I. The Lower Silurian Series
i. Eruptions probably of Arenig Age
It is in that part of Ireland which lies east of a line drawn from Strabane to Dungarvan Harbour that the records of Lower Silurian volcanic activity are to be found. In the north the development of volcanic rocks resembles that in Scotland, in the south it corresponds rather with the volcanic districts of Wales.
The Irish Silurian volcanic rocks have been traced with more or less detail on the maps of the Geological Survey. Since these maps were published, however, great advances have been made in the study of the petrography of volcanic rocks, as well as in the art of tracing their structure upon maps. Much, therefore, now remains to be done to bring our knowledge of the older volcanic history of Ireland abreast of that of the rest of the British Isles. In the following summary I have had to rely mainly on my own traverses of the ground, guided by the maps and memoirs of the Survey, and with the personal assistance of some of my colleagues.
The remarkable zone of crushed cherts, igneous rocks and sandstones, probably of Lower Silurian age, which I have referred to ([p. 201]) as wedged in between the schists and the Old Red Sandstone along the southern margin of the Highlands of Scotland, reappears in Ireland. It occupies an area in the County Tyrone, about 24 miles long and about 9 miles broad at the broadest part, but disappearing towards the north-east and south-west.[268] Lying between the Palæozoic formations on the south and the schists on the north, it occupies a similar position to the Scottish belt, but presents a much broader area, and thus affords greater facilities for examining the rocks. It presents the same indefinite or faulted boundaries as in Scotland, so that its relations to the rocks along its flanks have not been satisfactorily determined. That the rocks of this area are older than the Silurian strata to the south of them seems to be established by the occurrence of fragments of them in these strata, and that they are younger than the schists may be inferred from their non-foliated character. But they have undoubtedly undergone considerable crushing by powerful terrestrial movements which have placed them in their present position.
[268] This area was mapped by Mr. J. Nolan for the Geological Survey, and was described by him in the Geol. Mag. for 1879. I visited it in company with my colleagues, Mr. B. N. Peach and Mr. A. M'Henry, in 1890 and again in 1894. My first conclusion was that the volcanic rocks should be regarded as part of the schistose series lying to the north of them (Pres. Address Geol. Soc. 1891, p. 77). But on the second visit, after having studied the rocks of the border of the Scottish Highlands, I formed the opinion stated in the text.
The special feature of interest in this Irish area is the remarkable development of volcanic materials which is there to be seen, spreading over a far wider area than in Scotland. The rocks include lavas associated with tuffs and agglomerates, likewise a varied series of intrusive masses.
The lavas are chiefly dull greenish, fine-grained rocks, having the general character of diabases and "porphyrites." They are sometimes quite slaggy, and where the amygdaloidal kernels remain, these are usually of calcite. Under the microscope, the diabases show in some parts that their lath-shaped felspars, and the augite which these penetrate, are tolerably fresh, while in other parts fibrous chlorite, granular epidote and veins of calcite bear witness to the metamorphism which they have undergone.
One of the most conspicuous features in some of these lavas is the occurrence of the same sack-like or pillow-shaped structure which has been already referred to as so marked among the Arenig lavas of Scotland. Though the vesicles of these rocks are often quite uncrushed, showing that there has been no general subsequent deformation of the whole mass, there occur local tracts where evidence of considerable movement may be noticed. Thus close to a mass of gneiss, and elsewhere along their margin, the lavas are apt to be much jointed and broken with numerous lines of shear, along which the crushed material assumes more or less of a schistose structure. Yet in the solid cores between these bands of crushing the original forms of the vesicles are retained.
These greenish lavas are occasionally interleaved with grey flinty mudstones, cherts and red jaspers, which are more particularly developed immediately above. In lithological character, and in their relation to the diabases, these siliceous bands bear the closest resemblance to those of Arenig age in Scotland. But no recognizable Radiolaria have yet been detected in them.
Besides the more basic lavas, there occur also, but less abundantly, platy felsitic rocks which have suffered much from shearing, and consequently have acquired a fissile slaty structure.
The agglomerates are made up of angular, subangular and rounded fragments imbedded in a matrix of similar composition. This matrix has in places become quite schistose, and then closely resembles some parts of the "green schists" of the Scottish Highlands. Of the inclosed stones the great majority consist of various felsites, which, weathering with a thick white opaque crust, are internally close-grained, dull-grey or even black, sometimes showing flow-structure, and of all sizes up to eight inches in diameter or more. There are also fragments of the basic lavas, and likewise pieces of chert and jasper. On many of the rocky hummocks no distinct bedding can be made out in the agglomerate, but in others the rock is tolerably well stratified.
The tuffs are fine silky schistose rocks, and seem to have been largely derived from basic lavas. They have suffered more than any of the other rocks from mechanical deformation, for they pass into green chloritic schists. Some portions of them are not unlike the slaty tuffs of Llyn Padarn in Caernarvonshire.
Accompanying the fragmental volcanic rocks, some ordinary sedimentary intercalations are to be found—red shales and pebbly quartzites, that seem to have escaped much crushing. The true order of succession in the volcanic series has not yet been determined. But apparently above this series come some dark shales, such as might yield graptolites, pale grits and occasional limestones.
