THE ORIGIN OF MOUNTAINS. 
79 
the stratification, which it may cross at any angle. 
Heim distinguishes three forms of cleavage. Firstly, 
that due to the formation of slickensides as just 
described (ank, p. 75). The second kind of cleav- 
age is due to the minute particles in the rock being 
flattened by, and arranged at right angles to, the 
pressure, as shown in Figs. 20 and 21.* 
The third is produced by all the laminae or 
elongated particles being arranged by the pressure 
in lines of least resistance, so that they are forced to 
lie parallel to one another. 
It is, however, by no means always easy, especi- 
ally in the crystalline rocks, to distinguish cleavage 
from stratification. The structure of the rock, which 
forms the base of the Windgalle, and which Heim 
regards as partly stratification, is considered by some 
geologists to be all cleavage. 
The fact that cleavage has been produced by 
pressure was first demonstrated by Sharpe, and after- 
wards with additional evidence by Sorby and Tyndall. 
In fact, under great pressure solid rock behaves very 
much like ice in a glacier. 
Cleavage and folding are both due to the same 
* Geikie, 7'ext-book of Geology. 
