44 



tuent of the pebbles and sand, which form the third stratum I have 

 mentioned. It is difficultly acted on by any acid, except that 

 which is obtained by distillation from Derbyshire fluor spar ; and 

 which is termed fluoric acid. It is, however, very powerfully acted 

 on by the alkalies, which promote its fusion, and, when thus com- 

 bined with it, in a certain degree, form glass. It is this earth, which 

 chiefly forms the common flint, and horn-stone ; the more pure chal- 

 cedony ; and the still purer quartz, or rock crystal; the variously 

 figured agate; the arborised mochoa; the beautifully marbled jas- 

 per ; the sportive Egyptian pebble ; and the finely veined wood- 

 stone, still retaining the mark of every fibre, which regulated its 

 pristine structure. Combined with a certain portion of alumine, 

 it forms the chief constituent of the amethyst, topaz, and various 

 other stones, which, from their brilliancy, hardness, and colour, 

 are estimated as gems. This earth, also, chiefly helps to constitute 

 the granite,, of which the primitive mountains are formed. This 

 stone frequently displays, in a most beautiful and distinct manner, 

 the three substances of which it is composed. 'The feldspar, the con- 

 stituents of which are silica, alumine, and magnesia, will be seen, 

 generally, in oblong, curd-like masses, of various sizes. The mica, 

 composed of the same constituents, but in different proportions, will 

 appear generally in grains about the size of a pin's head, of different 

 colours, but, most commonly, black ; and sometimes in white flakes 

 of a metallic lustre. The quartz will be found interposed between 

 these generally of a greyish colour ; and appearing to be the me- 

 dium by which the other two substances are agglutinated together. 

 LIME is the earth which chiefly forms the various lime-stones, 

 which are placed, in different layers, beneath the beds of clay and 

 sand. This earth is rendered tolerably pure, when, by intense heat, 

 it is made into quick-lime. It fuses, when combined with flint and 

 clay ; and readily combines with acids. It is soluble in 700 times 



