Crystalline Schists

Crystalline Schists, a term now generally extended to the whole series of intimately associated rocks that includes the gneiss, mica-schists, and other foliated rocks underlying the fossiliferous and distinctly stratified Cambrian system (q.v.). Among these are the crystalline limestones, serpentinites, and layers of graphite seen in the Laurentian rocks of Canada, and quartzites, halleflintas, and ironstones in Wales and Scandinavia, in addition to the distinctly foliated or schistose gneisses, mica-schists, talc-schists, etc., that form most of the Scottish Highlands. The total thickness of these rocks is unknown, and individual layers are very inconstant in thickness, though the series as a whole presents much the same characters all over the world. No undoubted trace of life occurs in it, Eozoon (q.v.) being a much-debated structure either mineral or foraminiferal, and the fibrous texture of some graphite not being necessarily a sign of vegetable origin. According to one theory of their origin, these rocks are the result of metamorphism of ordinary, possibly Cambrian, sedimentary, and volcanic material; the limestone may have been of animal origin; the graphite metamorphosed coal; and the iron oxides may have been precipitated by organic acids. Against this view may be urged the uniformly complete character of the supposed metamorphism and the fact that fragments of these rocks exhibiting the same texture as the main mass are found in Cambrian conglomerates, arguing that if any such metamorphism occurred it must have been in pre-Cambrian times. The opposing view seems to be that these rocks represent part at least of the primitive crust of the earth or were thrown down as precipitates or sublimates from a primeval atmosphere under conditions of temperature and pressure unlike anything that has since prevailed. It is chemically possible that carbonate of lime and oxide of iron may be so thrown down and graphite possibly formed directly from hydrocarbon vapour; but the interlamination of limestone and serpentine is one among several serious difficulties in the way of this view.