GEOLOGY OF AGRICULTURE. 51 



76. Rocks of the tertiary class are characierized by 

 containing,- among other fossil remains, species of ani- 

 mals, which are identical with those now on the earth. 

 These overlay the secondary, and abound more than 

 either of the others in fossil remains. 



77. Over the tertiary rocks, and covering large por* 

 tions of the earth, is what geologists have called drift — 

 boulder rocks, rounded stones and pebbles, coarse 

 and fine gravel, sand and clay, forming, in many 

 cases, the soil which we now cultivate. This, all over 

 the northern half of the globe, seems to have been 

 transported, by some astonishing power, acting from 

 the north, and carried in a southern direction, from a 

 few rods to several hundred miles, from the rocks, in 

 which it had its origin. 



78. Since the drift period, various changes have 

 taken place, and are still going on, as the result of 

 causes now in operation, such as the running of streams, 

 the filling up of ponds, and others. Strata, formed by 

 these existing causes, are called alluvial, 



79. We have then, as t]|j3 most recently formed 

 strata, alluvial deposits, next drift, next tertiary rocks, 

 next secondary, and then primary, resting on the up- 

 per portion of the igneous rocks. 



80. Among the igneous, or un stratified rocks, are 

 granite, trap rocks, and the older and more recent 

 lavas. These appear to have been ejected in a state 

 of fusion by heat, at different epochs, from the bowels 



