-lines and river-beds. The sandstones recall the familiar features of sandy sea-bottoms not far from land. The shales bear witness to the fall of fine sediment in stiller water, such as now takes place in the deeper parts of seas and lakes. Notwithstanding their vast antiquity, the strata themselves exhibit no exceptional peculiarities of origin. They seem to be just such familiar deposits as are gathering under fitting conditions at the present time.
Some writers have speculated on the far greater intensity of all geological activities in the early times of the planet's history. But if we may interpret the record of the stratified formations by the phenomena of to-day, there is for these speculations no confirmation in the sedimentation of the oldest stratified deposits. It is of course quite intelligible, if not probable, that many geological forces may have been more vigorous in primeval times than they afterwards became. But of the gigantic tides, prodigious denudation and violent huddling together of the waste of the earth's surface, which have been postulated for the early Palæozoic ages, there is assuredly nowhere any indication among the stratified formations. In those vast orderly repositories, layer succeeds layer among thinly-laminated shales, as gently and equably as the fine silt of each tide sinks to-day over the floor of a sheltered estuary. At the primeval period of which these sediments are the memorial, the waters receded from flat shores and left tracts of mud bare to the sky, precisely as they do still. Then as now, the sun shone and dried such mud-flats, covering their surfaces with a network of cracks; the rain fell in heavy drops, that left their imprints on the drying mud; and the next tide rose so gently as to overflow these records of sunshine and shower without effacing them, but spreading over them a fresh film of sediment, to be succeeded by other slowly-accumulating layers, under which they have lain preserved during the long cycles of geological history.
That organized creatures had already appeared upon the earth's surface before the beginning of the Cambrian period cannot be doubted. The animal remains in the lowest Cambrian strata are far from being the simple forms which might be expected to indicate the first start of animal life upon the surface of the earth. On the contrary, though they are comparatively scanty in types, and often rare or absent throughout a thick mass of sedimentary deposits, they show beyond dispute that, when they flourished, invertebrate life had already reached such a stage of advancement and differentiation that various leading types had appeared which have descended, in some cases with generic identity, down to our own day. There must have been a long pedigree to these organisms of the oldest known fossiliferous rocks. And somewhere on the earth's surface we may yet hope to find the remains of their progenitors in pre-Cambrian deposits.
The researches of many explorers in Europe and North America have brought to light an interesting series of organic remains from the Cambrian system. Of the plants of the time hardly any traces have survived, save some markings which have been referred to sea-weeds. The earliest known sponges and corals occur in this system, likewise the ancestors of the graptolites, which played so prominent a part in the life of the next or Silurian period. There were already representatives of crinoids and star-fishes, besides examples of the extinct group of cystideans. Sea-worms crawled over the muddy and sandy sea-bottom, for they have left their trails and burrows in the hardened sediments. Molluscs had by this time appeared in their four great divisions of Brachiopods, Lamellibranchs, Gasteropods and Cephalopods, though the forms yet discovered among Cambrian rocks are comparatively few. The most abundant and characteristic inhabitants of the Cambrian seas were the trilobites, of which many genera have been disinterred from the strata. In the lowest fossiliferous Cambrian group the trilobitic genus Olenellus, already referred to, is the characteristic form. Higher up Paradoxides is predominant, while towards the top of the system the most characteristic genus is Olenus.
From the organic remains which have been preserved, we may legitimately infer the existence of others which have entirely disappeared. There seems no reason to doubt that the leading grades of invertebrate life which are wanting in the known Cambrian fauna were really represented in the Cambrian seas. The chance discovery of a band of limestone may any day entirely alter our knowledge as to the relative proportions of the several divisions of the animal kingdom in the earliest Palæozoic rocks. Sand is rather adverse to the preservation of a varied representation of the organisms of the overlying sea-water. Mud is generally favourable, but calcareous accumulations are greatly more so, and they usually consist almost entirely of organic remains. Thus in the Cambrian series of the north-west of Scotland the quartzites that form the lower group, though sometimes crowded with worm-burrows, contain hardly any other sign of organisms. The overlying shales, besides their abundant worm-castings, have yielded perfect specimens of Olenellus and other fossils. But in the uppermost group, consisting of limestones, every particle of the sediment appears to have passed through the intestines of worms, and as it gathered on the sea-bottom it enclosed and has preserved a varied and abundant assemblage of organisms, including trilobites, gasteropods and a number of cephalopods. While in the Cambrian rocks of Europe calcareous bands are comparatively rare, in those of North America they are not infrequent. Hence it is largely from American deposits that our knowledge of the Cambrian fauna has been derived.
Not a vestige of any vertebrate organism has yet been detected among the earlier Palæozoic sediments. So far as we know, there were no fishes in the Cambrian seas. The highest organisms then existing were chambered shells, a once abundant and singularly varied class, of which the living Nautilus is now the sole representative.
In trying to realize the general geographical conditions of Cambrian time, the geologist finds himself entirely without any evidence as to the character of the terrestrial vegetation. We can hardly doubt that the land was clothed with plants, probably including lycopods and ferns, possibly even cycads and conifers. But no remains of this flora have yet been recovered. Nor have any traces of land-animals been detected. All that we yet know of the life of the period has been gleaned from marine sediments, which show that the invertebrate population by which the sea was then tenanted embraced some of the leading types of structure that have survived through all the long vista of geological time down to our own day.
Some of the shore-lines of the Cambrian waters may still be traced, and it is possible to say where the land of the time stood and where lay the sea. In the British area the largest relic of Cambrian land is found in the far north-west of Scotland. Formed partly of the Lewisian Gneiss and partly of the Torridon Sandstone, it takes in the whole chain of the Outer Hebrides and likewise part of the present western seaboard of Sutherland and Ross. Along the margin of that northern land the white sand was laid down which now gleams in sheets of snow-like quartzite on most of the higher mountains from Cape Wrath to Skye. The sea lay to the east and, so far as we know, may have stretched across the rest of Scotland, and the north and centre of England. Another vestige of the land of this ancient era occurs in Anglesey. There, and likewise over scattered tracts in the Midlands, and in the south-west of England, the geologist seems to descry the last relics of islets that rose out of the Cambrian sea, and are now surrounded with its hardened sediments.
