MICA SLATE. CLAY SLATE. 227 
Mountains of gneiss are not so steep as those of 
granite, and their summits are usually rounded. Ben 
Lomond and others in Scotland, and mount Rosa in 
Italy, are almost wholly of gneiss, as well as the 
middle part of the Pyrennees. It is not an uncommon 
rock, but in Britain is of less frequent occurrence than 
granite. 
Many valuable metallic ores are found in veins of 
gneiss. This rock also sometimes contains crystals of 
garnet (70), and tourmaline (69). 
256. MICA SLATE, or MICACEOUS SCHISTUS, 
is a primitive rock of slaty structure, consisting principally 
of quartz (76) and mica (123). 
Like gneiss, it is rich in ores. It often contains beds 
of magnetic ironstone (235), galena (239), copper, 
blende (244), cinnabar (228), and sometimes even 
gold. It frequently has garnets, and sometimes tour- 
malines (69), interspersed in different parts of it. 
Mica slate occurs in many parts of Scotland ; the 
mountain of Schehallien, and the rocky adjacent coun- 
try, are in a great degree composed of it. 
257. CLAY SLATE is a primitive rock generally of dM 
blue colour, more 0?' less compact > always slaty, and always 
stratified. 
Under the appellation of clay slate are included 
roofing slate (120), tvhet slate (122), drawing slate (121), 
and some other kinds already described. 
Few rocks abound more in veins and beds of valuable 
metals than slate. In different countries it contains 
ores of tin, lead, cobalt (24?7), silver, and copper; and 
gold, and mercury (228) sometimes occur in it. The 
celebrated quicksilver mines of Idria (228), and the im- 
mense mass of copper at Parys mountain in the island 
of Anglesea (230), and in clay slafee. Crystals of py- 
rites (236), and sometimes garnets (70), and thin layers 
of quartz (76), and felspar (110), are all occasionally 
found embedded* in it. 
