60 ANNUAL REPORT 
ous or graphitic material. But even after decomposition the clays are 
subject to various changes, due to the chemical and physical conditions. 
to which they may be exposed and geological disturbances which may 
arise. ‘These changes are often very complicated, acting upon the clays: © 
simultaneously and reacting also upon each other. But in order that 
we may have a proper conception of the changes possible, we must con- 
sider each of the various influences and study the effect wrought by it 
upon the clays. It is evident, of course, that each of these actions has a 
different effect on different clays. : | 
The influence to which clays may be subjected ‘may be grouped. 
together as follows: 
1. Wearing away and redeposition. 
heat, 
2. Metamorphism due to < pressure, 
| steam and other gases. 
Bee) Cheniical action: 
4. Solution and crystallization. 
5. Infiltration. 
6. Leaching. 
7. Concretionary action. 
8. Glacial action. 
Wearing Away and Redeposition.—No material is able to with- 
stand the attacks of the great weathering agents, water and air, assisted 
by the physical factors, heat and cold, and durability is hence only a 
relative term of no significance geologically. As soon as a clay is de-' 
posited it is again subject to changes, more or less radical. It is worn 
away and transported by water, either to be redeposited as clay or to. 
become part of another rock. If the clay happens to be pure it is liable 
to be rendered impure by this process, if already impure it stands some 
chance of being purified by the removal of coarse constituents, sand, 
organic matter, concretions, etc. Such is the case with certain glacial 
clays where a heterogeneous jumble of materials is worn away and re- 
deposited free from all coarse mineral matter. This is the case in 
certain localities of northern Ohio, as at Milbury, where a glacial clay 
was by this: process redeposited as a fine grained material in a fresh 
water lake and was thus freed from the rock debris peculiar to bowlder 
clays. | 
Metamorphism.—In the mountain-forming periods of the earth’s 
crust, when the rock strata underwent crushing and crumbling, both 
heat and pressure were brought to bear upon the rocks, changing their 
structure and character. Similar metamorphosis is experienced by the 
strata when coming in contact with fused masses of lava, where heat 
brings about the alteration. In this manner shale is readily transformed 
