8 



on account of its tenacity, is comparatively a light soil in weight. Peaty 

 soils are light in both senses of the word, being loose, or porous, and 

 having little actual weight.* (See Table III.) 



GEOLOGICAL CLASSIFICATION OF SOILS. 



Whatever their composition and texture, soils are, from a geological 

 standpoint, mainly of two sorts: soils of disintegration and soils of 

 transport. Under the former are comprehended such as arise from the 

 waste and decay of the immediately underlying rocks, the limestones, 

 traps, granites, and the like, together with a certain admixture of vege- 

 table and animal debris; and which are directly influenced in their 

 composition, texture, and drainage by the nature of the subjacent rocks 

 from which they are derived. Under the latter are embraced all drift 

 and alluvial materials, such as sand, shingly debris, miscellaneous silt, 

 and clay, which have been worn from other rocks by atmospheric agencies 

 and transported to their existing positions by winds, waters, or ancient 

 glacial action .f 



DIFFERENCE BETWEEN THE SOIL AND THE SUBSOIL. 



Besides the soils proper, which come immediately under cultivation, 

 there are in most places a set of subsoils, differing from the true soils? 

 and which cannot be ignored. The true soils are usually of a darker 

 color, from the larger admixture of humus, whilst the subsoils are lighter 

 in hue, yellow, red, or bluish, from the greater preponderance of the 

 iron oxides. The soils are more or less friable in their texture, whilst 

 the subsoils are tougher, more compact, and more largely commingled 

 with rubbly and stony debris. The soils are usually a little more than 

 mere surface covering, whilst the subsoils may be many feet in tbiek- 

 ness.t 



WEATHERING OF THE ROCKS AND FORMATION OF THE SOIL. 



All exposed rocks break up in course of time under the continued 

 action of atmospheric agencies, however hard and refractory they may 

 be ; these agencies act both chemically and mechanically. The rain, 

 owing to the absorption of' carbonic acid from the atmosphere, acts 

 chemically on the rocks by its solvent action, and also from its oxygen 

 combining with substances not yet fully oxidized. Its mechanical ac 

 tiou appears in its washing away the finer portions of the disintegrated 

 rock or soil from higher to lower ground. The changes in temperature 

 have a loosening influence by causing alternate expansion and contrac- 

 tion. The atmosphere itself acts chemically upon the rocks by the 

 slow oxidization of those minerals which can absorb more oxygen, and 

 the production of carbonates and bicarbonates whose solubility still 



* The Soil of the Farm, 

 t Page's Economic Geology. 



