100 ELEMENTARY GEOLOGY 
composed of many crystals of calcite ; or a clay stratum 
may be metamorphosed to a slate (Fig. 44), in which 
the dense rock becomes harder, although at the same 
time an ability to split easily in one direction is intro- 
duced. This is called the slaty cleavage (Fig. 44). 
The condition of the cleavage in slate, illustrates 
Fic. 44. 
A piece of slate split along plane of slaty cleavage. The banding shows strati- 
fication planes nearly at right angles to the cleavage. 
one of the features of metamorphism. It splits readily 
because of many plates of a micaceous mineral, devel- 
oped in the rock as a result of the heat or other agent 
of alteration. In this change, one of the conditions 
is great pressure; and, as the minerals develop, they 
grow in a plane at right angles to the direction of the 