Later than the lavas and the pyroclastic material are various intrusive masses, which in bands and bosses form numerous craggy hills throughout the area. So far as I have been able to observe, these rocks include two groups. Of these the older consists of basic injections, such as gabbros and allied rocks, some of which remind me of the so-called "hypersthene-rock" of Lendalfoot, in Ayrshire. The coarser varieties, as at Carrickmore or Termon rock, are sometimes traversed by fine-grained veins from an inch to several feet in breadth. Portions of the slaggy diabases may be observed inclosed in these intrusive masses. The younger group is of more acid composition (granite, quartz-porphyry, etc.), and sends veins into the older.
ii. Eruptions of Llandeilo and Bala Age
Into the east of Ireland the Lower Silurian rocks are prolonged from Scotland, from the Lake District and from Wales. Though greatly concealed under younger formations across the breadth of the island, and occasionally interrupted by what are regarded as older strata of Cambrian age, they nevertheless occupy by much the larger part of the maritime counties from Belfast Lough to the southern coast-line of Waterford, even as far as Dungarvan Harbour. With the same lithological types of sedimentary deposits as in other parts of the United Kingdom, they carry with them here also their characteristic records of contemporaneous volcanic action. Though nowhere piled into such magnificent mountain-masses as in Westmoreland and North Wales, these records become increasingly abundant and interesting as they are traced southwards, until they are abruptly terminated by the coast-line along the south of the counties of Wexford and Waterford.
While much remains to be done, both in the field and in the laboratory and microscope-room, before our acquaintance with the Irish Silurian volcanic rocks is as complete as our knowledge of their equivalents in other portions of the United Kingdom, a serious preliminary difficulty must be recognized in the fact that the several geological horizons of these rocks have only been approximately fixed. Great difficulty was experienced by the Geological Survey in drawing any satisfactory line between the Llandeilo and Bala formations. This arose not so much from deficiency of fossil evidence as from the way in which the fossils of each group seemed to occur in alternating bands in what were regarded as a continuous series of strata. Indeed, in some localities it almost appeared as if the occurrence of one or other facies of fossils depended mainly on lithological characters indicative of original conditions of deposit, for the Llandeilo forms recurred where black shales set in, while Bala forms made their reappearance where calcareous and gritty strata predominated.[269] More recent work among the Silurian formations in England and Scotland, however, indicates that the parallel repetition of the two types of fossils is due to rapid and constant plication of the rocks, whereby the two formations, neither of them, perhaps, of great thickness, have been folded with each other in such a way that without the evidence of an established sequence of fossils, or the aid of continuous sections, it becomes extremely difficult to make out the stratigraphical order in any district. When the ground is attacked anew in detail, with the assistance of such palæontological and lithological horizons as have permitted the complicated structure of the southern uplands of Scotland to be unravelled, we may be enabled to tabulate the successive phases of the volcanic history of the region in a way which is for the present impossible. We have as yet no palæontological evidence that in the Silurian region of the east of Ireland, which extends from Belfast Lough to the south coast of County Waterford, any of the anticlinal folds bring up to the surface a portion of the Lower Arenig formation, though possibly some of the lowest visible strata may be of Upper Arenig age. A considerable part of the region must be referred to the Llandovery and other Upper Silurian formations, but the precise limits of the two divisions of the Silurian system have not yet been determined, except for the region north of Dublin, which has recently been re-examined for the Geological Survey by Mr. F. W. Egan and Mr. A. M'Henry.
[269] Jukes was disposed to regard the two faunas as essentially coeval, but inhabiting different kinds of sea-bottom. See his note, Explanation of Sheets 167, 168, 178, 179, p. 30.
These observers have ascertained that, as in Southern Scotland, by far the larger part of the Silurian region of the north-east of Ireland is occupied by strata belonging to the upper division of the system. The Lower Silurian formations, including the Llandeilo and Bala groups, form a belt varying up to six miles in breadth, which stretches from the coast of Down, between the mouth of Belfast Lough and Copeland Island, in a south-westerly direction to near the valley of the Shannon in County Longford. South of this belt the Lower Silurian rocks rise to the surface only here and there on the crests of anticlinal folds, and it is in these scattered "inliers" that the volcanic and intrusive rocks are found. So far as the available evidence goes, the volcanic history of this part of Ireland is entirely to be assigned to Lower Silurian time, and more especially to the interval between the beginning of the Llandeilo and the close of the Bala period. I must for the present content myself with this general limit of geological chronology, and make no attempt to trace the relative antiquity of the igneous rocks in the several districts in which they are distributed.[270]
[270] The task of revising the Irish maps and tracing out the respective areas of Upper and Lower Silurian rocks over the whole island is now in progress by the Geological Survey, Mr. Egan and Mr. M'Henry being entirely engaged on it.
