352 ELEMENTARY GEOLOGY 
of mountains are melted by the folding of the strata, 
this folding being powerful enough to produce suffi- 
cient heat for actual liquefaction. Then this molten 
lava is squeezed out through fissures and forced up 
to the surface by steam. Again, it has been thought 
that water has passed down to regions of melted 
rock, and by its expansion, has furnished sufficient 
energy for the eruption. Another theory suggests 
that chemical changes in the rock have caused melting 
and eruption. 
Hence we see that there are many questions con- 
cerning the origin of volcanoes, which at present can- 
not be answered, for the problem deals with conditions 
so far from thé surface that their cause and action are 
but matters of speculation. Still, steam produces the 
eruption, and mountains open fissures through which 
the lava can rise. What produces the lava, steam, and 
the mountains, are the unanswerable questions. If we 
suppose that the great heat is originated by the moun-. 
tains, we appeal to a cause whose nature is not under- 
stood. So since the answer is not at hand, it is neces- 
sary to leave the question indefinite. 
