588 
ELEMENTS OF GEOLOGY. 
CHAPTER XXXIV. 
METAMORPHic ROCKS— Continued. 
Definition of slaty Cleavage and Joints.—Supposed Causes of these Struc¬ 
tures.—Crystalline Theory of Cleavage.—Mechanical Theory of Cleavage. 
—Condensation and Elongation of slate Rocks by lateral Pressure.—Lam¬ 
ination of some volcanic Rocks due to Motion.—Whether the Foliation 
of the crystalline Schists be usually parallel with the original Planes of 
Stratification.—Examples in Norway and Scotland.—Causes of Irregular¬ 
ity in the Planes of Foliation. 
We have already seen that chemical forces of great inten¬ 
sity have frequently acted upon sedimentary and fossilifer- 
ous strata long subsequently to their consolidation, and we 
may next inquire whether the component minerals of the al¬ 
tered rocks usually arrange themselves in planes parallel to 
the original planes of stratification, or whether, after crystal¬ 
lization, they more commonly take up a different position. 
In order to estimate fairly the merits of this question, we 
must first define what is meant by the terms cleavage and 
foliation. There are four distinct forms of structui'e exhibit¬ 
ed in rocks, namely, stratification, joints, slaty cleavage, and 
foliation; and all these must have different names, even 
though there be cases where it is impossible, after carefully 
studying the appearances, to decide upon the class to which 
they belong. 
Slaty Cleavage. —Professor Sedgwick, whose essay “ On the 
Structure of large Mineral Masses ” first cleared the way to¬ 
wards a better understanding of this difficult subject, ob¬ 
serves, that joints are distinguishable from lines of slaty 
cleavage in this, that the rock intervening between two 
joints has no tendency to cleave in a direction parallel to 
the planes of the joints, whereas a rock is capable of indefi¬ 
nite subdivision in the direction of its slaty cleavage. In 
cases where the strata are curved, the planes of cleavage are 
still perfectly parallel. This has been observed in the slate 
rocks of part of Wales (see Fig. 624), which consists of a 
hard greenish slate. The true-bedding is there indicated 
by a number of parallel stripes, some of a lighter and some 
of a darker color than the general mass. Such stripes are 
found to be parallel to the true planes of stratification, 
wherever these are manifested by ripple-mark or by beds 