Viewing the volcanic region of Eastern Ireland as a whole, we are first struck by the feebleness of the manifestations of eruptivity in the north, and their increasing development as we advance southwards. At the northern end of the Silurian area in County Down, thin bands of "felstone" and "ash" have been mapped by the Geological Survey as interstratified with the highly inclined and plicated Silurian rocks.[271] As the latter are plainly a continuation of the strata which have been mapped out zone by zone in the south of Scotland, their igneous intercalations may be looked upon as probably equivalents of some of those in the Silurian districts of Wigtonshire and Kirkcudbrightshire. But in County Down no representative has yet been detected of the Arenig and Llandeilo volcanic series of the southern uplands of Scotland. Nor has more precise petrographical examination confirmed the reference of any of the igneous rocks in the Silurian area of that district to truly contemporaneously intercalated volcanic rocks. All the eruptive material appears to be of an intrusive character. It occurs in the form of dykes of lamprophyre or mica-trap belonging to the groups of minettes and kersantites. Nothing definite is known of the age of these intrusions: they are possibly referable to the time of the Lower Old Red Sandstone.[272]
[271] See Sheet 49 Geol. Survey, Ireland, and Explanation thereto (1871), pp. 16, 37, 39. The so-called "ashes" of the Explanation are probably parts of dykes which have been more or less crushed.
[272] Guide to the Collection of Rocks and Fossils belonging to the Geological Survey of Ireland, by Messrs. M'Henry and Watts, Dublin, 1895, p. 74.
Far in the interior several bands of "felspathic ash" and "massive agglomerate" are shown on the Survey map as running through the counties of Monaghan and Cavan.[273] In one locality south of the Drumcalpin Loughs a large exposure of this ash is visible: "brown crumbly beds, with small rounded pebbles, give place to a massive bed of agglomerate, the enclosed blocks of which are always of one species of felstone, sometimes measuring 10 × 12 × 18 inches, and not always rounded." South of Carrickatee Lough, and a few miles farther to the south-west, near Lackan Bridge, considerable exposures of these rocks occur. One crag in particular displays a thickness of more than 70 feet of "tough flaky breccias," "thick agglomerates with small and large blocks of felstone," and "thin beds of fine pale green compact grit without pebbles, and a few flags." "One of the flaky beds contains numerous white worn crystals of felspar"; "the imbedded blocks of felstone are of the usual kind—pale compact matrix showing dark oblong patches, vesicular and amygdaloidal, the cavities being filled with chlorite."
[273] Sheet 69 Geol. Survey, Ireland, and Explanation of Sheets 68 and 69, pp. 9, 13, 15.
Further south a more extensive area of igneous rocks has been mapped on the borders of Louth and Meath, where, according to the Geological Survey map, a group of lavas and tuffs extends for about twelve miles near Slane.[274] Other bands of "ash" and "felstone" have been mapped in the Silurian area south of Drogheda. Thus at Hilltown, west from the racecourse, a "bluish crystalline felstone, showing in places lines of viscous flow," is stated to be overlain by "indurated felspathic ash and tuff, felstone, and indurated shale" in alternating beds.[275] On a recent visit to this locality I found that the "porcellanite or indurated shale" is a greenish-grey chert, full of Radiolaria and finely-diffused volcanic dust. This association of radiolarian chert with contemporaneous volcanic activity is of much interest, as showing the extension of the same physical conditions of the Lower Silurian sea from Scotland into Ireland. The Lower Llandeilo age of the volcanic intercalations in County Meath is further indicated by the occurrence of Didymograptus Murchisoni in grey shales in the same neighbourhood with the radiolarian cherts. In the Lower Silurian district of Balbriggan numerous intrusive bosses and sills have been mapped by the Geological Survey. I have found, however, that among these rocks there occur bands of volcanic breccia, containing abundant angular fragments of a minutely-vesicular pumice, and also that some of the diabase-masses display the pillow-structure and amygdaloidal texture. Hence, though most of the igneous rocks are no doubt intrusive, they appear to include lavas and tuffs of Bala age.
[274] Ibid. Sheets 81 and 91. These rocks are chiefly augitic andesites, a few are basalts, and some seem related to felstones. Probably many of them are intrusive sills of uncertain age. The "ashes" contain fragments of felsite and porphyrite often of considerable size (Guide to Irish Rock-Collection, p. 36).
[275] Ibid. Sheets 91 and 92 and Explanation to these Sheets (1871), p. 10; Guide to Irish Rock-Collection, p. 36. Some of these lavas are andesites, others are felsites. Mr. M'Henry has contended that certain "ashes" and "agglomerates," particularly those exposed on the coast at Portraine, opposite Lambay Island, are "crush-conglomerates" due to terrestrial disturbances, which have affected both intrusive igneous rocks and the sedimentary series into which these have been injected.
When the numerous Silurian cores of the mountain-groups in the interior of Ireland shall have been searched for traces of contemporaneous volcanic action, it is not improbable that these will be found. One of the smaller Silurian inliers which diversify the great Carboniferous plain, that of the Chair of Kildare, has long been known to have igneous rocks associated with its abundantly fossiliferous Bala limestone.[276] On recently visiting this locality I found that, besides the amygdaloidal and porphyritic andesites and basalts described by Jukes and Du Noyer, the fossiliferous conglomerates contain pebbles of rocks like those of the Chair, together with worn crystals of felspar, while intercalated with them are thin courses of volcanic tuff. There is thus evidence here of contemporaneous volcanic activity during the accumulation of the Bala group of strata. The limited area over which the rocks are exposed, however, affords merely a glimpse of this volcanic centre.
