42 THE SOIL. III. 



divided clay falls out. The dissolved matters, consisting 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, sandstones, 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 ingre- 



dient 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 sandstones). 



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



but also contain finely divided silica, particles of im- 

 perfectly decomposed felspar, and often considerable 

 quantities of ferric oxide. 



3. Limestones, including chalk and magnesian limestones. 



Here the chief ingredient is calcium carbonate, but mag- 

 nesium, 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, when calcareous deposits such as tufa, traver- 

 tine, sinter, result; or by evaporation, by which gi/psnui, rock 

 salt, the Stassfurth deposits, &c., were probably formed. 

 Then, too, by the agency of animals, phosphatic deposits, e.g., 

 guano, coprolites, bone-earth, &c., have been produced, while 

 the remains of plants have given rise to the important rocks 

 coal, lignite, peat, &c. 



Metamorphic rocks partake of the nature of both igneous 

 and sedimentary, many having been formed from the latter by 

 chemical and physical changes produced by great pressure, or 



