ORIGIN OF SOILS 



61 



at all, the other being that the rock from which the soil was 

 derived was exactly the same as the underlying rock. As neither 

 assumption is strictly true, the method gives merely approximate 

 results. 



The following figures, secured by the method outlined above, 

 are compiled from Merrill's "Rocks, Rock Weathering and 

 Soils." 



CALCULATED PERCENTAGE Loss IN WEATHERING. 



The order in which these constituents are lost varies with the 

 rock and the conditions ; the following is the mean order in seven 

 cases : 



(i) Lime, (2) potash, (3) magnesia, (4) soda, (5) iron, 

 (6) silica, (7) alumina. That is, the greatest loss is usually of 

 lime, the next greatest is potash, and so on. The figures given 

 in the table preceding are sufficient to show the profound change 

 which may occur in the transformation of rock into soil and the 

 large amount of material which is carried away during weather- 

 ing, probably for the most part dissolved in water. 



Sedentary and Transported Soils. A sedentary soil is a soil 

 derived from the weathering of a rock in the present location of 

 the soil. On making an excavation, if the soil is sedentary, we 

 find the following : First, the surface soil ; then the subsoil, 

 lighter in color but of the same general character ; and at a lower 

 depth, we find the subsoil mixed with fragments of partly weath- 

 ered rock. The fragments increase in quantity until finally we 

 come to the solid rock. We thus observe a gradual transition 

 from soil to rock, and therefore infer that the overlying soil is 

 derived from the decomposition of rock which formerly occu- 



