Rocks and Their Origins

by Grenville A. J. Cole

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.

MICA AND HORNBLENDE SCHISTS

Schists are the ordinary foliated rocks of fine or medium grain. The folia are really flattened lenticular mineral aggregates, often bent and waved, lying on and against one another, with their platy surfaces in parallel planes. They result (i) from the deformation under pressure of objects already present in the rock, such as pebbles or crystals; or (ii) from the development of minerals under pressure during the process of metamorphism, such minerals being allowed greater facilities for growth in directions perpendicular to that from which the pressure is exerted; or (iii) from the development of minerals, notably mica, along the planes of weakness provided by stratification or by cleavage.