50 GEOLOGY OF AGRICULTURE. 



CLASSIFICATION OF ROCKS. 



73. As we come above tlie igneous, or unstratified, 

 into the stratified rocks, we find them of many va- 

 rieties, all of which have been arranged into three 

 principal classes — primary^ secondary^ and tertiary, 



74. The primary rocks either lie nearly horizontally 

 upon the igneous, or lean with a gentle slope against 

 them. In cases of the latter kind, it is believed, that 

 the igneous rocks have been forced upwards by inter- 

 nal convulsions of the earth, and have raised the pri- 

 mary rocks along with them, inasmuch as all stratified 

 rocks, having been deposited by the agency of water, 

 must originally have been nearly horizontal. These 

 primary rocks are generally hard. They have been 

 subjected to immense pressure. Many of them bear 

 marks of having been intensely heated since their de- 

 position. Some of them are highly crystalline. They 

 are nearly destitute of fossil remains, and the few they 

 contain are entirely unlike any plants or animals now 

 on the globe — an additional proof that, though not as 

 old as the igneous rocks, on which they lie, they are 

 older than other rocks which lie above them, and 

 which contain fossil remains more like existing species. 



75. Rocks of the secondary class overlay those of 

 the primary ; they contain more fossil remains ; and 

 the fossil remains found in them, though unlike exist- 

 ing species, bear a nearer resemblance to them than 

 those in the primary rocks. These facts show them 

 to be of later origin than the primary. 



