aEOLOGY OF AGRICULTURE. 6B 



secondary and the tertiary ; and then above these the 

 drift and the alluvial deposits. It is not to be sup- 

 posed, however, that each of these forms an entire, un- 

 broken layer or coating around the whole earth. This 

 is probably true of the igneous formations (the trap 

 and the granite). It is different with the aqueous for- 

 mations. The primary rocks have been broken in 

 many places, and forced asunder by the ejectment of 

 igneous matter from below. In other places they have 

 been lifted up, by internal heavings of the earth, so 

 high that no secondary rocks have been formed above 

 them. Consequently the secondary formation is more 

 broken than the primary. The tertiary is still more 

 broken, covering but comparatively small portions of 

 the earth. The drift is of very unequal thickness, 

 having been lodged by the agency that distributed it, 

 more in valleys, less on high grounds, and not at all 

 on mountains. The alluvial deposits are of very limit- 

 ed extent, confined mostly to the banks of rivers, which 

 have deposited them ; to peat swamps, formed by de- 

 caying vegetable matter ; and to the slopes and valleys 

 about volcanoes, furnished by volcanic matter from 

 the bowels of the earth. Any deposits, which are 

 the results of causes now in operation, are considered 

 as alluvial. It will be seen from the foregoing state- 

 ments, that what we call the soil {the cultivable portion 

 of the earth's surface, some 10 or 12 inches deep) may 

 lie on either of the aqueous, or stratified rock forma- 

 tions, or even on granite, or trap beds, with nothing 

 but drift intervening. If the soil lie thus above gran- 

 ite, we call that a granite region, as New Hampshire ; 

 if it lie above the primary, stratified rocks, we call it 



