30 
CLASSIFICATION OF ROCKS. 
sediments are disposed to arrange themselves in layers or strata. The materials in this 
case lie in parallel beds, varying greatly in thickness; all, however, separable from each 
other through the planes of deposition, each of which may be distinguished bylines 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. Thus, when we find regular forms as rhom¬ 
boids marked upon rocks, they 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 
regular forms are therefore the effects of crystallization. In some masses, however, both 
kinds of planes may 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 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 obtuse than those in slates and shales; they are frequently nearly square 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 bear the impress of design: 
they 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 thus 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 observe in this place, that all rocks divide by different kinds of 
planes. Those which 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 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 
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 the sediments, in their position, 
and in the character of the organic beings of the time and place. It will be conceived, 
then, that the lines of demarkation between 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. 
