26 



LYELL'S ELEMENTS OF GEOLOGY. 



Mineral Composition of stratified Eocks. 



sandstone are usually rounded, as if by the action of running 

 water ; but they sometimes, though more rarely, consist of sma'lt 

 crystals, as if they had been chemically precipitated from a fluid 

 containing silex in solution. 



Sandstone is an aggregate of such grains, which often cohere 

 together without any visible cement, but more commonly are 

 bound together by a slight quantity of siliceous or calcareous 

 matter, or by iron or clay. In nature there is every intermediate 

 gradation, from perfectly loose sand, to the hardest sandstone. 

 In micaceous sandstones mica is abundant ; and the thin silvery 

 plates into which that mineral divides, are arranged into 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 grit. 

 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 conglom- 

 erate, therelbre, 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 the substance called alumine, or argil ; but, in common lan- 

 guage, any earth which possesses sufficient ductility, when 

 kneaded up with water, to be fashioned like paste by the hand, 

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

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

 than mud derived from the decomposition or wearing down of 

 various 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 is almost always mixed 

 with quartz.* Shale has also the property, like clay, of be- 

 coming plastic in water : it is a more solid form of clay, having 

 been probably condensed by pressure. It usually divides into 

 thin laminae. 



One general character of all argillaceous rocks is to give out 

 a peculiar odour when breathed upon, which is a test of the pre- 

 sence of alumine, although it does not belong to pure alumine, 

 but, apparently , to the combination of that substance with oxide 

 of iron.f 



Calcareous rocks. — This division comprehends those rocks 

 which, like chalk, are composed of lime and carbonic acid. 



* The kaolin of China consists of 71.15 parts of silex, 15.86 of alumine, 1.92 of 

 lime, and 6.73 of water, (W. Phillips, Mineralogy, p. 33.) ; but other porcelain 

 clays differ materially, that of Cornwall being composed of 60 parts of alumine 

 and 40 of silex. 



t See W. Phillips's Mineralogy, " Alumine." 



