506 A TREATISE ON METAMORPHISM. 



ates, nitrates, and sulphates in great abundance within the soil, and thus 

 the processes of carbonation, nitration, and sulphation of the minerals of 

 the rocks are rapid. The carbonates, nitrates, and sulphates furnish food 

 for the chlorophyll-bearing plants. The plants are food for the animals. 

 After a time these plants and animals die, decompose, and thus supply 

 additional material to carry on the process of decomposition of the rocks. 

 Therefore there is action and interaction between the chlorophyll-bearing 

 plants and the bacteria, and between these and the animals alive; interac- 

 tion between these plants and animals dead and bacterial plants alive, and 

 interaction between all live and dead plants and animals and the rocks. 

 Decomposition of rocks produces plant food; the food builds the plants": 

 the plants build the animals; these in turn, both while alive and after 

 death, produce decomposition of the rocks, and so round indefinitely. 



It is now clear why it is so difficult to produce a good soil where such 

 a soil does not exist, and why a soil once formed may be retained indefi- 

 nitely by proper tillage. Where the soil is • good and vegetation is 

 abundant, the complicated machine is in full motion and is able to produce 

 new soil as fast as the old soil is transported elsewhere by wind or water. 



The cycles above considered do not include the whole case. The full 

 geological cycle does not occur in the belt of weathering alone. As fully 

 explained elsewhere (pp. 538-539, 612 et seq.), a portion of the salts dis- 

 solved during the decomposition of the rocks in the belt of weathering is 

 transported to the belt of cementation and there precipitated; another 

 portion goes to the sea. It is only when these portions are followed that a 

 full comprehension of the complicated and far-reaching geological effect of 

 weathering through plants and animals may be obtained. Here it will only 

 be remarked that of the salts of the sea, calcium carbonate is produced by 

 carbonation of the silicates, but this process is mainly dependent upon the 

 plants. Thus the work of the plants furnishes the material for the hard 

 parts of sea animals; these hard parts are the source of the great limestone 

 formations; as a result of geological revolutions limestone formations built 

 below the sea are raised and become land areas; upon these limestones 

 soils of unusual fertility are formed, and this results in luxuriant plant and 

 animal life, thus completing a cycle, one among the many geological cycles 

 connected with plants and animals. 



While life is especially potent in chemical work, the power of life in 

 mechanical work is considerable. 



