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 ([241]), cinnabar ([228]), and sometimes even gold. It frequently has garnets, and sometimes tourmalines ([69]), interspersed in different parts of it.
Mica slate occurs in many parts of Scotland; the mountain of Schehallien, and the rocky adjacent country, are in a great degree composed of it.
257. CLAY SLATE is a primitive rock generally of dull blue colour, more or less compact, always slaty, and always stratified.
Under the appellation of clay slate are included roofing slate ([120]), whet 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 ([247]), silver, and copper; and gold, and mercury ([228]) sometimes occur in it. The celebrated quicksilver mines of Idria ([228]), and the immense mass of copper at Parys mountain in the island of Anglesea ([230]), are in clay slate. Crystals of pyrites ([236]), and sometimes garnets ([70]), and thin layers of quartz ([76]), and felspar ([110]), are all occasionally found embedded in it.
This is a widely-extended rock; it sometimes forms whole mountains, and even chains of mountains; but these usually have a gentle acclivity. The summit of the celebrated mountain called Skiddaw in Cumberland is of clay slate.
258. PRIMITIVE LIMESTONE is a simple mountain rock of crystalline or granular structure; and generally of white, yellowish, greenish, or reddish colour.
To this species of rock belong many of the rich and beautiful kinds of marble already described (143, &c.). Carrara, or statuary marble ([146]), is a familiar instance of it. Whole mountains in Stiria, Carinthia, Carniola, and the Pyrenees, and three mountains in Switzerland, 10,000 feet in height, are of primitive limestone. The mountain of Filabres in Spain, is said to consist of one block of white granular marble, 2,000 feet high, and three miles in circuit; without intermixture of other earths or stones, and almost without a fissure.
Various mineral ores, in beds and veins, as lead, zinc ([241]), and iron, are occasionally found in this kind of rock.