328 ELEMENTARY GEOLOGY 
The most serious objection urged against this theory is, that 
mere contraction is not sufficient to account for the known results ; 
but there is grave cause for doubting the validity of this objec- 
tion. The formation of mountains occurs in only a few com- 
paratively small parts of the whole earth. In these places is 
concentrated the uplifting energy of all the rest of the sphere. 
Even then the elevation is very slow; and until better proof is 
brought to the contrary, we may fairly assume, as basis of the 
hypothesis, that the effect of contraction is sufficient. 
Although it has seemed. best to present this theory here to the 
exclusion of others, it should be admitted that contraction may 
be aided by various causes, such as have sometimes been sug- 
gested. It is particularly possible that the loss of gas from the 
earth may assist in the volume of contraction, just as the skin of 
a drying apple wrinkles whenever the water vapor passes from 
the pulp. Uplft may be greatly aided by the expansion of 
heated water which enters into the deep, lower parts of the earth, 
where the temperature is high. Even taken alone, each of these 
may be a cause for mountains; but more probably, if they act at 
all, they merely aid contraction, which appears to be the really 
unportant, and perhaps in many cases, the sole cause for moun- 
tain growth. 