While such was the general aspect of the region of the British Isles during Cambrian time, volcanic action manifested itself at various localities over the area, breaking out on the sea-bottom, and pouring forth sheets of lava and showers of ashes, which mingled with the sand and silt that were settling there at the time. In the northern or Scottish tract no trace of this subterranean activity has been found; but in the English Midlands and over much of Wales abundant evidence has been obtained to show that in those districts the Cambrian period was marked by frequent and prolonged eruptions.
As its name denotes, the Cambrian system is typically developed in Wales. It was there that Sedgwick first worked out the stratigraphical relations of its ancient sediments, and that Murchison demonstrated the succession of organic remains contained in them, applying to them the principles of classification laid down by William Smith in regard to the Secondary formations. It was there too that some of the earliest and most memorable achievements were made in the investigation of ancient volcanic rocks. Sedgwick and Murchison, besides the admirable work which they accomplished in establishing the stratigraphy of the older Palæozoic formations, clearly recognized that among these formations there were preserved the records of contemporaneous submarine eruptions. Sedgwick showed that the mountainous masses of eruptive rock in North Wales were really lavas and ashes, which had been discharged over the sea-floor at the time when the ancient sediments of that region were deposited, while Murchison established the same fact by numerous observations in the east and south of Wales, and in the bordering English counties. De la Beche had found similar evidence among the "grauwacke" rocks of Devonshire.[82]
[82] For early researches on the older Palæozoic volcanic rocks of Britain, see Sedgwick, Proc. Geol. Soc. vols. ii. (1838) pp. 678, 679, iii. (1841) p. 548, iv. (1843) p. 215; Quart. Journ. Geol. Soc. vols. i. (1845) pp. 8-17, iii. (1847) p. 134. Murchison, Proc. Geol. Soc. vol. ii. (1833-34) p. 85; Silurian System (1839) pp. 225, 258, 268, 287, 317, 324, 401; Siluria, 4th edit. (1867) p. 76 et seq. De la Beche, Mem. Geol. Survey, vol. i. (1846) pp. 29-36. A. C. Ramsay in the Maps and Horizontal Sections of Wales published by the Geological Survey; also Descriptive Catalogue of the Rock-Specimens in the Museum of Practical Geology, 1st edit. (1858), 2nd edit. (1859), 3rd edit. (1862); "The Geology of North Wales," forming vol. iii. of Memoirs of the Geological Survey, 1st edit. (1866), 2nd edit. (1881).
Following in the track thus opened up by these great masters, the officers of the Geological Survey were enabled to unravel, as had never before been attempted, the complicated structure of the old volcanic regions of Wales. At the outset of the following discussion I wish to express my admiration of the labours of the early pioneers who thus laid for us the foundation of our knowledge of volcanic action in the Palæozoic periods. To De la Beche and his associates in the Survey a special measure of gratitude is due from all who have followed in their steps and profited by their work. When we consider the condition of geological science, and especially of the department of petrography, in this country at the time when these early and detailed investigations were carried on, when we remember the imperfection of much of the topography on the old one-inch Ordnance maps (which were the only maps then available), when we call to mind the rugged and lofty nature of the ground where some of the most complicated geological structures are displayed, we must admit that at the period when these maps and sections were produced they could not have been better done; nay, that as in some important respects they were distinctly in advance of their time, their publication marked an era in the progress of structural, and especially of volcanic, geology. The separation of lavas and tuffs over hundreds of square miles in a mountainous region, the discrimination of intrusive sheets and eruptive bosses, the determination of successive stratigraphical zones of volcanic activity among some of the oldest fossiliferous formations, were achievements which will ever place the names of Ramsay, Selwyn, Jukes and their associates high in the bede-roll of geological science. No one ever thinks now of making a geological excursion into Wales without carrying with him the sheets of the Geological Survey map. These form his guide and handbook, and furnish him with the basis of information from which he starts in his own researches.
But science does not stand still. The most perfect geological map that can be made to-day will be capable of improvement thirty or forty years hence. The maps of the Geological Survey are no exception to this rule. In criticizing and correcting them, however, let us judge them not by the standard of knowledge which we have now reached, but by that of the time when they were prepared. It is easy to criticize; it is not so easy to recognize how much we owe to the very work which we pronounce to be imperfect.
The ancient volcanoes of Wales, thanks mainly to the admirable labours of my former friend and chief, Sir Andrew C. Ramsay, have taken a familiar place in geological literature. But a good deal has been learnt regarding them since he mapped and wrote. The volcanic history, as he viewed it, began in the Arenig period. The progress of subsequent inquiry, however, has shown that there are volcanic rocks in Wales of much older date. I shall show that the Cambrian period, both in South and North Wales, was eminently volcanic.
Much controversy having arisen as to the respective limits and nomenclature of the older Palæozoic rocks, let me state, at the outset of the inquiry into the volcanic eruptions of Cambrian time, that under the term "Cambrian" I class all the known Palæozoic rocks which lie below the bottom of what is termed the Arenig group. It was maintained by Sir Andrew Ramsay and his colleagues on the Geological Survey that on the mainland of Wales no base is ever found to the Cambrian system. More recently certain conglomerates have been fixed upon as the true Cambrian base, both in South and North Wales, and endeavours have been made to trace an unconformability at that line, all rocks below it being treated as pre-Cambrian. But conglomerates do not necessarily mark a stratigraphical discordance, and in South Wales there is no trace of any unconformability between the strata above and below the supposed line of break.[83] Professor Bonney has shown that in North Wales several zones of conglomerate have been erroneously identified as the supposed basal platform of the Cambrian series, and more recently Mr. Blake has pointed out that some of these conglomerates are unquestionably Lower Silurian.
