I A G R I C IM. T U R A L G E O I. O G Y . 5 



. )lanes are the effects of crystalization. Those which appear in 

 granite, in trap, serpentine and primary limestone, are never planes 

 )f deposition. The forms which these rocks give us are more ob- 

 usethan those in slates and shales; they are frequently nearly 

 quare blocks. All these planes serve an important purpose ; and 

 hough they are really produced by the operation of a constant 

 aw in the inorganic world, yet they bear the impress of design : 



: bey facilitate the dissolution of the mass, and by that means as- 



■ 1st in preserving a due balance in matters above and below water ; 

 ^ |iey are highly important as a means of separating and raisino^ 

 ' |ie layers from their beds, and thus aid in quarrying. Withou^t 

 ■- iem, it would be impossible to raise stones for flagging, and for a 



■ jiriety of other useful purposes. 



i JThe first great division of rocks, then, is into Primary and 

 -- \edimentary. The former are divided into two kinds : those 

 jhich are inassive, or destitute of planes analogous to planes of 

 - ^position, as granite; and those which are stralified, as o-neiss 

 ft ica slate, etc. » j 



It is proper, however, to observe in this place, that all rocks 



'^•ide by different kinds of planes. Those which are not the 



] incs of deposition, are termed joints ; and hence a rock is said 



: l,be jointed, when planes exist in a direction different from that 



(!• the planes of deposition. 



Sedimentary rocks are subdivided into several systems. By the 

 -t,m system, is meant a series of rocks formed and deposited in 

 ;.tp course of a single period or era, during which nearly the same 

 ,: (^ers of organic beings existed ; each system being marked, both 

 aits coming m and going out, by some great change in the con- 

 dion of things. The outgoing and the incoming of a system is 

 iiticated by changes in the sediments, in their position, and in 

 iM character of the organic beings of the time and place. It will 

 bie-onceived, then, that the lines of demarcation between systems 

 a^ the most important of all. The most instructive study is that 

 Oi^he diversity of these systems ; as from it we learn the history 

 oithe earth, its revolutions and changes. We are not, however 

 toreccive all the doctrines which are. advanced in relation to 

 cringes and revolutions as fully proved. At the time when or- 

 g£nc beings first existed, certain essentials in organization were 

 nt^ssary. A physical system was then established, and to this 



