378 THE BEGOLITH 



upon the extent to which weathering has been carried. The 

 ultimate product of weathering of rocks of any but the purely 

 siliceous type is a more or less ferruginous clay, which may 

 be contaminated or admixed with coarser, foreign particles. It 

 is the extent of decomposition, more than its lithologieal deriva- 

 tion, that determines both the chemical composition and physical 

 characteristics of any soil. 



Rocks of essentially the same type so far as composition is 

 concerned, regardless of structural modifications induced by 

 either to the Archsean or older Palseozoic formations, but this 

 metamorphism, have been formed and re-formed throughout 

 every period of the earth's history. As has been already 

 indicated, the greater portion of the granitic, gneissie, or highly 

 metamorphosed crystalline schists and calcareous rocks belong 

 merely because they, being older, have been longer subjected to 

 metamorphosing agencies, and not because in themselves they 

 possess essential differences. It is true that some authorities 

 lay stress on the supposed abundance of animal remains in cer- 

 tain Palasozoic formations, but no one but the veriest amateur 

 would now dream of attempting to discriminate between either 

 igneous or aqueous rocks of the same nature, but of different 

 geological ages, on purely chemical grounds. 



It is a fact, however, that within certain climatic limits, the 

 rocks of any one horizon may impart such characteristics to a 

 residual soil as shall render it adapted to plant growth of a 

 particular kind. Thus,^ throughout the central portion of Ken- 

 tucky, where, within the distance of a few miles, rocks occur of 

 several distinct geological horizons, each bearing its mantle of 

 residual soil, each horizon may be traced for long distances, 

 though the rocks themselves are wholly obscured, merely by the 

 character of its forest growth. This feature is, however, prob- 

 ably dependent more on physical than chemical characteristics. 



(8) Soils as affected by Plant and Animal Life. — There are 

 various forms of animal and plant life the action of which is 

 worthy of note in connection with the subject of decomposition ; 

 but since it is probable that they are of greater efficiency in 

 promoting changes in soils once formed than in bringing about 

 the preliminary rock disintegration, their consideration has been 

 left to form a portion of the present chapter. 



Ants, by means of their numerous borings, penetrating at 



^ As the writer is informed by Mr. J. E. Proetor. 



