462 SLIGIDINES, STOW SING OYRINIGS, 
The amount of shortening of the area of a rock-mass neces- 
sary to produce the property of cleavage may not be great in a 
slate, as shown by the amount of distortion of the bedding. In 
proportion as the shortening becomes greater the rock is apt to 
change from a slate to a schist. If the process of shortening 
continues until a schist is produced, it is often difficult or impos- 
sible to estimate the amount of horizontal shortening of the 
strata, but it is certain from observation that it is usually con- 
siderable, perhaps to one-half or one-third of the original. 
This is shown by the plications of layers of a slightly different 
color which are cut by the schistosity. If plicated layers such 
as often occur in schists were straightened out, they wou!d 
require a distance across the schistosity two or three times as 
- great as at present. 
It is very rare indeed that any rock is so homogeneous that 
the unmodified law of normal flow perfectly applies as above 
given. Among the sediments argillaceous rocks most nearly 
approach homogeneity. The massive rocks, however, still more 
nearly approach homogeneity. But even these are not strictly 
homogeneous, being composed of mineral particles of different 
sizes and characters. However, these particles in many instances 
are uniformly flattened and readjusted, so that the mass in a 
large way almost perfectly obeys the law. A mashed massive 
rock having cleavage but not fissility is a solid, strong schist, 
and it is in this class of rocks that cleavage in a uniform direc- 
tion as the result of normal plastic flow is best exemplified. 
Rarely rocks show an almost equal capacity to part in any 
direction parallel to fibers. In these cases the microscope 
shows that the mineral particles are very long in the fibrous 
direction, and have about the same average magnitude in all 
directions at right angles to their greatest diameters, instead of 
having two definite directions of mean and minimum diameters 
at right angles to each other, as is ordinarily the case in slaty 
cleavage. As pointed out by Professor Hoskins, this structure is 
explained by the deforming or maximum pressure being equal or 
nearly equal throughout a circle of revolution in all directions 
