12 AGRICULTURE 



solid rock. From such solid rock, whether 

 it be igneous or sedimentary, the soil that we 

 cultivate in our fields has all been derived, 

 sometimes directly, in other cases through 

 the agency of running water, as in the case 

 of alluvial soils. 



The sequence of changes from solid rock 

 to cultivated soil can be best studied by 

 inspecting a section that is exposed in the 

 operations of quarrying. Proceeding upwards, 

 we find that immediately over the solid rock 

 there are large masses of stone with but little 

 soil between them. These stones were at 

 one time part of the solid rock, but, by the 

 operations of certain weathering agents, they 

 have become detached from the main body 

 of rock. As we go farther up we find that 

 the stones become scarcer and smaller in 

 size, so that the spaces between the stones 

 are larger and the volume of soil greater. 

 Then, still farther up, the stones are even 

 smaller and fewer, and in the top 8 or 10 inches 

 which is the material stirred by the plough 

 and other agricultural implements large 

 stones may be entirely absent or difficult to 

 find. 



This gradual conversion of solid rock 

 into tillage soil is brought about by the opera- 

 tions of natural forces, few in number and 



