THE ROCKS THE ORIGIN OF SOILS. 





CHAPTER XIX. 



THE ROCKS. THEIR COMPOSITION AND INFLUENCE 

 UPON THE SOIL. 



Rocks are divided by geologists into two great classes ; 

 one termed primary ; igneous ; or unstratified ; such as gran- 

 ite; quartz, &c: the other, secondary; stratified; or sedi- 

 mentary; as sandstones slates &c.; by which is meant that 

 the latter has been formed from the debris of the former as 

 has been explained in the previous chapter. One other 

 class is termed, generally, the tertiary or third formation ; 

 and this consists, of the water worn pebbles; gravels ; marl 

 beds ; clays and sandstones which have been formed by the 

 later changes on the earth's surface and since animals of 

 the kinds which now exist appeared on the globe. For this 

 class of rocks are distinguished by the frequency of animal 

 remains in them, which are similar to or identical with 

 species which now exist. 



These three classes are divided into various sub-classes 

 called systems and these again into formations; each of 

 these having some common resemblance, which shows that 

 they were deposited under nearly the same general physi- 

 cal conditions of the earth's surface. Thus there is the car- 

 boniferous system, consisting of a series of limestones; sand- 

 stones ; iron stones; and beds of coal ; which contain animal 

 and vegetable remains of the same species, and are thus 

 shown to have been formed at one special era of the earth's 

 history. From the characteristics and formation and order of 

 deposition of these beds, the geologist or an attentive intelli- 

 gent student, can form as clear an idea of what occurred 

 during the age in which these plants grew and these ani- 

 mals lived, and these rocks were deposited and formed, as 

 if he had the open volume before him in which he might 

 , read the history. This is a study of the most intense inter- 



