Rocks and Their Origins

by Grenville A. J. Cole

THE SCENERY OF IGNEOUS ROCKS

Volcanic landscapes, where activity is very recent or still in progress, present a number of characteristic surface-forms. The cones that have accumulated round the vents surpass all other hills in regularity of outline, and the crater in the summit is often relatively large. Lava-cones may be steep-sided bosses when formed of protrusions of viscid rocks rich in silica, like the remarkable domes in the north of Bohemia, or they may present very gentle slopes where fluid basic lavas have been extruded.

Tuff-cones are liable to be breached on one side, owing to the outflow of lava which the crater-wall could not sustain, and they then assume the form of a mountain in which glacial influences have hollowed out a cirque.

Rain washes down the loose materials from great volcanic cones, and emphasises the concave curve of the mountain sides, the form that is so beautiful in Fuji-yama in Japan, and which Hokusai, with pardonable and affectionate exaggeration, reproduced in a hundred illustrations. Ultimately, however, grooves appear on the flanks of the cone, in which permanent streams gather, and the slopes are dissected and worn away. During this process, the tuffs yield steep and fantastic forms, and wall-like dykes weather out among them. The dykes are usually the last features to decay.

Where the vent has been plugged with lava at the close of its activity, the neck of rock often remains standing above the surrounding country. The site of cone after cone can be picked out in this way in the Cainozoic volcanic areas of central Germany. The jutting crag of trachyte or of basalt has often been seized on as the site of a feudal castle, under which the dependent agriculturists still gather at nightfall in their red-roofed town. The group of sheer-sided necks in the Hegau in southern Württemberg, the Hohentwiel, Hohenkrähen, and the rest, form a very striking landscape amid undulating Cainozoic lands.

The lava-beds that cover wide areas are naturally of basic composition. Basalts thus form enormous plains with rugged surfaces, on which at last a red-brown soil collects. When exposed to denudation from the edge of the region inwards, they develop a marked terrace-structure, through which the rivers cut steep and grim ravines. Grass may grow on the ledges and the tables; but the scarps, controlled by the well-marked vertical jointing of the lavas, remain sharp and prominent, and the rock falls away from these walls in whole columns at a time. This structure is characteristically seen in northern Mull and the adjacent smaller isles, and is still more impressive from the centre to the north of Skye, where the rain swept terraces covered by grass and bog and scanty oatfields, and the black steps of rock between them, present a scene of strange monotony and desolation.

In regions less exposed to stormy weather, the lava-plateaus may provide good soils. For instance, after the great seaward scarp of the basalts has been crossed in the counties of Antrim and of Londonderry, the lava-fields, dropped by faults towards Lough Neagh, are seen to be occupied by prosperous farms. In arid countries, however, the savage surface of the flows merely becomes modified by red dust and scoriaceous gravel, worn by wind and changes of temperature from the upstanding portions of the land.

Where a stratified country has been freely invaded by sheets of lava along its planes of bedding, the stratification is emphasised in any part exposed to weathering. The resisting igneous rock stands out in scarps along the hills, and marks out any folds that have been formed since the epoch of its intrusion.

When the beds remain fairly level, and are also uplifted, flat-topped hills are formed by the intrusive sheets, like those that may be carved out of a country flooded over by lava-streams. The crystalline rock, very probably a dolerite, protects what lies below it. The kopjes north of the Great Karroo in the centre of the Cape of Good Hope are thus level on the crest and bounded by a steep wall or krans of rock.

The edges of similar "sills" of igneous rock have controlled much of the scenery between the Highland border of Scotland and the Tyne. A fine example is the indented scarp of the Great Whin Sill, a sheet of dolerite intruded among the Carboniferous strata of Northumberland. This mass forms a platform for Bamburgh Castle against the wild North Sea, and is traceable south-westward across the country towards Carlisle. North of Hexham, its escarpment is occupied by Hadrian's wall, and the town of Borcovicus was planted on the edge, overlooking all Northumbria.

