Rocks and Their Origins

by Grenville A. J. Cole

Many interesting features of the hills called drumlins cannot be discussed here. Their arrangement with their longer axes in the direction of the movement of the ice shows that they were moulded in large measure within the ice itself, and came to light as it melted away from above downwards. They may be regarded as originating in tough and mixed materials, ice and stones and clay, from the lower layers of the ice-sheet, which became associated with the purer upper ice in certain episodes of the flow. Such mingling may occur at an ice-fall, or where shearing over an obstacle takes place. In the former case, the upper ice descends into the lower layers; in the latter, masses from below are pushed up into higher levels. As the forward flow proceeds, the masses representing the lower and stone-filled layers are treated just as "eyes" of coarser material are treated in a fluidal lava or in a rock deformed by metamorphic pressures. The purer and more plastic ice moves past and round them, and they assume an elongated form[56]. When final stagnation and melting have gone on, these masses are still separated from one another as rounded hills. Their bases have settled down upon the ice-worn surface, but their flanks and crests retain traces of the moulding action of the purer portions of the complex body styled an ice-sheet.

In recent years great interest has been aroused by researches on boulder-clays of ancient date, especially those of Permo-Carboniferous age[57]. These compacted deposits contain abundant striated boulders, and rest on glaciated rock-surfaces, which have a surprisingly modern aspect when laid bare by denudation. The grey-green Dwyka Conglomerate that is so widely spread throughout South Africa forms "kopjes" on the borders of the Great Karroo, with spiky crests and irregularly weathered cliffs; but its original deposition as a boulder-clay has been amply verified. It has now, moreover, been paralleled by a very similar rock discovered by A. C. Coleman in the Huronian beds of Canada.

CHAPTER V
IGNEOUS ROCKS

INTRODUCTION[58]

Igneous rocks, those varied masses that have consolidated from a state of fusion, attracted attention in the eighteenth century through their active appearance in volcanoes. James Hutton in 1785 showed that the crystalline granite of the Scottish highlands "had been made to invade that country in a fluid state." More than a hundred years, however, elapsed before geologists on the continent of Europe were willing to connect superficial lavas with the materials exposed by denudation in consolidated cauldrons of the crust.

It is interesting therefore to note that G. P. Scrope in 1825 treated of granite, without apology or hesitation, in a work entitled "Considerations on Volcanoes." So far from separating deep-seated from superficial products, Scrope wrote of the molten magma in the crust as "the general subterranean bed of lava." He conceived this fundamental magma, "the original or mother-rock," to be capable of consolidating as ordinary granite. Successive meltings and physical modifications of this granite gave rise, in his view, to all the other igneous rocks. Scrope laid no stress, however, on chemical variations within the magma, but urged that the transitions observable between different types of igneous material established a community of origin.

The connexion between lavas and highly crystalline deep-seated rocks, so simply accepted by Scrope, was worked out some fifty years later by J. W. Judd for areas in Hungary and in the Inner Hebrides. The features displayed in thin sections under the microscope were used by Judd, in a series of papers, to substantiate his views; but in France and Germany these features became the source of subtle distinctions between the igneous rocks of Cainozoic and pre-Cainozoic days. The lavas, in which some glassy matter could be traced, were said to be typically post-Cretaceous, and essentially different from those earlier types in which glass was replaced by finely crystalline matter; while the coarsely crystalline igneous rocks were uniformly regarded as pre-Cainozoic. Glassy rocks, such as pitchstone, interbedded contemporaneously in Permian or Devonian strata, were described as "vitreous porphyries," while those known to be of post-Cretaceous date might be styled andesites, trachytes, or rhyolites. Luckily common sense has recently triumphed in this matter, and the relative scarcity of glassy types of igneous rocks in early geological formations has been recognised as due to the readiness with which glass undergoes secondary crystallisation. The discussion has ended by showing that we have no evidence of world-wide changes in the types of material erupted during geological time.

At the present day, attention has been focused on the processes that go on in subterranean cauldrons, in the hope of explaining the differences between one type of extruded rock and another. Doctrines of descent have been elaborated, and one of the most subtle systems of classification[59] has been based upon characters that the igneous rock might have possessed, had circumstances not imparted others to it during the process of consolidation. The principle of this classification is, however, obviously correct, if we wish to trace back a rock bearing certain characters at the present day to the molten source from which it came.

CHARACTERS OF IGNEOUS ROCKS

The characters of igneous rocks vary considerably according as they have consolidated under atmospheric pressure only, or under that of superincumbent rocks. We must remember also that submarine lavas have to sustain a pressure of an extra atmosphere for every thirty feet of depth, or 400 atmospheres at 2000 fathoms, and that such rocks have a claim to be regarded as deep-seated. The gases that igneous rocks contain, probably as essential features of the molten magma, and at a temperature above their critical points, escape to a large extent near or at the surface of the earth. The bubbles raised in lava, whereby it is rendered scoriaceous, and the clouds of vapour rising from cooling lava-flows and from the throat of a volcano in eruption, are sufficient evidences of this process. The extremely liquid lavas of Kilauea in Hawaii, which emit very little vapour, are notable as exceptions. In the case of masses that cool underground, the retention of gases, and ultimately of liquids, until a very late stage of consolidation retards crystallisation until temperatures are reached lower than those at which it starts in surface-flows. As A. Harker points out[60], "the loss of these substances, by raising the melting-points in the magma, may be the immediate cause of crystallisation, quite as much as any actual cooling."