378 ELEMENTARY GEOLOGY 
Kinds of Metamorphism. — While pressure, heat, and water are 
commonly combined to cause metamorphism, there are cases in 
which one of these acts alone, or where the importance of one 
greatly overbalances that of the others. Where the changes 
have resulted mainly from pressure, the term employed is dy- 
namometamorphism; where water has been the most important 
factor, hydrometamorphism; and where heat has predominated, 
thermometamorphism. 
In the vicinity of igneous masses of intruded rock, the changes 
in the neighboring strata have been most marked close by the 
intrusion, or exactly at the contact. This alteration is recognized 
as contact metamorphism. But in great and extensive mountain 
folding, the alteration has been much more widespread, producing 
regional or general metamorphism. During this widespread change 
over great areas, as for instance over nearly the entire state of 
Vermont, the western part of Massachusetts and Connecticut, and 
the eastern border of New York, a condition of mountain folding 
of an extensive sort has changed the strata so decidedly that 
their original condition is often quite impossible of determina- 
tion. Here the profound, and at present even mysterious forces 
of nature, have conspired to produce changes, whose character we 
are but just beginning to understand. 
OreE DEPOSITS 
The Original Source of Ores. — Determinations of the 
specific gravity of the earth, show that the average 
weight is greater than that of the rocks at the surface. 
This leads to a common belief that the interior is either 
metallic, or else contains a greater percentage of heavy 
metals than of the lighter elements. This is also indi- 
cated by the fact that many igneous rocks are bringing 
