128 MICA-SCHIST. /- 



mica-schist ; by the prevalence of quartz, into quartz-rock ; 

 and of hornblende, into hornblende-schist. As regards 

 geological position, it reposes on granite, and is succeeded 

 by the other primary strata, frequently alternating in large 

 masses with one or other of these. It occurs in the 

 "Western Islands, and North-west Highlands of Scotland. 



MICA-SCHIST. Mica-schist may be regarded as gneiss 

 deprived of its feldspar, being a crystalline compound of mica 

 and quartz united in different proportions. It has a laminar 

 structure, which is more or less perfect, according to 

 the proportion and mode in which the mica enters into 

 its composition. Its colour is determined by the quantity 

 of the same mineral which it contains, and as that sub- 

 stance varies in tint from white, to dark brown, or black, 

 the quartz being usually white or, colourless, the prevailing 

 hue of the rock is, therefore, grey. Like the rest of the 

 primary deposits, it graduates into other members of the 

 same series, descending by the admixture of feldspar into 

 gneiss, and ascending by the preponderance of quartz, into 

 quartz-rock. In geological position it follows the gneiss, 

 and is succeeded by various schistose or slaty rocks of 

 crystalline texture, frequently passing into some of these. 

 Its chief localities are the north-west of Ireland, and the 

 Scottish Highlands, especially the picturesque district 

 termed the Trossachs. 



The subordinate rocks occurring among the primary strata, 

 may be studied, in some work exclusively devoted to the 

 subject; as Dr. M'Culloch's "Western Isles of Scotland, and 

 his Classification of Bocks. They are chiefly of schistose 

 or slaty texture, and are formed by the substitution of one 

 mineral for another, which constitutes the characteristic 

 ingredient of the rock. 



Thus, chlorite-schist is composed of the mineral substance 

 termed chlorite and quartz, differing from mica-schist, by its 

 dingy, green aspect, and soapy feel. It graduates by the 

 addition of feldspar into a variety of gneiss, by that of mica 

 into mica-schist, and occasionally assimilates with argillaceous 

 slate. 



Talcose- slate is composed of talc and quartz. It differs 

 from mica-schist, and chlorite-schist, chiefly in the presence 

 of talc, and in its colours, which are leaden, dingy white, or 