The farmers of North Britain and Ireland have long known upstanding igneous dykes as unprofitable "whinstones." The regularity of direction among dykes over very wide areas points to their intrusion along cracks produced by stretching of the crust. Radial grouping of dykes, such as one finds near volcanic necks, or, on a gigantic scale, round Tycho on the moon, may be due to explosive action; but the majority of dykes seem to have followed upon earth-movement. In the north of Ireland, from the coast of Down to that of Donegal, a series of compact rocks of Devonian age occurs in dykes lying almost invariably north and south. The post-Cretaceous dykes of the same region have a still more uniform trend, from north-west to south-east. Such series of dykes modify the scenery of coasts by forming promontories and serviceable piers for boats.

The offshoots near the surface of a great intrusive mass are far less regular. We are here close to the zone of attack, the "shatter-zone," and the structures or regular fracture-planes of the overlying rock only partially control the position taken up by the intrusive magma. Irregular knots and bosses appear in place of far-spreading sheets, and a network of crossing veins occurs, instead of a system of co-ordinated dykes. The resulting country is hummocky and broken, and, where the cauldron itself has become exposed, striking contrasts of surface are seen as we pass from the igneous core to the older and frequently stratified rocks upon its flanks.

Some large bodies of intrusive rock have, however, been formed sheet by sheet, and a bedded sill-like structure is then revealed in them on weathering. Sir A. Geikie[89] calls attention to this in his description of the heart of the black gabbro mass in Skye. But, as a rule, the continuity of structure in batholites, and their characteristic joint-planes set at angles to one another, cause them to appear as massive blocks in the landscape, untraversed by any regular lines.

Granite, with its broad tabular jointing, which is often developed parallel to a surface of cooling, forms rounded slopes and domes after long-continued weathering. When reared high into the zone of frost-action, it develops spires and pinnacles, as in the huge "aiguilles" of Mont Blanc. But, as decay goes on, the uniform descent of boulders and sand forms spreading taluses, banked against the lower slopes, while the curving joints, not too closely set, promote a smoothness on the higher lands. These joints, moreover, divide the rock into boulders almost ready-made. Tabular structure sometimes predominates; but even in this case the exposed ends of the layers soon become rounded, as the felspar crystals pass into a powdery state. Commonly, a rough spheroidal structure prevails, as may be traced in many of the Dartmoor "tors," and the blocks that slip away through widening of the joints become more and more rounded as their surfaces crumble on the talus ([Fig. 17]).

Fig. 17. Weathering Granite. Lundy Island.

In tropical lands, granite exfoliates under the alternations of clear hot days and clear cold nights, and the joint-structure allows of the formation of great round-backed surfaces, on which spheroidal boulders appear poised. These boulders are the relics of an overlying layer of granite, most of which has slipped away to the hill-foot. Their surfaces crumble, owing to the unequal expansion of the constituent minerals. When the rainy season sets in, the decomposed crust is washed away; during the dry season it falls off in flakes and powder. In this way the magnificent series of monoliths that surround the grave of Cecil Rhodes in the Matopo Hills have become separated out from a continuous sheet of granite. They stand now like glacial boulders on a surface almost as smooth as that of a roche moutonnée ([Fig. 18]). The landscape for miles around is fantastic with huge fallen masses, and with high-perched blocks that seem about to fall. Similar scenery is well known in central India, and exfoliation controls the form of mountain-domes in California and Brazil. J. C. Branner [90] lays most stress on temperature-changes in the surface-zone, and little on original spheroidal jointing, in promoting the exfoliation of the rounded boulders.

Fig. 18. Granite weathering under tropical conditions. Rhodes's Grave, Matopo Hills, S. Rhodesia. The blocks like boulders are residues of a sheet of granite that once overlay the hill.