[276] See Explanation to Quarter Sheet 35 N.E. (Sheet 119 of newer numeration) of Geol. Survey Ireland (1858), p. 16. (See note, p. 256.)
Crossing over the broad belt of Carboniferous Limestone through which the Liffey flows into Dublin Bay, we come to the great continuous tract of older Palæozoic rocks which stretches southward to the cliffs of Waterford. Through this tract runs the huge ridge of the Wicklow and Carlow granite. On the west side of this intrusive mass, bands of "greenstone-ash," as well as "felspathic ashes," have been traced among the Silurian rocks by the Geological Survey. But it is on the south-east side of the granite that the volcanic intercalations are best displayed. Indeed, from Wicklow Head to Dungarvan Harbour there is an almost continuous development of igneous rocks, rising into rocky eminences, trenched into ravines by the numerous streams, and laid bare by the waves in fine coast-cliffs. It is in this south-eastern region, comprising the counties of Wicklow, Wexford and Waterford, that the Irish Lower Silurian igneous rocks can best be studied.
There are obviously various distinct centres of eruption in this long belt of country. The Rathdrum and Castletimon tract forms one of these. Another of less size culminates in Kilpatrick Hill, a few miles to the southward. Arklow Head marks the position of a third. The lavas and tuffs which set in a few miles to the south of that promontory, and may be said to extend without interruption to the south coast, were probably thrown out by a series of vents which, placed along a north-east and south-west line, united their ejections into one long submarine volcanic bank. There can be no doubt that the most active vents lay at the southern end of the belt, for there the volcanic materials are piled up in thickest mass, and succeed each other with comparatively trifling intercalations of ordinary sedimentary material. Some of these vents, as I shall relate in the sequel, have been cut open by the sea along a range of precipitous cliffs.
The comparatively feeble character of the volcanic energy during Lower Silurian time over the greater part of the south-east of Ireland is shown by the great contrast between the thickness of the volcanic intercalations there and in Wales and the Lake country, but still more strikingly by innumerable sections where thin interstratifications of fine tuff or volcanic breccia occur among the ordinary sedimentary strata, and are sometimes crowded with Bala fossils. Some interesting illustrations of this feature are to be seen in the Enniscorthy district, where layers of fine felsitic tuff, sometimes less than an inch in thickness, lie among the shales. In some of the tuffs the lapilli are fragments of trachytic or andesitic rocks.
A striking example of rapid alternations of pyroclastic material with ordinary sediment lies far to the south in County Waterford, close to Dunhill Bridge, where a group of fine volcanic breccias and grits has been laid bare by quarrying.[277] These strata consist of coarser and finer detritus, enclosing angular fragments of felsites and grey and black shale. The felsite-lapilli vary in texture, some of them presenting beautiful flow-structure. The stones are stuck at random through each bed, the largest being often at the bottom. The beds of breccia vary from a few inches to a foot or more in thickness. There can, I think, be little doubt that each of these breccia-bands points to a single volcanic explosion, whereby felsitic fragments were thrown out, mingled with pieces of the Silurian strata through which the vents were drilled. In a vertical thickness of some fifty feet of rock there must thus be a record of ten or twelve such explosions.
[277] See Explanation of Sheets 167, 168, 178 and 179, Geol. Surv. Ireland, p. 56.
Nearer the active vents the fragmental deposits become, as usual, coarser and thicker. But I have not observed any thick masses of tuff like those of North Wales. So far as my examination has gone, the tuffs are mainly felsitic. The so-called "greenstone-ash" of the Survey maps is certainly in many cases not a true tuff. This term was proposed by Jukes for certain apple-green to olive-brown flaky fissile rocks only found "in association with masses of greenstone."[278] Some years ago I had occasion to make a series of traverses in Wicklow and Wexford, and then convinced myself that in that part of the country the "greenstone-ashes" were probably crushed bands of basic sills. Dr. Hatch has proved this to be their origin from a series of microscopic slides prepared from specimens collected by himself on the ground.[279] In other cases the "greenstone-ashes" seem to be excessively-cleaved or sheared felsites, which have acquired a soapy feel and a dull green colour; but they also do include true tuffs. Thus, in one instance, at Ballyvoyle cross-roads, in the south of County Waterford, a "greenstone-ash" is a dull green tuff full of fragments of felspar (chiefly plagioclase) and pieces of dark andesitic lavas. Another example may be found to the west of the Metal Man, near Tramore, where the tuff is full of fragments of felspar and shale cemented in a greenish-yellow material which may be palagonite.
[278] Explanation of Sheets 129, 130, p. 13 (1869).
[279] Explanation of Sheets 138, 139.
