Page:The American Cyclopædia (1879) Volume VII.djvu/705

Rh distinguished from those crystalline stratified rocks in which the bedding is ill defined, either from having been obscure from the first or else obliterated by subsequent crystallization. There are strong reasons for believing that the stratified crystalline rocks, by a process of softening and subsequent displacement or eruption, gave rise to the unstratified rocks with which they are often mineralogically identical; and hence the names of indigenous and exotic crystallines have been proposed by Dr. Hunt to designate respectively the stratified and the eruptive rocks. A third class of crystalline rocks is also to be distinguished, viz.: those which occur as veinstones in the fissures of other rocks, and have probably been deposited from watery solutions. Such are the quartz and spars which form the gangue of many metallic ores, and a large part of the so-called granite veins. The rocks of this third class, from their mode of formation, are designated by Dr. Hunt as endogenous crystallines. It is in some cases impossible to determine from its mineralogical characters to which of these three classes a given crystalline rock belongs. The unstratified crystalline or eruptive rocks include the modern volcanic lavas, which are evidently the products of igneous fusion, and the whole class is therefore sometimes designated as igneous rocks. It is supposed however that many of these rocks, as for example the exotic granites, have never been in a state of igneous fusion, but have assumed a plastic condition by the intervention of water under great pressure and at a temperature far below that of fused lavas. They have hence been called by some geologists plutonic and by others hypogene rocks, the latter name signifying rocks generated beneath, in allusion to their obvious subterranean source. The distinctly stratified and sedimentary character of the great formations of crystalline rocks, and the obvious analogies which they present in this respect to the uncrystalline formations, early attracted the attention of geologists. In both occur intercalated layers of limestones, argillites, and conglomerates; and the question naturally arose as to the origin of the gneisses, mica schists, diorites, serpentines, chlorite schists, and talc schists, which are the characteristic rocks of these crystalline stratified formations. That the elements of these had in some way been deposited from water, like the beds of sand, mud, and carbonate of lime of uncrystalline strata, seemed obvious; and hence the conclusion that they were once, like the latter, uncrystalline strata, which had subsequently changed their form. In accordance with this notion, they were designated metamorphic strata, and this term is by many geologists used as synonymous with stratified crystalline rocks. It was noticed that in some instances uncrystalline sediments had assumed a crystalline character in the immediate vicinity of certain erupted rocks; the effect of heat, or more probably of the heated solutions impregnating the last, having generated

in the midst of the contiguous sediments crystalline mineral species. It was then possible that a formation uncrystalline in one part of its distribution should elsewhere become crystalline, or in other words metamorphic; and it was conjectured that great areas of such rocks might be the stratigraphical equivalents of formations which are elsewhere uncrystalline sediments. In the Alps, for example, it was supposed that the gneisses and other crystalline schists were of mesozoic and even of cenozoic age, and similar rocks in other regions were declared to be palæozoic; till at length it seemed, such was the extension of the doctrine of rock metamorphism, that the sediments of any age might assume the characters of the primitive crystalline schists. In fact, the crystalline schists of the Alps, the British islands, and the Appalachians have all in turn been claimed as altered strata of palæozoic or more recent times. But these views have been controverted, and it has been shown that the crystalline strata which are now found in the Alps, superposed upon the uncrystalline fossiliferous sediments, are really ancient strata which were crystalline before the deposition of the latter, and in their normal position underlie them, but by great foldings and inversions have been brought to overlie them. In some instances in this region beds of apparently crystalline rocks are met with in which occur fossils like those of the uncrystalline sediments. These were regarded as further evidences of the metamorphic process which had proceeded so far as to develop a crystalline structure in the newer beds, without however obliterating their organic remains. But it has been shown that these pseudo-crystalline rocks are really sediments of the newer periods, made up of the ruins of the older and truly crystalline rocks. In many other cases, as in Wales and in eastern North America, it is found that the broken-up materials of the crystalline schists enter into the composition of the oldest palæozoic schists, which are themselves uncrystalline. While, therefore, it is clear that the crystalline schists were deposited from water, and, as will subsequently be seen, under conditions which, although chemically somewhat different from those of later times, did not prevent the development of organic life, it is now affirmed by one school of geologists that the great bodies of crystalline schists do not result from the alteration of any known series of uncrystalline strata; so that the division between the two established by Werner may still be retained as a fundamental one. This view is now sustained by Favre of Geneva, Sterry Hunt, Gümbel, Credner, and others; but the opposite view, which maintains a wide-spread metamorphism of palæozoic and more recent rocks, has been taught by very eminent names, and is still maintained in the principal geological text books and treatises. The partisans of the latter view, while asserting the comparatively recent origin of many crystalline schists, have always