Ca II.] MINERAL COMPOSITION OF STRATIFIED ROCKS. 11 



jind, lastly, the calcareous rocks or limestones consist of carbonic acid 

 and lime. 



Arenaceous or siliceous rocks. — To speak first of the sandy division : 

 beds of loose sand are frequently met with, of which the grains consist 

 entirely of silex, which term comprehends all purely siliceous minerals, 

 as quartz and common flint. Quartz is silex in its purest form ; flint 

 usually contains some admixture of alumine and oxide of iron. The 

 siliceous grains in sand are usually rounded, as if by the action of running 

 water. S^mdstone is an aggregate of such grains, which often cohere to- 

 gether without any visible cement, but more commonly are bound togethei- 

 by a slight quantity of siliceous or calcareous matter, or by iron or clay. 



Pure siliceous rocks may be known by not effervescing when a drop 

 of nitric, sulphuric, or other acid is applied to them, or by the grains 

 not being readily scratched or broken by ordinary pressure. In nature 

 there is every intermediate gradation, from perfectly loose sand, to the 

 hardest sandstone. In micaceous sandstones mica is very abundant ; 

 and the thin silvery plates into which that mineral divides, are often ar- 

 ranged in layers parallel to the planes of stratification, giving a slaty or 

 laminated texture to the rock. 



When sandstone is coarse-grained, it is usually called pit. If the 

 grains are rounded, and large enough to be called pebbles, it becomes a 

 conglomerate^ or pudding-stone, which may consist of pieces of one or of 

 many different kinds of rock. A conglomerate, therefore, is simply 

 gravel bound together by a cement. 



Argillaceous rocks. — Clay, strictly speaking, is a mixture of silex or 

 flint with a large proportion, usually about one-fourth, of alumine, or 

 argil ; but, in common language, any earth which possesses suflicient 

 ductility, when kneaded up with water, to be fashioned like paste by 

 the hand, or by the potter's lathe, is called a cla,y ; and such clays vary 

 greatly in their composition, and are, in general, nothing more than mud 

 derived from the decomposition or wearing down of rocks. The purest 

 clay found in nature is porcelain clay, or kaolin, which results from the 

 decomposition of a rock composed of felspar and quartz, and it is almost 

 always mixed with quartz."^ Shale has also the property, like clay, of 

 becoming plastic in water : it is a more solid form of clay, or argillaceous 

 matter, condensed by pressure. It usually divides into laminse, more or 

 less regular. 



One general character of all argillaceous rocks is to give out a pe- 

 culiar, earthy odor v.^hen breathed upon, which is a test of the presence 

 of alumine, although it does not belong to pure alumine, but, apparently, 

 to the combination of that substance with oxide of iron.f 



* The kaolin of China consists of 71-15 parts of silex, 15-86 of alumine, 1-92 of 

 linae, and 673 of water (W. Phillips, Mineralogy, p. 33) ; but other porcelain clays 

 differ materially, that of Cornwall being composed, according to Boase, of nearly 

 equal parts of silica and alumine, with 1 per cent, of magnesia. (Phil. Mag. voL 

 S. 1837.) 



f See W. Phillips's Mineralogy, " Alumine." 