The felsites of the south-east of Ireland form by much the largest proportion of the whole volcanic series. They occur as lenticular sheets from a few feet to several hundred feet in thickness, and occasionally traceable for some miles. On the whole, they are compact dull grey rocks, weathering with a white crust. A geologist familiar with the contemporary lavas of North Wales cannot fail to be struck with the absence of the coarse flow-structure so often characteristic of the felsites in that region. This structure, indeed, is not entirely absent from the Irish rocks, but it occurs, so far, at least, as I have seen, rather as a fine streakiness than in the bold lenticular bands so common in Caernarvonshire. In like manner the nodular structure, though not entirely absent, is rare.[280]
[280] In Waterford nodular felsites occur with concretions varying from the size of a pea to several inches in diameter. Explanation to Sheets 167, 168, 178 and 179, p. 11.
Until these felsites have been subjected to more detailed investigation, little can be said as to their petrography, and as to the points of resemblance or difference between them and those of other Lower Silurian districts in the United Kingdom. An important step, however, in this direction was taken by Dr. Hatch, who studied them on the ground, in the laboratory, and with the microscope. He found that some of them were soda-felsites or keratophyres (with albite as their felspar), that others were potash-felsites (with orthoclase as their felspar), while a third group contained both soda and potash, the last-named greatly preponderating.[281] The existence of soda-felsites had not been previously detected among British volcanic rocks, and it remains to be seen how far they may occur in the large and somewhat varied group of rocks combined under the general term "felsites." Dr. Hatch believed that these rocks probably graduate into the normal or orthoclase felsites; but it has not yet been possible to test this view on the ground, nor to ascertain whether there is any essential difference between the mode of occurrence of the two types.
[281] Explanation of Sheets 138, 139, p. 49; and Geol. Mag. 1889, p. 545.
Besides the more abundant felsites, occasional bands of andesite have been detected. Various other eruptive rocks occur, probably in most or all cases intrusive. Such are quartz-mica-diorites, quartz-diorites, augite-diorites or proterobases, dolerites, gabbros, diabases and epidiorites.[282]
[282] Guide to Irish Rock-Collections, pp. 34, 35.
I have said that the chief theatre of eruption lay towards the south-west end of the volcanic belt of the south-east of Ireland. The coast-line of County Waterford, from Tramore westward to Ballyvoyle Head—a distance of nearly fifteen miles—presents, perhaps, the most wonderful series of sections of volcanic vents within the British Islands. No one coming from the inland is prepared for either the striking character of the cliff scenery or the extraordinary geological structure there presented, for the country is, on the whole, rather featureless, and much of it is smoothed over and obscured by a covering of drift, through which occasional knobs of the harder felsites protrude. The cliffs for mile after mile range from 100 to 150 or 200 feet in height, and present naked vertical walls of rock, trenched by occasional gullies, through which a descent may be made to the beach. Throughout the whole distance agglomerates and felsites succeed each other in bewildering confusion, varied here and there by the intercalation of Lower Silurian shales and limestones involved and pierced by the igneous rocks. Hardly any bedded volcanic material is to be recognized from one end to the other. The sea has laid bare a succession of volcanic vents placed so close to each other that it will be difficult or impossible to separate them out. A careful study and detailed mapping of this marvellous coast-section, however, is a task well worthy of the labour of any one desirous of making himself acquainted with some of the conditions of volcanism during older Palæozoic time.
At the east end of the section, black shales containing Llandeilo graptolites, and calcareous bands full of Bala fossils, dip westward below a group of soda-felsites and felsitic tuffs, which seem to lie quite conformably on these strata. Here, then, we start with proof that the volcanic eruptions of this locality began during some part of the Bala period. But immediately to the west, these bedded igneous rocks are broken through by a neck of coarse agglomerate stuck full of chips and blocks of shale, some of them a foot long, with abundant fragments of scoriform and flinty felsites. Some columnar dykes of dolerite cut through the neck, and a larger intrusion seems to have risen up the same funnel. The bedded tuffs appear again for a short distance, but they are soon replaced by a tumultuous mass of agglomerate. And from this part of the coast onwards for some distance all is disorder.
The agglomerates are crowded with blocks of various felsites and micro-granites sometimes 18 inches in diameter, many of them presenting the most exquisite streaky flow-structure. The angularity of these stones and the abrupt truncation of their lines of flow prove that they were derived from the shattering of already consolidated rocks. In other places the ejected materials consist almost wholly of black shale fragments, but with an intermixture of felsite-lapilli.
It is difficult to convey an adequate idea of the way in which the agglomerates are traversed by dykes, veins and bosses of various felsites, and of how these break in endless confusion through each other. Some of the intrusive rocks are compact and amorphous, others are vesicular, others close-grained and columnar. Again and again they present the most perfect flow-structure, and it is noticeable that the lines of flow follow the inequalities of the walls of the fissure up which the rock has ascended, and not only so, but even of the surfaces of detached blocks of shale or felsite which have been caught up and enclosed in the still moving mass.
A few of these intrusive rocks were examined in thin slices by Dr. Hatch. Most of them appear to be soda-felsites, but they include also rather decomposed rocks, some of which are probably diorites and quartz-diorites. Occasionally, thoroughly basic dykes (dolerite) may be observed.
