The fourth and last great division of rocks are the crystalline strata or schists, called gneiss, mica-schist, clay-slate, chlorite-schist, marble, and the like, the origin of which is more doubtful than that of the other three classes. They contain no pebbles or sand or scoriae, or angular pieces of imbedded stone, and no traces of organic bodies, and they are often as crystalline as granite, yet are divided into beds, corresponding in form to those of sedimentary formations, and are therefore said to be stratified.
The beds sometimes consist of an alternation of substances varying in colour, composition, and thickness, precisely as we see in stratified fossiliferous deposits. The materials of these strata were originally deposited from water in the usual form of sediment, but they were subsequently altered by subterranean heat, so as to assume a new texture. It is demonstrable, in some cases at least, that such a complete conversion has actually taken place.
Alterations, such as might be produced by intense heat, are observed in strata near their contact with veins and dikes of volcanic rocks. These, however, are on a small scale; but a similar influence has been exerted much more powerfully in the neighbourhood of plutonic rocks under different circumstances, and perhaps in combination with other causes. The effects thereby superinduced on fossiliferous strata have sometimes extended to a distance of a quarter of a mile from the point of contact.
Throughout the greater part of this space the fossiliferous beds have exchanged an earthy for a highly crystalline texture, and have lost all traces of organic remains. Thus, for example, dark- limestones, replete with shells and corals, are turned into white statuary marble, and hard clays into slates called mica-schist and hornblende-schist, all signs of organic bodies having been obliterated.
Although we are in a great degree ignorant of the precise nature of the influence here exerted, yet it evidently bears some analogy to that which volcanic heat and gases are capable of producing; and the action may be conveniently called plutonic, because it appears to have been developed in those regions where plutonic rocks are generated, and under similar circumstances of pressure and depth in the earth. Whether electricity or any other causes have co-operated with heat to produce this influence, may be matter of speculation, but the plutonic influence has sometimes pervaded entire mountain masses of strata. The phenomena, therefore, being sometimes on so grand a scale, we must not consider that the strata have always assumed their crystalline or altered texture in consequence of the proximity of granite, but rather that granite itself, as well as the altered strata, have derived their crystalline texture from plutonic agency.
In accordance with this hypothesis we use the term ” Metamorphic” for the altered strata, a term derived from pern, meta, and trans, morphe, forma.
Comments on this entry are closed.