III. THE SOIL 37 



Another method of classification is based upon structure. Thus we 

 have 



A. Crystalline rocks, e.g., granite. 



B. Vitreous rocks, e.g., obsidian. 



C. Colloidal rocks, e.g., silicious sinter, nodules. 



D. Fragmentary rocks, e.g., sandstones, shales. 



Bocks of classes A and B are generally igneous in origin (exception 

 certain limestones, rock salt, gypsum, etc., which may be classed as 

 crystalline). Class C have probably been deposited from solution. 

 Class D are sedimentary in origin. 



Class 1 include the oldest rocks, from which probably the other 

 rocks have been formed. Bocks of this class generally contain several 

 minerals. 



The oldest igneous rocks probably consisted mainly of silicates and 

 silica ; granite, syenite, basalt, diorite and trap, 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 alkalies (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 sus- 

 pension. In the sea and rivers the suspended matter speedily settles 

 to the bottom, the heaviest and coarsest portion, consisting of large 

 fragments of quartz and some felspar, mica, etc., first, then the more 

 finely divided quartz, etc., and lastly, after a long time and only in 

 deep water, the very finely divided clay. The dissolved matters, con- 

 sisting of compounds of potash, soda, lime and magnesia, remain in 

 solution until removed by the agency of living organisms, e.g., shell- 

 fish, coral polyps, or sea-plants. 



In this way are formed accumulations in the bed of the ocean 

 which, under pressure subsequently applied, will furnish grits, sand- 

 stones, shales and limestones respectively, the latter being mainly 

 composed of the debris of marine Crustacea. The dissolved silica is 

 also removed by diatoms, whose silicious skeletons sink to the bottom 

 and remain admixed with the calcareous materials. 



Sedimentary rocks are thus divided into the three great classes 



1. Sandstones, grits and conglomerates, whose main ingredient is 

 quartz, almost always mixed with some felspar and mica, and having 

 their grains cemented together by either calcium carbonate (calcareous 

 sandstone), clay (argillaceous sandstone), ferric oxide (ferruginous sand- 

 stone), or soluble silica (silicious sandstone). 



2. Shales or clays. These are mainly composed of kaolin, but also 

 contain finely divided silica, particles of imperfectly decomposed felspar 

 and often considerable quantities of ferric oxide. 



3. Limestones, including chalk and magnesian limestones. Here 

 the chief ingredient is calcium carbonate, but magnesium, silica, iron, 

 aluminium, phosphoric acid and other substances are almost always 

 present in varying proportions. 



In addition to these are some few rocks formed in other ways. 

 Thus, by precipitation from solution, either by loss of carbonic acid, 