In the midst of this tumultuous assemblage of volcanic masses, representing the roots of a group of ancient vents, there occur occasional interspaces occupied by ordinary stratified rocks. In the eastern part of the section these consist mainly of black shale, sometimes with calcareous bands, from which a series of Bala fossils has been obtained.[283] A very cursory examination suffices to show that these intercalations do not mark pauses in the volcanic eruptions. They are, in fact, portions of the marine accumulations under the sea-floor through which the vents were blown; they have been tossed about, crushed and invaded by dykes and veins of felsite.
[283] But see the Geol. Survey Memoir on Sheets 167, 168, 178 and 179, Ireland (1865), p. 28, for a description of the association of Bala and Llandeilo fossils on that coast-line.
But certain other intercalated strips of stratified rocks present a special interest, for they bring before us examples of volcanic ashes that gathered on the sea-floor, but which were disrupted by later explosions. Thus, at the Knockmahon headland, well-bedded felspathic grits and ashy shales occur, thrown in among the general mass of eruptive material. As I have already remarked, it is difficult or impossible to fix the horizons of the stratified patches that are involved among the igneous ejections of this coast-section, save where they contain recognizable fossils, but the intercalation of true bedded tuffs among them is a proof that volcanic action had been in operation there long before the outbreak of the vents which are now laid bare along the cliffs.
In the south-east of Ireland there is the usual association of acid and basic sills with the evidence of a superficial outpouring of lavas and ashes. But these intrusive masses play a much less imposing part than in Wales. They may be regarded, indeed, as bearing somewhat the same proportion to the comparatively feeble display of extrusive rocks in this region that the abundant and massive sheets of Merionethshire and Caernarvonshire do to the enormous piles of lavas and tuffs which overlie them.
Among the acid intrusive sheets the most conspicuous are those mapped by the Survey as "elvans." These rocks, as they occur in Wicklow and Wexford, have been examined by Dr. Hatch, who finds them to be micro-granitic in structure, occasionally exhibiting micropegmatitic or granophyric modifications.[284] The true stratigraphical relations of these rocks have not yet been adequately investigated. Those of them which occur on the flanks of the great granite ridge are not improbably connected with that mass, and if so are much younger than the Lower Silurian volcanoes.[285]
[284] Explanation of Sheets 138 and 139, p. 53.
[285] The Leinster granite is certainly later than the Lower Silurian rocks and older than the Carboniferous rocks of the south-east of Ireland. It may belong to the great epoch of granite protrusion during the Old Red Sandstone period.
The basic sills, or "greenstones," consist largely of diabase, frequently altered into epidiorite; they include also varieties of diorite.[286] That they were intruded before the plication and cleavage of the rocks among which they lie is well shown by their crushed and sheared margins where they are in thick mass, and by their cleaved and almost schistose condition where they are thinner. The intense compression and crushing to which they have been subjected are well shown by the state of their component minerals, and notably by the paramorphism of the original augite into hornblende.
[286] Dr. Hatch, op. cit. p. 49.
The scarcity of dykes associated with Silurian volcanic action is as noticeable in the south-east of Ireland as it is in Wales. I have observed a considerable number, indeed, but they are confined to the line of old vents on the Waterford coast, and, but for the clear cliff-sections cut by the sea, they would certainly have escaped observation, for they make no feature on the ground in the interior. They are sometimes distinctly columnar, and vary from less than a foot to many yards in width. They traverse both the agglomerates and the intrusive felsites. Most of them are of felsite, sometimes cellular; but in some cases they are dolerites. There is obviously no clue to the dates of these dykes.
That some at least of the vents along the south coast of County Waterford may be vastly younger than the Lower Silurian rocks through which they have forced their way is suggested, if not proved, by a section which is in some respects the most extraordinary of the whole of this remarkable series. The occurrence of a group of red strata was carefully noted by the late Mr. Du Noyer at Ballydouane Bay, when he was engaged in carrying on the Geological Survey of that part of the country. At first he regarded them as belonging to the Old Red Sandstone, which comes on in great force only a few miles to the west; but he subsequently arrived at the belief that they are really an integral part of the Lower Silurian rocks of the district. Professor Jukes had previously expressed himself in favour of this latter idea, which was thought to receive support from the occurrence of some reddish strata in the Lower Silurian rocks of Tagoat, County Wexford.[287]
[287] Explanation of Sheets 167, 168, 178 and 179 of the Geological Survey of Ireland (1865), pp. 10, 59.
The occurrence of red rocks among Silurian strata, which are not usually red, might quite reasonably be looked for in the neighbourhood of Old Red Sandstone, Permian or Triassic deposits. If these deposits once spread over the Silurian formations, a more or less decided "raddling" of the latter may have taken place. But in the present instance, though the Old Red Sandstone begins not many miles to the west, no such explanation of the colour of the strata is possible. The cliffs of Ballydouane Bay consist of red sandstone, red sandy shale and conglomerate. The red tint is of that dull chocolate tone so characteristic of the Lower Old Red Sandstone. The conglomerates are immense accumulations of ancient shingle, consisting largely of pieces of white vein-quartz and quartzite, sometimes a foot long and often well water-worn. Some of the sandy beds are full of large scales of white mica, as if derived from some granitic or schistose region at no great distance. Taken as a whole, the strata are much less indurated and broken than the Silurian grits and shales of the district; some of them, indeed, weather into mere incoherent sand that crumbles under the fingers. There does not appear to be any positive proof that the red rocks are truly bedded with the ordinary Silurian strata, the junctions being faulted or obscured by intrusive igneous masses.
