Rocks and Their Origins

by Grenville A. J. Cole

The base of a massive lava-flow, however, cools under much more uniform conditions, and the columns, stretching upwards from the ground and produced by slow contraction, give rise to the regular prismatic structures long ago known as "giants' causeways." The original Giant's Causeway in the county of Antrim is the lower part of a basaltic flow, exposed by denudation on the shore. Fingal's Cave in Staffa owes its tough compact roof to the preservation of that portion of the flow which cooled downwards from the upper surface. G. P. Scrope[63] long ago observed this dual structure in columnar lavas.

The columns, or the more irregular joint-blocks that sometimes represent them, are often subdivided by further contraction into spheroids, the coats of which peel off, as the rock weathers, like those of an onion. The curved cross-joints of massive columns, now convex upwards, now concave, represent the same tendency towards globular contraction.

A lava-flow is sometimes divided into large rudely spheroidal masses, which fit into one another, and which show signs of more rapid cooling on their surfaces. These were particularly observed on the mountains near Mont Genèvre by Cole and Gregory[64], who compared the forms to "pillows or soft cushions pressed upon and against one another." It was suggested that these forms were produced by the seething of viscid lavas, masses being heaved up and falling over, and the outer layers having time to cool in a glassy state before they were deformed by contact with others. This pillow-structure has been widely recognised, and J. J. H. Teall has remarked how often "pillow-lavas" are associated with radiolarian cherts. He regarded them, therefore, as of submarine origin. Sir A. Geikie[65], moreover, stated that the spheroidal sack-like structure was produced by the flow of such lavas into water or watery silt. This acute suggestion has now been verified by Tempest Anderson[66], who has observed in Samoa the chilling of the lobes of lava, as they are thrust off into the sea and washed over by the waves. H. Dewey and J. S. Flett[67] have pointed out that pillow-structure commonly occurs in lavas in which there has been a conversion of lime soda felspars into albite, a change frequent in a series of rocks which they call the "spilitic suite." The importation of soda is attributed to vapours entering soon after the consolidation of the rock, and it is urged that any excess of sodium silicate must have escaped into the sea-water in which the pillow-lavas were produced. Hence radiolaria will flourish in the neighbourhood (presuming that a decomposition of the silicate can be brought about), and their remains will in time form flint in the hollows of the lavas. The paper quoted contains numerous references to previous work, and is a suggestive example of how petrographic study may go hand in hand with the appreciation of rocks from a natural history point of view. It is only characteristic of the subject of petrology that G. Steinmann[68] has with equal ingenuity explained the relations between radiolaria and spilitic lavas by reminding us that gravity-determinations show an excess of basic material under the oceans and of lighter material, rich in silica, under continental land. Hence, when deep-sea deposits are crumpled by earth-movements, basic types of rock, graduating even into serpentine, become associated with radiolarian chert, partly as extruded lavas, but usually as intrusive sheets injected at the epoch of mountain-building.

The characters of igneous rocks in dykes, that is, of those types that have consolidated in fissures, resemble in many respects the characters of lava-flows. Chilling being usually equal on both surfaces, glassy or compact types of rock occur on both sides, and the dyke is, as previously observed, more crystalline in the centre. Columnar structures arise from both surfaces, the dyke also shrinking parallel to its margins. In the outer layers so formed, the columns are small, and they increase in diameter nearer the centre. In small dykes and veins, the columns may run continuously from side to side; in larger ones, they meet along a central surface, which forms, on weathering, a plane of weakness in the rock. Dykes may thus become worn away, decay spreading from the central region, and leaving the more resisting and more glassy portions clinging to the bounding walls.

Where, however, the surrounding rocks are more easily worn away than the igneous invader, as very often happens, the dykes stand out on the surface as great ribs and walls.

The rocks cooled in the deep-seated cauldrons, under what are styled plutonic conditions, have parted with their gases so slowly that they do not show scoriaceous structure. They may become very coarsely crystalline, like many of the Scandinavian granites; minerals, moreover, may be produced which are unstable or difficult to form nearer the surface. Crystals developed in plutonic surroundings become carried forward when the partially consolidated mass is pressed up to a volcanic orifice, and may undergo resorption on the way. Many, however, escape, and impart a porphyritic structure to lavas. The deep-seated rock, from causes that promote the growth of one mineral and the retention of another in solution, may also become "porphyritic" in situ, smaller crystals, or even a eutectic intergrowth, finally filling in the ground.

The viscidity of igneous rocks may cause any of the types to show a fluidal structure. Constituents already formed become dragged along in parallel series as the mass moves forward. Sometimes a group of spherulites, or a knot of "felsitic" matter caused by the dense growth of embryo-crystals, is stretched out into a sheet, and on fractured surfaces a banded structure characterises the mass. These banded rocks record, in their crumpled and obviously fluidal layers, the formerly molten condition of the mass. Even completely crystalline rocks may show parallel arrangement of their minerals, owing to flow during the last stages of consolidation, or to pressure from the walls of the cauldron, influencing the positions taken up by crystals that possess a rod-like or platy form.

Fig. 16. Granite invading Mica-Schist. Clifton, near Cape Town. Adjacent sections were studied by Charles Darwin (see [p. 156]).

The conspicuously banded structures in some crystalline rocks that are often grouped with the metamorphic gneisses may, however, be best explained by their composite origin, and the history of the structure is easily determinable in the field. A common case arises where a granite magma, perhaps already bearing crystals, is intruded, under pressure operating from a distance, into a well-bedded series of sedimentary rocks. The sediments open up like the leaves of a book and admit the invader along their planes of stratification. Even limestone may thus become interlaminated with an igneous rock, just as basalt has been known to separate the annual rings of trees involved in it. This intimate admixture permits of extensive mineral changes, and the two types of rock, probably very different in geological age, become welded together into a composite gneiss, both members of which have influenced one another by contact-metamorphism, often across a wide stretch of country ([Fig. 16]).