THE SOIL. 33 



for many centuries, it can be understood how what 

 was once solid rock has become as finely divided as 

 is the mineral part of the best soils. 



It is evident that comparatively a small portion 

 of the soil of the earth remains where it was when 

 it was rock. Analysis of some soils shows that 

 they have the same composition as the rocks which 

 underlie them. Such soils are usually of only 

 moderate depths. In most soils, however, there 

 are fragments of rook different from that found 

 underlying or in adjacent hills. It needs no argu- 

 ment to show that the surface soil at the foot 

 of hills, along the banks of many streams, or 

 where there was once a lake, has been carried there 

 by water. Sometimes valleys have been tilled to a 

 great depth and, later, a channel worn through this 

 filled valley by a river. Soils which remain where 

 they were formed are called sedentary soils, or soils 

 in place. Those which have been moved in, or after, 

 the process of soil formation, are called transported 

 soils. Those which have been carried by moving 

 water are called alluvial soils. These usually are in 

 layers or strata, sometimes differing in the kind of 

 material, almost always differing noticeably in the 

 fineness of division. Soils which were deposited by 

 glaciers are called drift. The most noticeable quality 

 of these is the presence of boulders. Wherever these 

 large, rounded rocks are found it may safely be de- 

 cided there has been glacial action in the remote past. 

 Sometimes drift and alluvial soils are mingled togeth- 

 er. Drift soils cover much of the United States north 

 of the 39th. parallel of latitude. 



