322 ELEMENTARY GEOLOGY 
perhaps because of the fissures that reach deep into the 
earth, volcanoes are common among mountains, espe- 
cially among those that are even now growing (p. 329). 
Because of the fissures which furnish the place for de- 
posit, and because of the presence of heat and heated 
water, caused by volcanic action, and possibly by the 
slippmg of rocks over one another, mineral veins 
(Chapter XX.) abound in mountainous regions. 
The movement of the strata sometimes causes a 
crushing; and the presence of great pressure and 
heat, together with the action of heated waters, often 
cause rocks to be changed or metamorphosed during 
mountain development (p. 366). Hence metamorphism 
is a common accompaniment of mountain growth; and 
in the central parts of these folds, as well as deep down 
in the roots of mountains, which later are sometimes 
exposed by denudation, it is common to find the strata 
so altered and changed that their original condition is 
almost or even completely masked. 
Cause of Mountain Growth. — Phenomena to be Ka- 
plained. Although this subject has long been under 
consideration, geologists are far from being agreed 
upon the cause of mountains. It is quite generally 
believed that their growth has to do with the heated 
condition of the interior of the earth; but just in 
what way this causes mountain uplift, is still a sub- 
ject for hypothesis. It is a question of great com- 
