CELA PT ERW20K/ 
ALTERATION AND. METAMORPHISM 
Rock-changes induced by Epigene Action. Deep-seated Alteration or 
Metamorphism. Degrees of Metamorphism. Thermal or Contact 
Metamorphism. Regional Metamorphism — Plutonic, Hydro- 
chemical, and Dynamo-metamorphism. 
Alteration by Epigene Action.—Very few rocks have not 
undergone some change since the time of their formation. 
At and for some distance down from the surface water passes 
more or less readily along the various planes of division by 
which all rocks are traversed—not only so, but it soaks into 
the rocks themselves, occupying their minutest pores and 
capillaries. In this way chemical changes of greater or less 
importance are effected, by which certain rocks tend to 
become disintegrated, while others, on the contrary, are more 
firmly consolidated. Crystalline igneous rocks, as a rule, are 
prone to decay—their felspathic and ferromagnesian con- 
stituents being readily broken up chemically, and some 
portion of their substance removed in solution. Many 
schistose rocks experience the same kind of change—a 
change which usually results in weakening a rock—its 
hardness and solidity becoming more or less impaired. 
Sedimentary rocks, on the other hand, being themselves the 
products of decay and disintegration, and consisting there- 
fore of more stable ingredients, are less liable to those 
chemical changes to which igneous and schistose rocks alike 
are subject. Instead of being weakened by the action of 
percolating water, they are often strengthened by the intro- 
duction into their pores and capillaries of various mineral 
substances which bind their ingredients more firmly together, 
To this general rule there are, as might have been expected, 
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