52 



Physics of the Soil. 



rocks made up of minerals of different kinds the crystal", 

 do not all expand at the same rate and this unequal expan- 

 sion and contraction tends to loosen crystals and fragments, 

 breaking the rock down, and thus form soil. 



FlG. 8. Section of limestone hill showing rock changing to soil. 

 (AfterChamberlin.) 



69. Formation of Soil From Limestone. If one will visit 

 any limestone quarry where the soil and rock are exposed 

 in section as represented in Figs. 8 and 9 it will be 

 clearly seen how the rock is slowly converted into soil. In 

 such cases as these, the water containing carbonic or other 

 acids dissolves away the lime and magnesia, leaving the 

 more insoluble portions of the lime rock to form the soil 

 mantle which is left. These more insoluble portions are 

 usually clay and very fine sand so that soils formed in this 

 way are oftenest clayey soils, sometimes containing even 

 less lime than other soils not derived from limestone. 



FIG. 9. Section of flat limestone surface showing rock changing to soil. 

 (After Chamberlin.) 



The mantle of soil seen above gravel beds in railroad 

 cuts and where hills have been graded down on wagon roads 

 has usually most of it originated from the decomposition 

 of the gravel in place in the same manner as a soil from 

 the limestone itself. So, too, in countries where granite 

 and other crystalline rocks lie beneath the soil, these have 



