1140. Sedimentary non-fossiliferous rocks. If powdered granite be mixed with water and then suffered to settle and the water poured off, the residaum, when dry, will closely resemble gneiss, which, as Professor Ansted observes, 'is, indeed, nothing else than stratified granite. If the water in which the pounded rock is thrown is moving along at a slow rate, and that part of the granite called feldspar happens to be somewhat decomposed, as it often is, then the feldspar (which is so truly clay, that it makes the best possible material for the use of the potteries), and the thin shining plates of mica will be carried farther by the water than the lumps of white quartz or flint sand, which with the other two ingredients made up the granite, and the two former will be deposited in layers, which, by passing a galvanic current through them, would in time become mica-schist. If the mica were absent, or if the clay were deposited without it, owing to any cause, then a similar galvanic current would turn the deposit into something like clay-slate.' (Ancient World, p. 19.) Thus were formed the first sedimentary or deposited rocks, still showing marks of their igneous origin, and 'often in themselves crystalline, but bearing evident marks of what is called 'mechanical structure,' or, in other words, of having been deposited from water.' (Ibid.) Gneiss and mica-slate often form mountain masses in association with the different varieties of granite, particularly in Scotland in the lofty mountain of Ben Nevis. The beds of gneiss in various parts of Great Britain are found of enormous depth, and they frequently contain large cracks which are filled up with trap, basalt, whin stone, and other igneous rocks. The different kinds of slates found associated with the gneiss are often considerably distorted, and sometimes they are found in wavy contortions. Associated with these rocks are those beautiful varieties of limestone known as statuary marble, which are generally white, but sometimes streaked by the admixture of various minerals.
