ORIGII^ Aiq^I) FORMATIOIS^ OF SOILS. 9 



Alluvium. — If no denudation took place, the soil of 

 every locality would be simply the decayed upper surface 

 of the rocks underneath it. But, in proportion to tlie 

 slope of the ground and the quantity of rain, the soil is 

 moved from higher to lower levels, so that m many cases 

 a good soil comes to lie upon rocks, which of themselves 

 would only produce a poor soil. During every fall of 

 ram, transportation of soil goes on, and the thicker soils 

 of the valley are partially formed in this way. 



The running water bears along the transported matter 

 and leaves it when the force of the current diminishes ; 

 the finer portion being carried further ; the extremely 

 comminuted material moviug as long as the current 

 moves at all. When a river reaches a level tract on 

 which its motion is slow, and over part of which it can 

 flow in flood, all the suspended material, consisting of 

 fine sand and mud, is deposited and constitutes the 

 alluvium, or new land formed by such deposits at the 

 river mouth. 



Marine alluvial soils have a similar origin. The rising 

 tide sweeps away the fine material from every exposed 

 bank or cliff, and becomes loaded with mud and extremely 

 fine sand, which, as it stagnates at high water, it deposits 

 in a thin layer on the surface of the flats. This layer, 

 which varies in thickness, is thus coarser and thicker at 

 the outer edge of the flats than near the shore. 



From the same cause, the earlier deposit of the coarse 

 sediment, the lower strata of the layer are arenaceous, 

 while the upper surface is fine and slimy. Thus the flats 

 continue to grow until they reach such a height that they 

 can be overflowed only by the high spring tides ; and they 

 at length become gradually covered w^ith the coarse 

 grasses and sedges which grow in such places. 



Drift or Boulder Clay.— In addition to river and 

 marine action, it must be borne m mind, however, that 



