CHAPTER XX 
METAMORPHISM AND ORE DEPOSITS 
METAMORPHISM 
Nature of Metamorphism. — Hardly are rocks depos- 
ited in beds, or otherwise accumulated, before they 
commence to undergo change. Sometimes it is con- 
solidation by the deposit of cement, again it is a differ- 
ence wrought in the minerals. At times the alteration 
causes decay, which weakens the rocks, while again 
the changes make them stronger. So rocks are con- 
stantly and steadily varying; but the term metamor- 
phism refers to a particular class of changes by which 
the minerals are altered, and one rock gradually evolved 
into another, which differs more or less completely from 
the original. 
Among the strata this process of metamorphism may 
be seen exhibited in various stages. In reality, nearly 
all rocks show some beginnings of metamorphism, the 
result of the constant work of the ever-busy agents. 
Let us look at some of the rocks which show decided 
changes of a metamorphic nature. Suppose it to 
be a limestone. This is naturally made of amor- 
366 
Oe to ‘. 
