44 



SLATE. GLAY. 



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. cr y S tals, 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- 

 times called argillaceous schistus, known generally 

 by the name of roof slate, as it is used for that pur- 

 pose. It consists of silex48, 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 fertility. 

 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. 

 Augite. — This is not a very abundant mineral, ex- 



