VOLCANOES 351 
The Cause of Volcanoes. — The immediate cause of 
voleanie eruption is undoubtedly the explosive action 
of steam, so that the volcanic outbreak is like a great 
boiler explosion; but the question of the origin of the 
heat is less easily answered. As in mountains, there 
can be little doubt that the heat is more or less directly 
associated with the high temperature of the earth’s 
Fic. 215. 
Mato Tepee, a voleanic neck rising 1100 feet above the river. 
interior. The theory that explains mountains will like- 
wise explain the original source of volcanic energy. 
Contraction of the earth, forming mountains, produces 
fissures in the rocks; and through these the lava rises, 
building volcanic cones. Perhaps the pressing action 
of folding strata causes the rise of liquid rock near 
enough to the surface to be expelled by steam. 
Some have suggested that the rocks in the roots 
