Page:A Treatise on Geology, volume 1.djvu/142

 126 of granulation it might constitute an arenaceous rock, and be even called sandstone. The former is, perhaps, almost really true with respect to clay slate; for this substance is not very distinct, chemically speaking, from decomposed felspar which has lost or changed the condition of its potash by the operation of water: hence it happens under particular circumstances (which permit the access of alkali and the agency of great heat), that powdered blue slate is actually transformed to white and glassy crystalline grains of felspar. This is one of the results of the yet uncompleted experiments on the effects of long continued heat, instituted by Mr. W. V. Harcourt in Yorkshire.

Clay slate, the simplest form of argillaceous fissile rock, is so uniform in its appearance, fineness of grain, colour, hardness and chemical composition, that mineralogists have often included it in their arrangements as a peculiar mineral species. Imbedded in it we sometimes find certain crystallised minerals, as chiastolite or hornblende (in Skiddaw), cubic pyrites (Dunolly, near Oban, Ingleton, in Yorkshire); its colour is black (Skiddaw), purple (Snowdon), green (Langdale), yellow (Charnwood Forest), mottled (near Ambleside): some varieties (Westmoreland) are translucent at the edges: others (N. Wales) opaque: there are variations of hardness, from the soft perishing slate of Skiddaw to the hard durable rocks of Langdale.

If we imagine the substance of clay slate diffused amongst and around grains of quartz, felspar, mica, bits of jasper or other minerals, and the whole indurated considerably, the general title applicable to the whole series of rocks thus composed is Grauwacke,—which varies in fineness of grain from what emulates clay slate to a conglomerate with quartz pebbles half an inch in diameter. Examples may be found in Ben Ledi,—the Lammermuir,—the Cavan district,—at Llanberis.

Structures.—Amongst these rocks the evidence of successive deposition is sometimes most clear and