36 CI-ASSIFICATION OF ROCKS. 



sediments are disposed to arrange themselves in layers or strata. The materials in this 

 case lie in parallel beds, varying greally in tliickness ; all, however, separalde from each 

 other through the planes of deposition, each of which may he distinguished by lines upon 

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

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

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

 boids marked upon rocks, tliey are not 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 

 reo-ular forms are therefore the effects of crystallization. In some masses, however, both 

 kinds of planes may be found. If tlie beds are horizontal, tlie upper and lower planes 

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

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

 produce rhomboids or other mathematical forms. In other cases, again, all the planes 

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

 primary limestone, are never planes of deposition. The forms which these rocks give us 

 are more obluse than those in slates and shales; they are frequently nearly scpiare blocks. 

 All these planes serve an important purpose ; and though they are really produced by the 

 operation of a constant law in the inorganic world, yet they l)ear the impress of design: 

 (hey facilitate the dissolution of the mass, and by that means assist in preserving a due 

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

 rating and raising the layers from their beds, and tlius aid in quarrying. Without them, 

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



The first great division of rocks, then, is into Primary and Sedimentary. The former 

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

 planes of deposition, as granite; and those which are stratified., as gneiss, mica slate, etc. 



It is proper, however, to oi)serve in this place, tliat all rocks divide by different kinds of 

 j)lanes. Tiiose whicli are not the planes of deposition, are termed joints ; and hence a 

 rock is said to be jointed, when planes exist in a direction different from tliat of (he planes 

 of deposition. 



Sedimentary rocks arc 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 

 coming in and going out, by some great change in the condition of things. The outgoing 

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

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

 tlien, that the lines of dernarkation lietween systems are the most important of all. The 

 most instructive study is that of the diversity of these systems ; as from it we learn the 

 history of the 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. 



