Schists

Schists, crystalline rocks in alternating layers or folia of different minerals. On the Continent the term is often extended to slates and shale (q.v.), the constltuents of which, though laminated or cleaved, are neither crystalline nor alternating. There seem sometimes to be transitions in the field on the one hand, from shales, slates, and sandstones to spotted slates - i.e. slates with scattered crystals, quartzites, and true schists; and, on the other hand, between confusedly-crystalline rocks and those that are foliated; but these transitions, even If demonstrated, are not conclusive as to modes of origin. The distinction between schists and foliated rocks - i.e. between schistosity and foliation - is one of minor importance, dependent as it is mainly on the presence in the former case of a laminated mmeral, such as mica or talc in masses of considerable surface. Thus gneiss often passes mto mica-schist. fhe most abundant schists are the light-coloured and but slightly greasy micas-schist, talc-schist, lighter and greasy to touch, often merely the result of the weathering of mica-Schist, the dark-green and soft chlorite-schist, and the harder hornblende-schist; but gneiss, quartzite, hornblende-rock, and other rocks which commonly occur in association with these schists, especia1ly in the series known as Archaean (q.v.), are often spoken of with them under the general term of "the Crystalline Schists." Of the two conflicting theories as to their origin, the alleged occurrence of fragments exhibiting the same structure as the main mass in conglomerates at the base of the Cambrian, as at Bangor, is a strong argument against their merely metamorphic origin; whilst it is difficult to explain the formation of gneiss or mica-schist, to say nothing of graphite and crystalline limestone, as precipitates direct from a heated primitive atmosphere. It is now generally admitted that gneiss and hornblende-schist may result from the alteration of granite and diorite, and quartzite is often obvlously only a partially-fused sandstone.