THE RELATIONS OF GEOLOGY TO AGRICULTURE. 547 



rocks does not differ much from that of the soils. Hence the conclusion 

 is that the latter are only the former comminuted, with the addition of 

 from three to ten per cent, of organic matter. 



Since the rocks differ considerably in composition, we should expect a 

 corresponding difference in the soils derived from them. And such is 

 the fact to a considerable extent, where the soil is simply the result of 

 the disintegration of the rock beneath it. It is enough so in many dis- 

 tricts to form characteristic soils. Thus, over quartz rocks and some 

 sandstones we find a very sandy and barren soil, though it is said that 

 in nearly all soils enough silicates of lime and magnesia are present to 

 answer the purposes of vegetation ; but the alkalies and phosphates may 

 be absent. When the rock is limestone, the soil is sometimes quite bar- 

 ren for the want of other ingredients, and also in consequence of the dif- 

 ficulty of decomposition. Clay, also, may form a soil too tenacious and 

 cold. The sandstones that contain marly beds, and some of the tertiary 

 rocks of analogous character, form excellent soils. So does clay slate, 

 and especially calciferous mica schist. The amount of potash and soda 

 in gneiss and granite often makes a rich soil from these rocks, and the 

 trap rocks form a fertile though scanty soil. 



But, in the third place, in most countries, aqueous and glacial agencies 

 have so mixed the soils together that their original peculiarities are lost, 

 and new and compound characters are given them. This is particularly 

 the case in northern countries, where the drift agency has swept over the 

 surface, and torn off and mixed together the disintegrated portions of the 

 several formations. Subsequently rains and streams have carried the 

 finer portions of the drift into the lowest places, and there formed allu- 

 vial meadows, and, although these are usually the best of soils, they are 

 often derived from many different rocks. The drift left upon the higher 

 grounds is generally quite barren, chiefly because of its coarseness. 



A fourth service which the geologist renders to agriculture is by the 

 discovery of fertilizers. Sometimes he can point out deposits of the 

 phosphates, either in a crystalline state, or as coprolites or guano. He 

 can also show what rocks contain carbonate of lime, or discover sulphate 

 of lime, or marl beds, or greensand, or decomposing fossil shells, or de- 

 posits of carbonaceous matter. He can also find what rocks contain 

 enough of potash or soda to be of service when pulverized. 