The basic rocks present far more rugged outlines. When a cauldron occupied by basic diorite or by gabbro comes under denuding action, the numerous crossing joints oppose the formation of domes or tables. The weather widens one groove here, another there; the rock breaks away in angular fragments rather than as a powder over a broad surface, and serrated edges and jagged pinnacles arise along the crests. The diorites among our old metamorphic rocks in Scotland or in Ireland can be recognised on the sky-line at considerable distances. Sir A. Geikie, in his "Scenery of Scotland," has made the contrast between granite and gabbro in the centre of the Isle of Skye familiar to all geologists. Here the two types of rock were erupted at no long interval, and they have been exposed to denudation under the same conditions. J. Macculloch dwelt in 1819[91] on the relative resistance of the gabbro and the rapid disintegration of the granite hills, quaintly remarking of the latter that "the loose stones, by their constant descent from the summits, obscure the rocky surface, covering the sides with long torrents of red rubbish even more unpleasing to the sight than their conoidal forms." Macculloch noted that the loose blocks in the gabbro region lay much as they had fallen, without the production of a sand.

In most mountain-chains produced by folding, igneous matter has been forced up as an accompaniment of the earth-movements. The local knots and laccolites, or the great cores admitted along certain anticlines, stand out on weathering among schistose or stratified hills. Their surfaces are marked by accidents, and each peak as it comes into view offers something of a new surprise. The wall of Mont Blanc from the angle near Entrèves, and the huge crag of the Matterhorn above the valley of the Visp, have illustrated to every traveller the dominance of igneous masses in the landscape. In our own islands, the granites of Ben Cruachan and Cairn Gorm have resisted long ages of denudation; an intrusive sheet of finer grain forms the long sheer wall of Cader Idris; while obsidian lava-flows, now grey and dull and crystalline, have furnished on Snowdon the finest scenery of Wales. The fortress-town of Edinburgh has arisen on the relics of a dead volcano; and the high moor of Leinster, so long the peril of the English, records an igneous cauldron that has been exposed to denudation from the opening of Devonian times.

CHAPTER VI
METAMORPHIC ROCKS

INTRODUCTION[92]

Under the term "metamorphism," considered philologically, any change may be included that is undergone by rocks after their original deposition. Van Hise, in his monumental treatise, covers processes of cementation and alteration by percolating waters, as well as those larger changes that accompany earth-movement and the transference of rocks into regions of igneous activity. It is, indeed, impossible to draw any just line in this matter; but there is a general agreement that "metamorphic rocks" are those that have been altered by heat or pressure or both, either on a local or a regional scale, with the result that new structures, or new minerals, or both, have arisen in the mass. The efficacy of heat alone or of pressure alone, of contact-metamorphism or of dynamo-metamorphism, in producing considerable changes has been much debated. Some of the thermal changes have been already referred to in the chapter on igneous rocks. While, moreover, the new structures and the development of mica in ordinary slate bring it into the metamorphic group, we have found it convenient to describe the slates in connexion with common clays. The rocks now to be dealt with give evidence of more extreme changes, and the crystalline character of their constituents is appreciable by the unaided eye. For the most part, then, this chapter treats of gneisses and schists. The wider use of the terms schiste and schiefer on the continent of Europe makes it necessary in most countries to style the metamorphic forms "crystalline schists."

Over wide areas of certain countries, and sometimes when we approach the localised cores of mountain-chains, the rocks show a parallel arrangement of their constituents, reminding us of sediments; but their constituents are all crystalline, and they are more interlocked with one another than is the case in ordinary strata.

Such rocks have long been said to be "foliated." The term was used by G. P. Scrope as far back as 1825; but this author, in common with most geologists of his day, regarded the mineral folia as resulting from sedimentation. D'Aubuisson de Voisins[93] had already referred the parallelism of the feuillets of mica in schists to some cause acting on them during the consolidation of the rock from a plastic state; but it was left for Charles Darwin[94], in his remarkable observations on metamorphic rocks in 1846, to separate clearly foliation from stratification.

In all cases of metamorphism, we have to bear in mind that the alteration may be both chemical and physical. Substances may have been removed from the rock, others may have been imported. The crystalline constituents that are now present do not necessarily result from the crystallisation of the original materials of the rock.