OF SOUTH CAROLINA. iG 
splits are due to slaty cleavage, and have no connection with those partings called planes of strati- 
fication, but, on the contrary, often make a considerable angle with them. 
; 
It is not always easy to distinguish these structures from each other, and yet it is quite important 
to be able todo so. Rocks having jointed structure have no tendency to split in lines parallel with 
the joints, and in this manner jointed structure may be distinguished from cleavage. 
In hand specimens it is difficult, if not impossible, to distinguish cleavage planes from those of 
stratification ; but when the rock is examined in place, there will generally be found a layer or 
bed presenting a different mineral composition or color, which will be seen to cross the cleavage 
planes, thus indicating the plane upon which it was deposited, or the true plane of stratification. 
The cause of cleavage is obscure, but it is supposed to result from the tendency of the simple 
component substances of the rock to arrange themselves in crystaline forms, at a time when the semi- 
fluid state of the mass permitted a certain degree of motion among its particles. Re-arrangement 
of the molecules of matter seems, however, to take place, even when the mass is in no degree fluid. 
The crystaline structure which metals assume under particular cireumstances, is illustrated in the 
axles of rail-road cars long in use; and I have found fossil shells on the coast of Horry District, 
where the passage of the ordinary structure of the shell into cale-spar may be traced. The sketch, 
Fig. 1, will exhibit these structures better than many words. ; 
Fig. 1. 
A mal on Gs 
ae id. nA 
The dark lines represent the planes of stratification, the lines crossing these, joints, and the short, 
vertical lines exhibit cleavage planes. 
Nal 
Primary StTatirtep Rocks 
The lowest of the stratified rocks—those which contain no organic remains—are included under 
this name. They are gneiss, hornblende-slate, mica-slate, talcose-slate, and clay-slate. 
In mineral composition these rocks are very dissimilar, but they agree in some very important 
points. ‘They comprehend vast beds or accumulations of sedimentary matter—such as might be 
derived from the preceding rocks by abrasion and other causes, and differ from recent sedimentary 
rocks, in their crystaline structure, in containing no distinct fragments of other rocks, and in the 
entire absence of any remains of organic bodies. 
Their crystaline structure is supposed to result from a degree of subterranean heat, which, 
although intense, did not destroy the lines of stratification or bedding, which they received at the 
time of their deposition. Such a degree of heat can readily be supposed to result from contact with 
the underlying, intensely heated granite. The alterations, both in texture and appearance, produced 
by this cause, have suggested, for these, the name of metamorphic rocks. 