[83] See a discussion of this subject in Quart. Journ. Geol. Soc. vol. xxxix. (1883), p. 305.
My own examination so far confirms the conclusions arrived at by these observers. Like my predecessors in the Geological Survey, however, I have been unable to detect anywhere in Caernarvonshire or Merionethshire a base to the Cambrian system, and I am compelled to agree with them in regarding as Cambrian (partly even as Lower Silurian) all the rocks from Bangor to Llanllyfni, which have more recently been classed as pre-Cambrian. But though thus supporting their general stratigraphy, I am bound to acknowledge that they failed to recognize the existence of a great volcanic series below the Arenig horizon. The existence of this series, noticed by Sedgwick, was first definitely stated by Professor Hughes,[84] and his statements have been confirmed and extended by subsequent observers, notably by Professor Bonney and Mr. Blake. The Cambrian period is thus proved to have been perhaps even more continuously volcanic than the Lower Silurian period was in Wales.
[84] Proc. Camb. Phil. Soc. vol. iii. (1877), p. 89. The Cambrian volcanic areas of North Wales are represented in [Map II.]
The following table shows the subdivisions of the Cambrian system now recognized in Britain:—
| Wales. (Ranging up to 12,000 feet or more.) | Western England. (About 3000 feet.) | N.W. Scotland. (About 2000 feet.) | ||
Upper or Olenus Zones. |  | Tremadoc Slates Lingula Flags (Lingulella, Olenus, etc. | Shineton Shales (Dictyograptus or Dictyonema, Olenus, etc.). | Limestones, about 1500 feet thick, divisible into seven groups (Archæocyathus, Maclurea, Ophileta, Murchisonia, Orthoceras, and vast quantities of annelid castings). |
Middle or Paradoxides Zones. |  | Menevian group (Paradoxides). | Conglomerates and limestones (Comley), with Paradoxides, etc. | |
Lower or Olenellus Zones. |  | Harlech and Llanberis group with basement volcanic rocks; bottom not seen. | Thin quartzite passing up into green flags, grits, shales and sandstone (Comley Sandstone), containing Olenellus. | Shales ("fucoid beds"), with Olenellus, Salterella, etc. Quartzites with annelid burrows. The base of the series lies unconformably on pre-Cambrian rocks. |
CHAPTER X
THE CAMBRIAN VOLCANOES OF SOUTH WALES
In the southern part of the Principality of Wales a remarkably varied display of British Cambrian volcanic rocks has been preserved. The district around St. David's has the distinction of being the first in which volcanic rocks of such high antiquity were recognized. As far back as the year 1842, Ramsay found that "felspathic volcanic ash" was associated with other proofs of igneous action, and this fact was recorded by him on the published Horizontal Sections of the Geological Survey. Unfortunately he afterwards came to regard the rocks as "altered Cambrian," thus following certain hypothetical views which, as will be further alluded to in the sequel, he had adopted in explanation of the phenomena in Caernarvonshire and in Anglesey. The volcanic nature of these ancient materials was subsequently rediscovered by Dr. Hicks, who has devoted much time and labour to their study. Distinguishing the volcanic series of St. David's by the name "Pebidian," he has contended that it forms a pre-Cambrian system separated by an unconformability from the base of the Cambrian formations. He likewise endeavoured to show that an older system of rhyolitic lavas, felsitic breccias and hälleflintas could be distinguished, which he termed "Arvonian"; and more ancient still, a core of granitoid or gneissic rocks, which he separated under the name of "Dimetian." My own investigation of the ground thoroughly convinced me that there are no pre-Cambrian rocks at St. David's; that the "Arvonian" and "Dimetian" series are merely intrusive rocks (quartz-porphyry, granite, etc.) which have invaded the volcanic series; and that the "Pebidian," instead of being a pre-Cambrian formation on which the Cambrian base rests unconformably, is a group of volcanic rocks into which the Cambrian strata pass down conformably, and which in the St. David's district constitutes the lowest group of the Cambrian system.[85]
[85] For Dr. Hicks' views, see especially his papers in the Quart. Journ. Geol. Soc. vols. xxxi. xxxiii. xxxiv. xl. My criticism of them will be found in op. cit. vol. xxxix. (1883), subsequently in the main confirmed by Prof. Lloyd Morgan, op. cit. xlvi. p. 241. See also Prof. Blake, op. cit. xl. (1884). Dr. Hicks in his more recent papers has merely reiterated his previously published opinions.
Click on image to view larger
Walker & Boutall sc.
Fig. 40.—Map of the volcanic district of St. David's.
The volcanic geology of St. David's possesses a special interest inasmuch as it embraces a tolerably full development of various features which characterize the volcanic groups of later Palæozoic systems. Though the rocks are chiefly tuffs, they include also sheets of lava, as well as sills, dykes and bosses. They show a remarkable range in chemical composition from quite basic to highly acid materials. They present the amplest proofs of having been erupted and spread out over the sea-bottom, and they likewise afford clear evidence of alternation with the ordinary non-volcanic sediment of the time to which they belong. In these respects they are particularly noteworthy, for they prove that in the earliest Palæozoic ages the essential features of volcanic action were already as well developed as in any subsequent epoch of geological history.
The volcanic group of St. David's attains a visible thickness of about 1800 feet. Its upper part graduates upward into purple and green Lower Cambrian sandstones. The base of the group is not seen owing to the plicated structure of the district. Hence the total thickness of volcanic material cannot be determined, neither can we tell on what it rests, whether on a still lower sedimentary series or on some platform of pre-Cambrian rocks.
