44 
SLATE. — CLAY. 
gray, red, and other colours. Common plaster of 
Paris has a laminated or granular structure, and is 
sometimes compact. It is softer than common 
limestone, and can be easily scratched with the 
nail. It does not efferversce with acids. Its con- 
stituent parts are, lime 32, sulphuric acid 46, water 
31. There is a variety which contains no water, 
and is therefore called anhydrous. When crystal- 
lized, gypsum can be separated into thin plates, like 
mica and talc, which break into four-sided 
Fig. 6. crystals, as in fig. 6. This mineral is 
used for various purposes, such as a ma- 
|\ \ nure, for cements, castings, &c. It is 
\^^^^ generally found in connexion with salt- 
springs or rock-salt. 
Slate and clay. — By the term slate, when employ- 
ed alone, is understood clay slate, or what is some- 
tira-es called argillaceous schistus, known generally 
by the name of roof slate, as it is used for that pur- 
pose. It consists of silex 48, alumine 23, and some- 
times a little carbon, potash, water, and oxide of 
iron. It is easily scratched by the knife, and on 
moistening, by breathing on it, gives out a peculiar 
odour. Its colours are usually bluish, greenish, 
gray, or reddish. The slate rocks have generally a 
slaty structure, and may be split in two directions, 
which have an acute angle with each other; but 
some varieties cannot be split in any direction. On 
account of the potash contained in some species of 
slate, it is easily decomposed on exposure to the 
atmosphere, forming a clayey soil of great fertihty. 
What is called shale by the English miners is only 
a soft, dark-coloured slate, containing more carbon 
than common slate. Slate is employed for roofing 
buildings, drawing slates, and pencils, and the clays 
for making bricks and pottery. The clays are of 
the same chymical composition as slate. Both are 
very abundant. 
Augile. — This is not a very abundant mineral, 6x- 
