GEOLOGY OF SOIL. 9 



ducts, subsequently distributed by water. The lime, 

 originally a part of igneous rocks — has been separa- 

 ted and combined with air, by animals or plants, by 

 a living process, called secretion. The modern pro- 

 duction of carbonate of lime, is still going on under 

 the forms of shells and corals. Though belonging 

 to neither division, the subject will be simplified by 

 referring limestone to the second class of rocks — but 

 it is truly a salt, and belongs to neither division and 

 it will be discussed hereafter. 



15. The chemical constitution of all rocks is 

 similar. If rocks are divided into two classes, the 

 first composed of the highly crystaline, usually cal- 

 led primary, such as granite, gneiss, mica slate, por- 

 phyry. ; and the second class, composed of rocks, 

 usually called trappean, as basalt, greenstone, trap, 

 then the great difference in their chemical constitu- 

 tion is this : 



The first class, or granitic rocks, contain about 20 

 per cent, more of silex, and from 3 to 7 per cent, 

 less of lime and magnesia and iron, than the second 

 or trappean class. 



16. If the language of geology is borrowed, and rocks 

 which present the appearance of layers, or a " strat- 

 ified structure," are divided into two classes, fossilifer- 

 ous and non-fossiliferous, or those which do, or do 



