SLATES. The substances belonging to this class may be distributed into the following species :— 1. Mica-slate, occasionally used for co vering houses.
2. Clay-slate, the proper roofing-slate. 8. Whet, slate.
4. Polishing-slate.
3. Drawing-slate, or black chalk.
6. Adhesive slate.
7. Bituminous shale.
8. Slate-clay.
Mica-elate.—This is a mountain rock of vast continuity and extent, of a schis tose texture, composed of the minerals mica and quartz, the mica being generally predominant.
Clay-slate.—This substance is closely connected with mica ; so that uninter rupted be found between these two rocks in mountain chains. It is a simple schistose mass, of a bluish gray, or grayish-black color, of various shades, and a shining, somewhat pearly internal lustre on the faces, but of a dead color in the cross fracture.
All the best beds of roofing-slate im prove in quality as they lie deeper under the surface ; near to which, indeed, they have little value.
A good roofing slate should split read ily into thin even laminm • it should not be absorbent of water either on its face or endwise, a property evinced by its not increasing perceptibly in weight after im mersion in water; and it should be sound, compact, and not apt to disintegrate.
Some of the first qualities of slate for roofing, are now found in Vermont, near Brattleboro'. It equals the Welsh, and
somewhat resembles it. It is also found in Worcester co., Harvard, and Pepperel, Mass. It occurs over a large tract in N. Carolina. We see by Arkansas papers that a valuable quarry of it has just been discovered in Eagle Town, in the Choctaw country. The slate is in two hills, about a hundred feet high, which, it is said, are composed wholly of slate.
Slates for roofs, are trimmed, shaped, and bored by the slater. The roof is boarded with feather-edged weather boards, and the slates are fixed with cop per or zinc nails. A roof should incline from 12° to 25°. The best slate is bluish gray ; light-gray is stony and does not shape easily ; black or dark absorbs wet,, and quickly decays. They from 15 inches by 5, to 3 feet by 2 feet. The Welsh, Switzerland, and Kendal, are the best and largest.. • It is also found in the vicinity of Bos ton, at Charleston, Quincy, and'Meldon. Taleose and chlorite slates are found in the New England states abundantly. These are the gangue of the gold in the southern states. Drawing-slates are found in Rhode Island.