364 



MICAS IN METAMORPHIC ROCKS. 



Micas in Metamorphic Rocks. Muscovite, according to Professor 

 Heddle, is chiefly found in granite veins, which he terms " exfiltra- 

 tive veins," but which we regard as vein-stones. Such veins, remark- 

 able for their tortuous course and branches, were by Jameson and the 

 older writers termed contemporaneous veins. Muscovite is specially 

 cited from the great vein of Capval, in Harris, where an abundance 

 of ferrous oxide gives it a green colour, though the finest specimens 

 are known from Bigsetter Voe, in Mainland, Shetland, and Glen 

 Skiag and Loch Glass, in Ross-shire. This mica is comparatively 

 rare in granite in Scotland. Margarodite is another potash mica, but 

 with more soda ; it is characteristic of gneissose rocks, specially in 

 Shetland, but it occurs in granular limestone in Aberdeenshire, Banff, 

 and the central parts of Scotland. 



Of the black micas, biotite i characteristic of crystalline lime- 

 stones, no other mica occurring in them in Scotland except marga- 

 rodite. Among the well-known localities for it are Glen Urquhart, 

 Glen Laggan in Inverness, Shinness in Sutherland, and Glen Beg in 

 Glenelg. Lepidomelane is another black mica, found in gneiss on 

 the north shore of Loch Shin, in Sutherland. Its colour is yellowish- 

 brown to chocolate-brown. This is the ordinary black mica of the 

 granites of Ireland. A blacker mica is Haughtonite, remarkable for 

 ' its large percentage of iron. It is found in veins in hornblendic gneiss, 

 in the graphic granite which occurs in the gneiss, at Rispond, Suther- 

 land, and in the micaceous gneiss of many localities in Koss and 

 Sutherland. This mica is frequently associated with oligoclase, 

 especially in veins, but it is an essential constituent of grey granite, 



