20 GEOLOGY OF SOIL. 



modern production 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 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 

 those usually called primary, such as granite, gneiss, mica- 

 slate, porphyry ; and the second class, composed of rocks 

 usually called trappean, as basalt, green-stone, trap, then the 

 great difference in their chemical constitution is this : 



The first or granitic class, contains 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 " stratified 

 structure," are divided into two classes, fossiliferous and non- 

 fossiliferous, or those which do, and those which do not 

 contain remains of animals or plants, it will be found, that 

 the fossiliferous are neither granitic nor trappean, yet are 

 they to be classed with the last, agreeing with these, in con- 

 taining less silica, and more lime, magnesia, and alumina. 



17. The stratified, non-fossiliferous rocks agree in chemical 

 composition with the granitic, and the fossiliferous with the 

 trappean and volcanic. 



18. The trappean and fossiliferous contain the most lime 

 and magnesia ; the granitic and non-fossiliferous, the most 

 silex. The great difference in chemical composition between 

 the two classes, is produced by lime and magnesia, two sub- 

 stances which, more than all others, have been thought to 

 influence the character of soil. 



19. The amount of this difference is about from 3 to 7 

 per cent. ; yet notwithstanding this, the general chemical 