Nowhere in the British Islands, so far as I am aware, is there a similar group of strata among the Lower Silurian rocks. If they belong to so ancient a series, they show that in the south of Ireland, during Lower Silurian time, there arose a set of peculiar physical conditions precisely like those that determined the accumulation of the Old Red Sandstone in the same region at a later geological period. And in that case it is hardly possible to conceive that these conditions could have been confined to the extreme south of Ireland. We should certainly expect to meet with evidence of them elsewhere, at least in the same Silurian region.[288]
[288] The nearest approach of any Silurian group of strata to the character of these conglomerates is furnished by the remarkably coarse conglomerates, boulder-beds and pebbly grits of the Bala and Llandovery series in the region between Killary Harbour and Lough Mask, to which further reference is made in a later part of this chapter.
While I hesitate to express a decided opinion in opposition to the conclusions of such experienced observers as Jukes and Du Noyer, I incline to believe that the rocks in question really belong to the Old Red Sandstone. If such shall finally be determined to be their geological position, they will supply evidence that some at least of the volcanic vents of the coast-line cannot be older than the Old Red Sandstone. They are pierced by masses of soda-felsite and by a coarse red agglomerate containing abundant pieces of felsite. These volcanic rocks belong to the same type as those which break through the undoubted Silurian rocks on either side. They may thus come to prove a recrudescence of volcanic energy in this same district at a much later geological period; and a new problem will arise to task the skill of the most accomplished field-geologist and petrographer—to unravel the structure and history of this chain of volcanic vents, and, in so doing, to detect and separate the eruptions of Lower Silurian time from those of the Lower Old Red Sandstone.
In the far west of Ireland, another group of Lower Silurian volcanoes has left its remains in the mountainous tract of country between the western shores of Lough Mask and Killary Harbour.[289] There appear to have been at least three separate centres of eruption along a line stretching in a north-easterly direction for about 16 miles from the western end of Lough Nafooey to the hamlet of Derrindaffdery beyond Tourmakeady, where the older rocks are unconformably overlain by the lower Carboniferous strata. As shown by the mapping of the Geological Survey, the most northerly area, which may be called the Tourmakeady centre, has a breadth of about a mile, and dies out southward after a course of nearly six miles. About a mile to the south-west of the last visible prolongation of its rocks, we encounter a second volcanic centre which occupies an area of about a square mile in the valley of Glensaul. The third centre stretches from the western shores of Lough Mask across Lough Nafooey, where it forms a mass of high rugged ground, and reaches a length of some six or seven miles before it finally dies out.[290]
[289] This group was placed in the Upper Silurian series by the officers of the Geological Survey who mapped the region (see Sheets 84, 85, 94 and 95 of the Geological Map of Ireland and accompanying Explanation), and on their testimony I formerly referred to the volcanic rocks as of Upper Silurian age. Mr. Baily, however, had pointed out that the limestone associated with the lavas and agglomerates contains Bala fossils. Yet, in spite of this palæontological testimony, the fossils were considered to be "derivative," and the rocks were removed from the series of formations to which they would naturally be assigned. A recent examination of the ground, in company with Mr. J. R. Kilroe of the Geological Survey, has satisfied me that the volcanic rocks are interstratified with sedimentary deposits of Bala age, and must consequently be grouped with the rest of the Lower Silurian series of Ireland. The results of this examination are given in the text.
[290] These areas were carefully mapped for the Survey by Mr. Nolan, and the lines of division marked by him fairly represent the general distribution of the rocks.
The rocks in each of these three areas are similar. One of their distinguishing features is the intercalation among them of a fossiliferous limestone and calcareous fossiliferous tuffs, which contain well-preserved species of organisms characteristic of the Bala division of the Lower Silurian rocks.[291] There cannot be any question that these organisms were living at the time the strata in which their remains occur are found. The most delicate parts of the sculpture on Illænus Bowmanni and Orthis elegantula are well preserved. Nor have the limestones been pushed into their present places by volcanic agency, or by faults in the terrestrial crust. They are not only regularly intercalated among the volcanic rocks, but the limestone in some places abounds in volcanic dust, while above it come calcareous tuffs, also containing the same fossils. It is thus clearly established that the volcanic series now to be described has its geological age definitely fixed as that of the Bala period.
[291] See the list of fossils as determined by Mr. Baily in Explanatory Memoir to accompany Sheets 73, 74, 83 and 84 of the Geological Survey of Ireland, p. 68 (1876).