The structure of the group, notwithstanding all that has been written about it, has never yet been adequately worked out. The unfortunate and barren controversy about supposed pre-Cambrian rocks at St. David's has tended to obscure the real importance of these rocks as the oldest well-preserved record of volcanic action in Britain. They deserve to be carefully surveyed on maps of a large scale, in the same detailed manner as has been so successfully applied to the elucidation of younger volcanic tracts. Until such detailed investigation is made, any account of them which is given can be little more than a general outline of the subject. The following description is the result of my examination of the ground in company with my colleague Mr. B. N. Peach, and afterwards with the late Mr. W. Topley.[86] A few additional observations, from the subsequent exploration of Professor Lloyd Morgan,[87] are incorporated in the narrative.
[86] Quart. Journ. Geol. Soc. vol. xxxix. (1883), p. 294 et seq. While the essential parts of the investigation are given in the following pages, I would refer the reader to this paper for details not transferred to the present volume.
[87] Op. cit. vol. xlvi. (1890), p. 241.
The geologist who traces these St. David's rocks in the field cannot fail to be struck with their general resemblance to volcanic masses of later Palæozoic date. Many of the lavas and tuffs are in outward characters quite indistinguishable from those of the Lower Old Red Sandstone and Carboniferous systems of Britain. So many points of detail may be observed to be common to the Palæozoic eruptive rocks all over the country from the Cambrian to the Permian periods as to indicate that volcanic phenomena must have recurred under much the same conditions throughout Palæozoic time.
By far the larger part of the Cambrian volcanic group of St. David's consists of bedded tuffs, though a few lavas are interstratified in it, particularly towards the top. The whole has subsequently been invaded by acid protrusions, and lastly by basic dykes.
1. Bedded Tuffs and Lavas.—The tuffs, which are the predominant members of the volcanic group, present many varieties of colour, from dark purple, through tints of brick-red and lilac, to pale pink, yellow and creamy white, but not unfrequently assume various shades of dull green. They vary likewise in texture from somewhat coarse breccias or agglomerates, through many gradations, into fine silky schists in which the tuffaceous character is almost lost. Generally they are distinctly granular, presenting to the naked eye abundant angular and subangular lapilli, among which broken crystals of a white, somewhat kaolinized, felspar and fragments of fine-grained felsite are often conspicuous. The greater part of the tuffs, particularly the purple, red and dark-green varieties, which constitute so large a proportion of the whole, has been derived from the explosion of basic rocks similar in character to the diabases now found associated with them. On the other hand, the paler varieties, both in the form of fine tuffs and of breccias, have probably resulted mainly from the destruction of more siliceous lavas, probably felsites (rhyolites) or other acid rocks.
That many of the tuffs are due to the destruction of diabase-lavas may be surmised from their close general external resemblance to these rocks, and from the way in which they are associated with the contemporaneous sheets of diabase. Some of the dull dark-purple tuffs might almost at first sight be mistaken for truly eruptive rocks. The analyses of two typical examples of these basic tuffs (Nos. I. and II.), and one (No. III.) of an intermediate variety containing an admixture of acid fragments, are given in the subjoined table.
| SiO2 | Al2O3 | Fe2O3 | FeO | MnO | CaO | MgO | K2O | Na2O | H2O and Loss on Ignition. | Total. | Specific Gravity. | |
| I. | 51·25 | 20·41 | 3·02 | 3·91 | 0·21 | 4·53 | 7·22 | 2·93 | 1·82 | 5·02 | 100·32 | 2·84 |
| II. | 48·11 | 13·30 | 3·70 | 8·10 | 1·43 | 8·48 | 9·51 | 1·57 | 1·96 | 4·21 | 100·37 | 2·92 |
| III. | 61·54 | 16·30 | 4·40 | 3·66 | 0·32 | 3·08 | 2·99 | 1·62 | 2·81 | 2·99 | 99·71 | ... |
| I. | Purplish-red shaly tuff from below olivine-diabase, Crag Rhosson. Analysis by Mr. J. S. Grant Wilson. |
| II. | Dull purple and green tuff from the lowest group of tuffs between Pen-maen-melyn and Pen-y-foel. Analysis by Mr. Wilson. |
| III. | Greenish shaly finely granular tuff, from road-side, north of Board Schools, St. David's. Analysis by Prof. A. Renard of Ghent. |
Although the majority of the tuffs are more or less basic, they frequently contain evidence in the form of small felsitic lapilli that acid lavas were present in the eruptive vents, while the pale tuffs show that at the time of their discharge it was these acid lavas and not the diabases that were blown out by the explosions. Appended are three analyses of the acid tuffs (Nos. IV. V. and VI.).
| SiO2 | Al2O3 | Fe2O3 | FeO | MnO | CaO | MgO | K2O | Na2O | H2O and Loss on Ignition. | Total. | Specific Gravity. | |
| IV. | 80·59 | 11·29 | 0·28 | 1·41 | trace | 0·52 | 0·95 | 2·98 | 0·72 | 1·96 | 100·70 | 2·55 |
| V. | 73·42 | 12·09 | 0·91 | 3·13 | 0·25 | 2·94 | 1·12 | 1·67 | 3·88 | 1·28 | 100·69 | 2·74 |
| VI. | 72·63 | 16·23 | 2·70 | 0·48 | ... | 0·18 | 1·36 | 3·35 | 0·15 | 3·00 | 100·12 | ... |
| IV. | Greenish felsitic breccia, Clegyr Hill; angular fragments of various felsites in a greenish base. Analysis by Mr. J. S. Grant Wilson. |
| V. | Grey granular felsitic tuff, Bridge over Allan River north from St. David's Board Schools. Analysis by Mr. Wilson. |
| VI. | Pale pinkish-white, finely schistose tuff—a characteristic sample of the "Porth-lisky schists." Analysis by Prof. Renard. |
Many varieties of texture can be traced among the tuffs, from coarse breccias or agglomerates, with blocks a yard or more in length, to fine schistose mudstones or sericitic schists. One of the most remarkable of the finer kinds, found near Pen-y-foel, is externally dirty-green, compact and tolerably homogeneous, but with distinct evidence of its clastic character. Under the microscope it is found to be composed mainly of lapilli of a peculiar rock, which is characterized by the abundance and freshness of its plagioclase (an unusual feature in the volcanic group of St. David's); by the large, well-defined crystals (one of which measured 0·022 inch by 0·0125 inch) of augite; by large crystals replaced by green decomposition-products, but having the external form of olivine; by the absence or scantiness of any base or groundmass; and, in many of the lapilli, by the abundance of spherical cells, either empty or filled up as amygdales with decomposition-products. These spherical vapour-vesicles, so characteristic of the basic or palagonitic lapilli in many Palæozoic volcanic vents, were found in one fragment, where they were particularly abundant, to range from a minimum of 0·0008 inch to a maximum of 0·0033 inch, with a mean of about 0·0018. The rock from which these lapilli have been derived comes nearest to one of the diabases from the same part of the district (which will afterwards be referred to), but shows a closer approach to basalt rocks.
