96 CLASSIFICATION OF ROCKS. 



sediments are disposed to arrange themselves in layers 01 Btrata. The materials in this 



lie in parallel beds, varying greatlj in thickness; all, however, separable from each 



other through the planes of deposition, each of which ma] be distinguished bj 1 1 m •-< upon 



the faces of a ledge, bj some diversity in the materials, or difference in the colors of two 



.111 beds. Other lines, however, appear both upon the ends or surfaces of beds, 



which arc not indicative of bedding planes. Thus, when we find regular forms as rhom- 



u marked upon rocks, they arc aol to be taken at all as the result of deposition. No 



difference of materials or difference of color can be discerned along these lines. Such 



regular tonus are therefore the effects of crystallization. In some masses, however, both 



kinds of planes maj be found. If the beds are horizontal, the upper and lower planes 



are those of deposition ; but they may lie in any other direction, as the vertical, or oblique 

 in carious degrees. The other lines course along upon the planes of deposition, and 

 I' duce rhomboids or other mathematical forms. In other caves, again, all the planes 

 ai ■ the effects of crystallization. Those which appear in granite, in trap, serpentine and 

 I oar) limestone, are never planes of deposition. The forms which these rocks give us 

 are more obtuse than those in dates ami shales; they an- frequently nearly square blocks. 

 All these plain's Berve an important purpose ; and though thej are really produced by the 

 operation of a constant law in the inorganic world, yet they hear the impress of design: 

 they facilitate the dissolution of the mass, ami by that means assist m preserving a due 

 balance in matters above and below watei ; thej are highly important as a means of sepa- 

 rating and raising the layers from then beds, and thus aid in quarrying. Without them, 

 it would be impossible to raise stones for flagging, and tor a variety of other useful purposes. 



The t'u st great division of rocks, then, is into Primary and Sedimentary. The former 



divided into two kinds: those which are massive, or destitute of planes analogous to 



plams of di . as granite; and those winch are stratified, as gneiss, mica slate, etc. 



It is proper, however, to observe in this place, that all rocks divide by different kinds of 



platen. Thus,- w Inch are not the planes of deposition, are termed joints; and hence a 

 rock to be jointed, w ben planes exist in a direction different from that of the planes 



of deposition. 



Sedimentary rocks are subdivided into several systems. By the term system, is meant 

 a series of rocks formed and deposited in the course of a single period or era, during which 

 nearly the same orders of organic beings existed ; each system being marked, both at its 



conniiL r in and going out, b_\ some great change in the condit tf things. The outgoing 



and the incoming of a system is indicated by changes in the sediments, in then position, 

 ami in the character of the organic beings of the time and place. It will be conceived, 

 then, that the lines of demarkation between systems an- the most important of all. The 

 most instructive study is that of the diversitj of these Bystems; as from it we learn the 

 historj of tho earth, its revolutions and changes. We are not, however, to receive all the 

 doctrines which are advanced in relation to changes and revolutions as fully proved. At 

 the time when organic beings first existed, certain essentials in organization were necessary. 



