508 
Van Rise—Earth Movements. 
geneous rocks, the mashing process may have gone so far as to 
impose on them nearly the same mass effects as in homogeneous 
rocks, the numerous layers being pressed until they lap back 
upon themselves, like the plications of a closed fan. The hard¬ 
est and most brittle pebbles of quartz or of jasper may be flat¬ 
tened so that they are several times as long as broad, or may be 
pressed into thin paper-like leaflets, or even so far that their 
original outlines are altogether lost. During the flowage, as a 
result^of the flattening and rotation of the particles, and of the 
formation of new minerals in the rock and their rotation, the 
rock may gain a capacity to part in one direction more readily 
than in others. This property is called cleavage, and is best 
illustrated by slates, which with a wedge may be split into thin 
layers. The cleavage may or may not accord with the original 
structure. 
Rock Alterations .—But perhaps the most surprising of all the 
earth movements are the minute ones. By our modern method 
of microscopical study of rocks in thin section, we look through 
even the black rocks as though they were transparent, and we 
see that no rocks are stable. We see one mineral changing into 
another. We see minerals grow. We see one mineral replaced 
by another. In short, we see one kind of rock transformed into 
another kind. Minerals stable under one set of conditions may 
not be stable under another. No rock is so dense that water may 
not penetrate to its innermost part. Water, driven by gravity, is 
the great transporting and transforming agent of the earth, as 
well within the mass of the rock as upon the surface. 
Wherever as a result of the forces of deformation the rocks 
are fractured, creviced, or brecciated, there waters freely enter. 
These waters may take the materials of the rocks in solution, 
and thus are explained the caves of the earth. Within the 
cracks, crevices, and openings in the rocks through which the 
percolating waters pass, a portion of the material in the solu¬ 
tion may be deposited, and thus the rocks be healed of their 
wounds, becoming as strong as before broken. But usually differ¬ 
ences between the original and secondary materials enables one 
to trace the histories of the transformations. The water de¬ 
posits in rock crevices are of great interest to man, for from 