Another interesting tuff is that of which the analysis (No. II.) has been given. It occurs not far from the horizon of the rock just described. Under a low power, it is seen to be composed mainly of fragments of diabase like the rocks of Rhosson and Clegyr Foig. These fragments are subangular, or irregular in shape, and vary considerably in size. They are sometimes finely cellular—the cavities, as in the case just referred to, being spherical. The plagioclase crystals in the diabase-lapilli are everywhere conspicuous; so also is the augite, which occurs in larger forms than in the rock of Rhosson or Clegyr Foig. Next in abundance to these basic fragments are rounded or subangular pieces of felsite. These weather out in conspicuous grey rough projections on the exposed face of the rock; under the microscope they are seen to consist of fine granular felsite, which shows a groundmass remaining dark between crossed nicols, but with luminous points and filaments, and an occasional spherulite giving the usual cross in polarized light. Lapilli of an older tuff may here and there be detected. A few angular and subangular grains of quartz are scattered through the rock. The lapilli are bound together by a finely-granular dirty-green substance.
As a typical illustration of the minute structure of the felsitic tuffs, I may refer to the rock No. V. of the foregoing analyses. It is composed mainly of fragments of various felsites, many of which show good flow-structure. Large, and usually broken, crystals of orthoclase are dispersed among the other ingredients. Here and there a fragment of diabase may be detected; but I could find no trace of pieces of the peculiar microcrystalline spherulitic quartz-porphyries of St. David's. There is but little that could be called matrix cementing the lapilli together. The presence of fragments of diabase may possibly reduce the proportion of silica and increase that of magnesia, as compared with what would otherwise have been present in the rock.
Some of the tuffs appear to have been a kind of volcanic mud. A specimen of this nature collected from the road-side section, north of the Board School, presents a finely-granular paste enclosing abundant angular and subangular lapilli of diabase, a smaller proportion of felsite (sometimes displaying perfect flow-structure), broken plagioclase crystals, and a greenish micaceous mineral which has been subsequently developed out of the matrix between the lapilli.
Though they lie in the sedimentary series above the main volcanic group, I may refer here to certain thin bands of tuff at Castell, on account of their interest in relation to the true Cambrian age of the volcanic group. They are not quite so fresh as the tuff that occurs in thicker masses, but their volcanic origin is readily observable. One band appears to be made up of the debris of some basic rock, like the diabase of the district, through which detached plagioclase crystals are scattered. The lapilli are subangular; and around their border a granular deposit of hæmatite has taken place, giving a red colour to the rock. Another band presents small angular lapilli, almost entirely composed of a substance which to the naked eye, or with a lens, is dull, white and clay-like, easily scratched, and slightly unctuous to the touch. Under the microscope, with a low power, it becomes pale greyish-green and transparent, and is seen to consist in large part of altered felspar crystals, partially kaolinized and partially changed into white mica and calcite. These scattered crystals are true volcanic lapilli, and have not been derived from the mechanical waste of any pre-existing volcanic rock. In the tuffs interstratified with the conglomerate, at the quarry above Porth-clais, though much decomposed, crystals of plagioclase can likewise still be traced. These strata are also true tuffs, and not mere detritus due to mechanical degradation (see [Fig. 41]).
The general result of the study of the microscopic structure of the Cambrian tuffs of St. David's may be briefly summed up as follows:—
1. These pyroclastic deposits are almost wholly composed of fragments of eruptive rocks, sometimes rounded, but usually angular or subangular. In the more granular varieties very little matrix is present; it consists of fine debris of the same materials. No detached microlites have been noted, such as are common among modern volcanic ashes; but there are abundant ejected crystals. In these respects the Cambrian tuffs resemble those of the other Palæozoic systems. A mingling of grains of quartz-sand may indicate the intermixture of ordinary with volcanic sediment.
2. They may be divided into two groups—one composed mainly of fragments of diabase or other similar basic rocks, the other of felsite. The former group has doubtless been derived from the explosion of such rocks as the diabase-sheets of the district. The felsitic tuffs have not been observed to contain any fragments of the microcrystalline quartz-porphyries of St. David's. They have been derived from true fine-grained felsites or rhyolites. There are various intermediate varieties of tuff, due to the mingling in various proportions of the two kinds of debris.
3. They are marked by the presence of some characteristic features of the volcanic vents of later Palæozoic time, and in particular by presenting the following peculiarities: (a) lapilli of a minutely-cellular pumice with spherical cells; (b) lapilli with well-developed flow-structure; (c) lapilli consisting of a pale green serpentinous substance resembling altered palagonite and probably originally glass; (d) lapilli derived from the destruction of older tuffs; and (e) lapilli consisting of ejected crystals, especially of felspars, sometimes entire, often broken.
4. They frequently show that they have undergone metamorphism, by the development of a pale greenish micaceous mineral between the lapilli, the change advancing until the fine tuffs occasionally pass into fine silky schists.
