There are two green varieties, called in the trade "sea-green" and "unfading green," the former being used for a cheap roofing slate; and there are purplish varieties.
Slate is obtained in large quantities from the departments of Maine-ct-Loire (Angers), Ardennes (Fumay) and Mayenne (Renaz).
The dark bituminous layers of clay slate, which occur intercalated among the quartzites, have led, here as elsewhere, to the hope of coming upon a seam of coal, but it is contrary to experience that coal of any value should be found in rocks of that age.
Many of the block mountains of the Great Basin are of complicated internal structure, showing rocks of all ages - slate, limestone, quartzites, granite, multi-coloured volcanic rocks, and large areas of lava overflow.
Iron, coal and slate are the chief products, and copper and cobalt may be added.