238 SOILS. 



soils. Many rocks contain ferrous oxide, which readily under- 

 goes further oxidation ; certain sulphides in rocks are oxidizable 

 under the ordinary conditions found in a moist atmosphere, and 

 in such cases the chemical action results in rendering the rocks 

 brittle. 



638. Water can easily transport the finer particles of soil 

 from where they were formed by disintegration of the rocks to 

 points at distances from their source, varying with their weight. 

 For this reason the particles accumulate in different degrees 

 of fineness at different points along water-courses. 



639. It is believed that during the Glacial period, when large 

 portions of the northern hemisphere were covered deeply with 

 sheets of moving ice, immense amounts of coarse and fine 

 soils were carried far from the places where they were formed, 

 and were heaped up more or less irregularly in the masses which 

 now form gravelly hills and ridges. The glacial action now 

 going on in the Alps shows how vast must have been the soil- 

 making and soil-carrying power of the glaciers which once cov- 

 ered so much of our continent. 



640. Soils which have not been carried by water or ice from 

 the place where they were formed by some of the agencies men- 

 tioned above are not generally of great depth, and their nature 

 can usually be made out by examination of the contiguous 

 rocks. 



641. Classification of soils. For our present purpose soils 

 may be classified as gravelly, sandy, clayey, calcareous, loam}', 

 and peaty. Gravelly soils differ widely in their chemical char- 

 acter, since the pebbles w r hich compose them may be either chiefly 

 quartz and fragments of rocks in which quartz predominates, or 

 there may be also a good proportion of limestone, or of feld- 

 spathic rocks. With the coarse pebbles is intermingled a 

 certain proportion of finer soil. Sandy soils are usually made 

 up of fine quartz with which some other matters are asso- 

 ciated, such as some compound of iron, grains of feldspathic 

 minerals, micaceous particles, etc. In a few cases, however, 

 the sandy soils differ widely from this composition ; for in- 

 stance, the green sand of New Jersey contains a large proportion 

 (more than fifty per cent) of green grains of a silicate of iron and 

 potassium. Clayey soils are generally derived from the dis- 

 integration of various feldspathic rocks, and are mixtures of 

 hydrated aluminic silicate with man}* other matters. Such soils 

 are generally adhesive, are retentive of water, and dry into 

 a hard mass ; these characters which belong to true clay an; 