In my study of the St. David's district, I was unable to observe any evidence that the basic and siliceous tuffs characterize two distinct periods of volcanicity. From the foregoing analyses it appears that some of the oldest visible tuffs which are seen between Pen-maen-melyn and Pen-y-foel contain only 48·11 per cent of silica; while a specimen from Porth-lisky yielded 72·63 per cent of that ingredient. Specimens taken even from adjacent beds show great differences in the percentage of silica, as may be seen in the analyses Nos. III. and V.
This alternation of basic and siliceous fragmental materials has its parallel in the neighbouring eruptive rocks, some of which are olivine-diabases containing only 45 per cent of silica, while others are highly siliceous quartz-porphyries. But all the siliceous eruptive rocks, so far as I have been able to discover, are intrusive, and belong, I believe, to a later period than that of the volcanic group; in no single instance do they appear to me to be true superficial lava-flows. On the other hand, the basic eruptive rocks occur both as contemporaneous sheets and as intrusive masses. The presence of both siliceous and basic lavas in the Cambrian volcanic reservoirs, however, is proved by the character of the tuffs. It would appear from the evidence at present known, that while the basic lavas were most abundant in the vents during the volcanic period recorded by the rocks of St. David's, furnishing the material for most of the fragmental eruptions, and issuing in occasional superficial streams of molten rock, the siliceous lavas did not flow forth at the surface, though their debris was copiously discharged in the form of dust and lapilli.
The rise of both basic and acid lavas at different periods in the same or adjoining vents, so familiar in recent volcanic phenomena, thus appears to have also characterized some of the oldest examples of volcanic action. An interesting parallel may be traced between the succession of events at St. David's and that which occurred in the volcanic group of the Lower Old Red Sandstone of the Pentland Hills, near Edinburgh, of which a detailed account will be given in [Chapter xx.] of this volume. It is also worthy of remark that in the latest of the volcanic episodes in British geology a remarkable similarity to the St. David's volcanic group may be observed. Some of the older Tertiary agglomerates are full of pieces of acid rocks (felsites, rhyolites or granophyres), while the lavas poured out at the surface were mainly basalts.
In the volcanic group of St. David's the tuffs contain evidence that ordinary sedimentation was not entirely interrupted by the volcanic discharges. Thus, in the Allan valley, west from the Cathedral, one of the schistose tuffs is full of well-rounded pebbles of white quartz. Occasional shaly bands indicate the deposit of mud with the tuffs.
Excluding the granites and porphyries (which are described at [p. 155]), two kinds of eruptive rocks are associated with the volcanic group. One of these is certainly intrusive and of late date, viz. dykes and veins of diabase, to be afterwards referred to. The other kind occurs in long parallel sheets, some of which, if not all, are true contemporaneous lava-streams, erupted at intervals during the accumulation of the volcanic group. They form prominent crags to the west of St. David's, such as Clegyr Foig, Rhosson, and the rocky ground rising from the eastern shores of Ramsey Sound. Their dip and strike coincide with those of the tuffs above and below them. It is possible that some of these sheets may be intrusive sills intercalated along the bedding of the tuffs; and in one or two cases I have observed indications of what, on further and more careful exploration, may prove to be disruption across the bedding.
But it is the interbedded sheets that possess the chief interest as superficial lava-streams of such venerable antiquity. They present many of the ordinary features of true lava-flows. In particular a slaggy structure may be detected at the bottom of a sheet, the vesicles being here and there lengthened in the direction of flow. Some of the sheets are in part amygdaloidal. The alternation of these sheets with tuffs, evidently derived from lavas of similar character, is another argument in favour of their contemporaneous date. One of the best localities for studying these features lies between Clegyr Foig and the coast, west of Rhosson.
The eruptive rocks thicken towards the south-west, as if the main vents had lain in that direction. There are doubtless intrusive as well as contemporaneously interbedded masses in the rough ground between Pen-maen-melyn and Treginnis. To separate these out would be a most interesting and beautiful piece of mapping for any competent geologist in possession of a good map on a sufficiently large scale.
The interbedded lavas, so far as I have had an opportunity of studying them, appear to present remarkable uniformity of petrographical characters. Megascopically they are dull, fine-grained to compact, sparingly porphyritic, ranging in colour from an epidote-green to dull blackish-green and dark chocolate-brown. Some of them are finely porphyritic from the presence of small glistening surfaces which present the colour and metallic lustre of hæmatite and yield its characteristic streak. Obviously basic rocks, they present, as I have said, a close external resemblance to many of the lavas of the Lower Old Red Sandstone and Carboniferous districts of Scotland. From their chemical composition and microscopic structure they may be most appropriately ranged among the diabases. The analyses of two of the most conspicuous diabases of this class in the district, those of Rhosson (VII.) and Clegyr Foig (VIII.), by Mr. J. S. Grant Wilson, are shown in the following table:—
| SiO2 | Al2O3 | Fe2O3 | FeO | MnO | CaO | MgO | K2O | Na2O | H2O and Loss on Ignition. | Insoluble Residue. | Total. | Specific Gravity. | |
| VII. | 45·92 | 18·16 | 1·18 | 9·27 | 0·19 | 7·19 | 10·07 | 1·78 | 2·12 | 4·22 | 0·04 | 100·14 | 2·96 |
| VIII. | 45·38 | 16·62 | 4·06 | 8·63 | 0·14 | 8·19 | 9·41 | 0·71 | 2·20 | 4·34 | 0·08 | 99·76 | 2·99 |
