84 THE CHEMISTRY OF THE FARM 



action of water, air and frost ; and, in the later stages 

 of their history, by the action of vegetable and animal 

 life, and their products. The original matter from 

 which all subsequent formations have been derived 

 consists of the igneous rocks, composed chiefly of 

 silica and alumina united with variable proportions of 

 oxide of iron, potash, soda, lime, magnesia, and small 

 quantities of many other substances. Such rocks always 

 contain some phosphoric acid, frequently as apatite. 



Some soils are derived directly from the decompo- 

 sition of igneous rocks, as in the case of soils derived 

 from lava, basalt, and granite. In most cases, how- 

 ever, the igneous rocks have undergone disintegration 

 during geologic ages, and have been redeposited on the 

 sea bottom, the deposit being generally associated with 

 the remains of vegetable and animal life. The sedi' 

 mentary rochs thus produced consist either of sand, 

 clay, or limestone, or mixtures of these, in various 

 states of aggregation. The sand consists of little 

 altered fragments of the hardest and most resistant 

 constituents of the original rock ; it is chiefly com- 

 posed of quartz, but generally contains besides small 

 quantities of felspar, mica, and other minerals. Clay 

 is a hydrated siHcate of aluminium ; it is a result of 

 the chemical decomposition of potash or soda felspars. 

 These felspars are decomposed by the prolonged action 

 of water containing carbonic acid ; the alkahes and a 

 part of the silica are removed in solution, and clay 

 remains. During the decomposition of igneous rocks 

 the lime and magnesia have been removed in solu- 

 tion, and have accumulated in the ocean; the pre- 

 cipitation of the lime and magnesia as carbonate has 

 been brought about through the agency of vegetable 



