36 MINERALS ROCKS 



notably in granite and gneiss. 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 different 

 forms, constituting, when crystallised, the various modifications of 

 calcite (rhombohedral) and arragonite (rhombic), and, when 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 replacing calcium. Eocks containing calcium 

 carbonate also invariably contain notable quantities of phosphates. 

 Limestones 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 (vide Chap. IV). 



Silicates of Magnesia are also extremely abundant. Many differ- 

 ent varieties exist, among the most common being talc and steatite, 

 6Mg0.4Si0 2 .H 2 O (usually containing ferrous oxide and alumina), 

 hornblende, asbestos and augite (Mg:Ca:Fe:Mn)O.Si0 2 , chlorite, 

 4Mg(Fe")0.2Si02.Al 2 O ? .3H 2 O, and olivine, 2(Mg:Pe)O.SiO g . Many 

 of these also contain silicate of alumina, and both ferrous and ferric 

 silicates. 



Clay, in its pure form, occurs as kaolin, Al 2 O 3 .2SiO.,.2H 2 O. Com- 

 mon clay, however, always contains iron (replacing the aluminium) 

 and generally some imperfectly decomposed felspar, so that it serves 

 as a source of potash and iron to plants. 



Rocks. Any detailed account of rocks, their origin and character- 

 istics would be out of place here ; but a few words may fitly be said 

 about the chemical composition and characteristics of a few typical 

 soil-yielding rocks. 



If we accept the nebular theory of the earth's origin, it is evident 

 that all rocks must have been formed out of the original intensely 

 heated matter which, ages ago, represented the earth. The greater 

 portion of the rocks at present forming the crust has probably been 

 through a succession of changes, at one time forming hard igneous 

 rock, then broken down by weathering into debris, which in course of 

 time was again consolidated into rock, in some cases to be again de- 

 nuded. The rocks now found are classified in various ways by geolo- 

 gists. One convenient method is based upon the process by which 

 they were formed. We thus get three principal class.es : 



1. Igneous rocks, subdivided into (i) Plutonic, (ii) Volcanic. 



2. Sedimentary rocks, subdivided into (i) Mechanically precipi- 

 tated, (ii) Chemically precipitated. 



3. Metamorphic rocks. 



These terms are almost self-explanatory. Class 1, generally hard 

 and silicious, often crystalline, comprises those rocks which have been 

 formed by solidification from a fused state ; class 2, those formed from 

 the fragments of previous rocks by deposition, generally under water ; 

 class 3, rocks (generally of class 2) which have been altered in char- 

 acter, since their deposition, by high temperature and pressure. 