The two rocks here analyzed, likewise that from the crag south of Castell and that from the cliffs at the southern end of the promontory between Ramsey Sound and Pen-y-foel, show under the microscope a general similarity of composition and structure. They present a variable quantity of a base, which under a ⅕ objective is resolved into ill-defined coalescent globulites and fibre-like bodies, that remain dark when rotated between crossed nicols. In some varieties, as in part of Rhosson Crag, the base is nearly lost in the crowd of crystalline constituents; in others, as in the crag south of Castell, it forms a large part of the whole mass, and may be seen in distinct spaces free from any crystalline particles. Through this base are diffused, in vast numbers, irregularly-shaped grains of augite, seldom showing idiomorphic forms. These grains, or granules, may perhaps average about 0·003 inch in diameter. Plagioclase is generally hardly to be recognized, though here and there a crystal with characteristic twinning may be detected in the base. Magnetite occurs abundantly—its minute octahedra, with their peculiar colour and lustre, being apparent with reflected light on the fresher specimens, though apt to be lost as diffused ferruginous blotches in the more decomposed varieties. But perhaps the most remarkable ingredient is olivine. Red hæmatitic crystals are visible, even to the naked eye, dispersed through the groundmass of the rocks. With a lens these may be observed to be orthorhombic in form, and to be evidently pseudomorphs after some mineral which has been converted chiefly into hæmatite. Such red pseudomorphs are common in Carboniferous and Old Red Sandstone lavas, where in some cases they appear to be after hornblende, and in others after augite, but occasionally are suggestive of olivine, though with no trace of the original substance of that mineral. In the lava associated with the tuffs at the south end of the promontory between Ramsey Sound and Pen-y-foel, however, certain large, well-developed pseudomorphs are undoubtedly after olivine. They have the characteristic contour of that mineral and its peculiar transverse curved and irregular fractures. The average length of these pseudomorphs was found, from the measurement of six examples, to be 0·023 inch, the largest being 0·034, and the smallest 0·014. Seen by transmitted light they present a structureless pale-green material nearly inert in polarized light, round the borders and across fissures in which an opaque substance has been developed, as serpentine and magnetite have been grouped in the familiar alteration of olivine. The opaque material is bright brick-red in reflected light, and is evidently now chiefly oxidized into hæmatite. Every stage may be traced, from orthorhombic forms with the incipient development of transverse fissures filled with iron-oxide, to others of distorted shapes in which the ferruginous matter occupies the whole, or nearly the whole, of the mould of the original crystal.
The rocks now described differ from the Palæozoic andesites or "porphyrites," with which I am acquainted, in their more basic composition, in the less abundance of their microscopic base, in the comparatively inconspicuous development of their felspars of later consolidation, in the absence of large porphyritic felspars of earlier consolidation, in the extraordinary prominence of the granular augite, and in the presence of olivine. In composition and structure they are essentially forms of olivine-diabase.
Fig. 41.—Section showing the interstratification of tuff and conglomerate above Lower Mill, St. David's.
Above the volcanic group of St. David's lies a band of quartz-conglomerate which has been taken by Dr. Hicks to mark the base of the Cambrian system. This rock, though mainly composed of quartz and quartzite, contains fragments of the underlying volcanic rocks. But that it does not mark any decided break in the sedimentation, much less the violent unconformability and vast interval of time which it has been erroneously supposed to do, is well illustrated by the occurrence of bands of tuff, as well as diffused volcanic dust, in the conglomerate and also in the green and red shales and sandstones which conformably overlie it. An example of this intercalation of volcanic material is represented in [Fig. 41]. On the left side vertical layers of fine reddish tuff (a) are succeeded by a band of quartz conglomerate (b) of the usual character. Parallel to this conglomerate comes a band, about six inches thick, of fine tuff (c), followed by ashy sandstone (d), which graduates into conglomerate (e). No more complete evidence could be desired of the perfect inosculation of the conglomerate with the volcanic group. On the coast at Nun's Chapel similar evidence presents itself. The conglomerate there contains some thin seams of tuff, and is intercalated between a series of dull green agglomerates and tuffs and some greenish shales and sandstones with layers of tuff.
Not less striking is the evidence of the contemporaneous eruption of fine volcanic dust in the overlying shales and sandstones.[88] Some of the red shales are full of this material, which here and there is gathered into the thin seams or ribs of which the microscopic characters have already been described. This diffused volcanic detritus marks, no doubt, the enfeebled discharges of fine dust towards the close of the volcanic episode in the Lower Cambrian period at St. David's. It would be difficult to find an instance of a more perfect transition from a series of thoroughly volcanic masses into a series of ordinary mechanical sediments.
[88] These are a portion of Dr. Hicks' "Caerfai group" in the Lower Cambrian series. They have yielded Lower Cambrian fossils.
2. Intrusive Granite and Quartz-Porphyry.—The heart of the volcanic group is pierced by a mass of granite which also cuts the conglomerate and overlying shales and sandstones on the east side. The age of this intrusive boss cannot be more definitely fixed than by saying that it must be later than the volcanic group. This rock has been the subject of a remarkable amount of description, and has been dignified by being actually elevated into a distinct Archæan "formation" composed of "highly crystalline gneissic rocks," with "bands of limestone, hornblende, chlorite, and micaceous schists."[89] Into this somewhat dreary chapter of English geological literature it is fortunately not necessary to enter here. I will only say that the rock is unquestionably a granite, with no essential differences from many other bosses regarding which there has been no controversy. It is a holocrystalline rock with a thoroughly granitic texture, and composed of the ordinary minerals of granite—quartz, orthoclase and plagioclase, among which a green chloritic mineral, more or less plentiful, probably represents original hornblende, biotite or augite. Sometimes the quartz and felspar in the body of the rock show a micropegmatitic arrangement, and the same structure occurs in veins that traverse it. This structure gives the rock some resemblance to the Tertiary granites and granophyres of Ireland and Scotland.
[89] See the papers cited on [p. 145] and my discussion of the relations of this granite in Quart. Journ. Geol. Soc. vol. xxxix.; also Prof. Lloyd Morgan, op. cit. vol. xlvi. (1890).