The lavas of the Lough Mask region consist of felsites and andesites with rocks of probably more basic composition. The felsites are generally quartziferous porphyries, which occupy a considerable space in each of the three districts. To what extent they are intrusive rather than interstratified remains for investigation. Some of them have undoubtedly invaded other members of the volcanic series. But, on the other hand, fragments of similar quartz-porphyries and felsites abound in the intercalated bands of volcanic breccia.
The andesites and more basic lavas are finely-crystalline or compact, dull-green to chocolate-purple rocks, often resembling the "porphyrites" of the Old Red Sandstone. Some of them are strongly vesicular, the cavities being filled with calcite on fresh fracture, though empty on weathered surfaces. The sack-like or pillow structure, already referred to as characteristic of many Lower Silurian lavas, appears conspicuously among some of these rocks. At Bohaun, nine miles south from Westport, where a prolongation of the volcanic series rises to the surface from under the overlying coarse conglomerates, I observed that, owing to the compression which the rocks have there undergone, the pillow-shaped blocks have been squeezed together into rudely polygonal forms, while their vesicles have been greatly drawn out in the direction of tension. Where the rocks have been still more sheared, the distinct pillow-shaped blocks with their vesicular structure disappear, while the more fine-grained crusts that surround them have been broken up and appear as fragments involved in a matrix of green schist.
Intercalated with the lavas are numerous bands of volcanic breccia and fine tuff. The stones in these breccias consist chiefly of various felsites with andesites and more basic lavas. But pieces of jasper, chert, shale and grit are not infrequent. In some places abundant blocks of black shale are to be noticed, probably derived from the Llandeilo group which exists below, and which has here and there been ridged up to the surface in the midst of the volcanic rocks.[292] Near Shangort I noticed in one of these breccias one block measuring 12 feet, another 20 feet in length and 3 or 4 feet thick, composed of alternating bands of grit and slate. It is interesting to note that these strata had already undergone cleavage before disruption, the bands of slate being strongly cleaved obliquely to the bedding. None of the Llandeilo or other rocks in the neighbourhood display this structure. The blocks seem to have been derived from some deeper group of strata. They are laid down parallel with the rude bedding of the breccia in which they lie.
[292] In re-examining this region, Mr. Kilroe has found in the stream west of the monastery, Tourmakeady, an uprise of graptolitic black shale containing forms belonging to the very lowest Llandeilo or Upper Arenig strata, and a similar band above Leenane, Killary Harbour.
The fine tuffs and thin ashy limestones associated with the thicker band of limestone show the renewal of volcanic explosions after the interval marked by the calcareous deposit which is sometimes 20 or 40 feet thick. In many places this limestone is brecciated and much mingled with volcanic dust and lapilli. At Shangort, for example, the thick tolerably pure limestone is truncated on the west and north sides by a coarse agglomerate probably filling a volcanic vent. A few hundred yards further north, beyond the interrupting agglomerate, the limestone reappears on the same line of strike, but is then found to be nodular and brecciated and much mingled with volcanic detritus. It lies among ashy grits and tuffs.
Fig. 64.—Diagram of the general relations of the different groups of rock in the Lower Silurian volcanic district along the western shore of Lough Mask.
a, Llandeilo shales, cherts and grits; b, Volcanic breccias; c, Felsites and andesites; d, Tuffs and ashy grits and shales; e, Limestone with Bala fossils; f, Calcareous tuffs and thin bands of ashy limestone with fossils; g, Coarse conglomerate and grits; h, Wenlock strata resting unconformably on the Bala rocks and passing southwards from these to overlie an older series of schists; *, Fault.
The general structure of the ground occupied by the Lough Mask volcanic rocks is diagrammatically represented in [Fig. 64]. The thickness of the volcanic series must amount to many hundred feet, but it has not been precisely determined. The uppermost parts of the series pass under a great thickness of coarse conglomerates and pebbly grits which form the ridge of Formnamore, and stretch thence westwards along Killary Harbour and through the Mweelrea mountains. These strata are classed as the Upper Silurian on the Geological Survey map. Since, however, they conformably overlie rocks containing Bala fossils, and in the Killary district include green shales which have yielded fossils of the same age, they doubtless belong in large part to the Lower Silurian division. The remarkable coarseness of these conglomerates towards the south, and their rapid passage into much finer grits and shales towards the north, probably indicate that they were formed close to the shores of a land composed of schistose rocks, quartzite and granite, of which the mountainous tracts of Connemara are the last relics.
A base to the volcanic series is found in the occasional uprise of a short axis of Llandeilo, or perhaps even upper Arenig strata, containing bands of dark chert and black graptolitic shales. Unfortunately the relations of these underlying rocks to the volcanic masses are not very clear, being obscured by superficial accumulations and also by faulting. It is thus hardly possible to be certain whether they pass up conformably into the base of the volcanic series, or are covered by it unconformably.
The position of this isolated volcanic district in the far west of Ireland, the abundance, variety and thickness of the erupted materials, and the definite intercalation of these materials in the Bala or highest division of the Lower Silurian series, acquire a special interest from the history of the nearest Silurian volcanic area which has now to be described—that of the western shores of the Dingle promontory.