18 . GEOLOGY OF SOIL. 



other rocks have been made, called secondary. The ruins 

 of the primary rocks have been transported by water, and 

 then gradually deposited layer upon layer. Under immense 

 pressure, these layers of mud, sand, fine gravel, rolled 

 stones, &c., have been hardened into solid rock, and have 

 formed sandstones, slates, or even rocks presenting the 

 crystalline structure or texture of granite, by the action of 

 heat, which the facts of modern geology teach exists in the 

 interior of our globe. 



6. This internal heat is supposed to be the cause of volca- 

 noes, and the primary rocks to have been the ejections under 

 circumstances unknown, of the melted mass of the globe ; 

 ejections similar in kind to those of modern lava, but greater 

 in degree. 



7. Intermediate between modern lava and primitive rocks, 

 and actually passing into either, is a large class of ancient 

 volcanic rocks, called trappean ; such are basalt, trap, green- 

 stone and highly crystalline porphyry. 



8. However named and classed are the rocks of the 

 earth's surface, they have had one common origin, the molten 

 matter of the globe. Hence, having a common origin, their 

 ultimate chemical constituents are similar. If granitic rocks 

 have a certain chemical constitution, then sandstone, slate, 

 &c., having been formed from worn-out and worn-down 

 granitic rocks, have a constitution chemically like them. 



9. To the agriculturist, the terms primary and secondary 

 are unnecessary. Equally so are all distinctions of soil 

 based on these terms. 



10. Soil is the loose material covering rocks, and it is 

 supposed to have been formed from their decay. Both are 

 to be classed by their origin. The origin of rocks refers not 

 only to the mode of their first formation, but to their subse- 

 quent arrangement. The origin of all rocks, geology 