This granite has undergone a good deal of decomposition, for its felspars are turbid, and its original ferro-magnesian constituents are always replaced by green chloritic aggregates, while epidote is also present. The rock tends to become finer in grain towards the margin, and then sometimes assumes a more decidedly pegmatitic structure, like graphic granite. At the northern end of the granite ridge, a gradation can be traced from the ordinary texture through increasingly fine-grained varieties until we pass into a microcrystalline spherulitic porphyry. After a careful examination of the ground I satisfied myself that the spherulitic quartz-porphyries, which form a conspicuous feature in the geology of St. David's, are really offshoots from this granitic core.[90]
[90] These apophyses from the granite constitute the "Arvonian" formation of Dr. Hicks' pre-Cambrian series of St. David's.
These spherulitic rocks have been fully described.[91] They consist of a base composed of a microcrystalline aggregate of quartz and orthoclase, which is distributed between the spherulites. These have been developed in remarkable beauty and perfection. While the microcrystalline structure is everywhere recognizable, the spherulites occasionally disappear. But their absence is merely local, and they may be found both in large dykes and narrow veins. A further porphyritic structure is given to the rocks by the presence in them of abundant quartz, which takes the form of conspicuous rounded blebs or worn crystals sometimes distinctly dihexihedral, but with somewhat blunted angles. Porphyritic plagioclase is also common. Flow-structure is occasionally traceable. Some parts of these rocks where the porphyritic elements are locally absent might be cursorily classed as felsites; but they all possess a microcrystalline and not a felsitic base. They cannot be confounded with the true felsites of which fragments occur in the tuffs.
[91] See, for example, J. Davies, Quart. Journ. Geol. Soc. vol. xxiv. p. 164, xxxv. p. 203; also the paper already referred to, op. cit. xxxix. p. 315; and Mr. Teall's British Petrography, p. 334.
In addition to the parallelism that may be traced between the earliest Palæozoic agglomerates and those of the youngest volcanic series of Britain, a close analogy may also be noticed between the acid intrusive rocks of the two widely-separated periods. In both cases we have a granitic core sending out apophyses which assume a spherulitic structure and traverse earlier volcanic products of the district.
These spherulitic quartz-porphyries of St. David's occur as bosses, dykes (elvans) or veins, cutting through all horizons of the volcanic group, and in one case apparently, if not actually, reaching the quartz conglomerate. One of the best exposures of this intrusive character may be seen in the cliff below Nun's Chapel, where the elvan runs along the face of the cliff through the uppermost zone of the volcanic group, cutting the strata somewhat irregularly. Apparently in connection with this dyke, a network of intrusions of decomposed quartz-porphyry may be observed in the shales along the face of the cliff immediately below Nun's Chapel. On the whole, the intruded material has forced its way along the bedding-planes of the shales, but has also broken across them, sending out finger-like branches.
3. Diabase Dykes and Sills.—The latest rocks of the St. David's district are dykes and intrusive sheets of diabase, which traverse all the other formations. The dykes are specially abundant in the granite. One or two may be detected in almost every artificial opening which has been made in that rock; while on the coast-section they are here and there profusely abundant. They are likewise frequent in the quartz-porphyries, and occur also in the volcanic group as well as in the sandstones and shales above the conglomerate, but become fewer in number as they recede from the granite centre.[92]
[92] The occurrence of these dykes is paralleled by that of the similar intrusions in the quartz-felsite of Llyn Padarn to be afterwards described.
In external characters, the rock composing these dykes and sheets may be described as usually a dull dirty-green or yellowish-brown mass, to which the old name of "wacke" might appropriately be given. It exhibits the texture and mode of weathering of the more distinctly crystalline members of the basalt family. It is occasionally amygdaloidal or cellular, the kernels or cavities being arranged parallel with the sides of the dyke. Here and there a rudely prismatic structure extends between the walls.
The microscopic structure of this rock has been described by Professor Judd, Mr. Davies and Mr. Tawney. It is a diabase, but more allied in structure to true basalt than the olivine-diabase of the volcanic group. It especially differs from the older rock in the abundance and freshness of its felspars, in the comparative scarcity of its augite, and in the absence of olivine. The magnesian silicates are very generally replaced by green decomposition-products diffused through the mass. An occasional crystal of hornblende, recognizable by its cleavages and dichroism, may be detected. Some of the diabase dykes present excellent examples of flow-structure. A beautiful instance occurs in a dyke that cuts the shales, in a small cove to the east of Nun's Chapel. The shale and eruptive rock are in contact; and the small acicular prisms of felspar, besides ranging themselves in line parallel to the side of the dyke, stream round the larger felspar crystals.
Some of the dykes or veins are only three inches broad. They send out fingers, and sometimes break abruptly across from one line to another. They appear generally to have followed the lines of joint in the granite, as Mr. Tawney has observed;[93] consequently they must be posterior to the development of the system of jointing in that rock.
[93] Proc. Nat. Hist. Soc. Bristol, vol. ii. part ii. (1879), p. 115.
Besides the abundant dykes, there has been a more limited extrusion of the same material in sheets parallel (or approximately so) to the bedding of the sandstones and shales. These sheets are well displayed at St. John's Point, where evidence of their being intrusive, and not truly bedded, may be seen along the fine cliffs which have been cut by the waves on this part of the coast-line.
The sedimentary series which overlies the volcanic group of St. David's, and contains the fossils of the lower part of the Cambrian system, gradually loses all trace of volcanic material, as its members are followed upward in stratigraphical order.[94] We thus learn that the eruptions of this district came to an end in an early part of the Cambrian period. But as we shall see in the following pages, volcanic activity was subsequently renewed at no great distance in the next or Silurian period.
[94] Dr. Hicks has noted the occurrence of "volcanic tuff" in the Lower Lingula Flags of Porth-y-Rhaw, a little to the east of St. David's (Quart. Journ. Geol. Soc. vol. xx. 1864, p. 240). This intercalation is marked as a "dyke" in the MS. notes of Sir A. C. Ramsay on a copy of the Geological Survey map of the district.