SOME IMPORTANT MINERALS. 41 



It is disintegrated by atmospheric agencies, but not so easily 

 as felspar. It furnishes plant food by virtue of the potash, 

 lime, and iron which it contains. 



Calcium carbonate. This occurs in a great variety of diffe- 

 rent forms, constituting, when crystallised, the various modifi- 

 cations of calcite (rhombohedral) and arayonite (rhombic), and 

 in the massive form the rocks chalk, limestone, and marble. 

 As already stated, these substances contain magnesium in 

 smaller or larger quantities, also iron, and often manganese, 

 the metals magnesium, iron, and manganese partially re- 

 placing calcium. Eocks containing calcium carbonate also 

 invariably contain notable quantities of phosphates. Lime- 

 stones thus furnish important constituents of plant food and 

 are almost indispensable in many of the processes which go on 

 in soils under the influence of bacteria (v. Chap. IV.). 



Silicates of magnesia are also extremely abundant. Many 

 different varieties exist, among the most common being talc and 

 steatite, 6Mg0.4SiO. 2 .H. 2 O (usually also contains ferrous oxide 

 and alumina), hornblende, asbestus, and auyite (Mg:Ca:Fe:Mn) 

 O.SiO,, chlorite, 4Mg(Fe")0.2SiO. 2 .Al,0 3 -3H. 2 0, an d oliuiiie, 

 2(Mg:Fe)O.Si0. 2 . Many of these also contain silicate of alu- 

 mina, and both ferrous and ferric silicates. 



Clay, in its pure form, occurs as kaolin, Al. 2 3 .2Si0. 2 .2H./). 

 Common clay, however, always contains iron (replacing the 

 aluminium) and generally some imperfectly decomposed fel- 

 spar, so that it serves as a source of potash and iron to plants. 



The oldest igneous rocks probably consisted mainly of sili- 

 cates and silica; granite, syenite, basalt, diorite, trap, &c., 

 are types of such old rocks. Under the denuding influences 

 of the weather, portions of their silicates are decomposed and 

 the whole rock disintegrates and is carried away by running 

 water to the sea, the alkalis (potash and soda), the greater part 

 of the lime, and portions of the magnesia and silica being in 

 solution ; much of the silica and the silicate of alumina in 

 suspension. In the sea and rivers the suspended matter 

 speedily settles to the bottom, the heaviest and coarsest por- 

 tion, consisting of large fragments of quartz and some felspar, 

 mica, &c., first, then the more finely divided quartz, &c., and 

 lastly, after a long time and only in deep water, the very finely 



