THE ORIGIN OF MOUNTAINS. 
59 
finally to A3 £3. Of course if the cooling of the 
surface and of the deeper portion were the same, 
then the strata between A and B would themselves 
contract, and might consequently still form a regular 
curve between A3 and B3. As a matter of fact, 
however, the strata at the surface of our globe have 
long since approached a constant temperature. Under 
these circumstances there would be no contraction of 
the strata between A and B corresponding to that 
in the interior, and consequently they could not lie 
flat between A3 and B3, but must be thrown into 
folds, commencing along any line of least resistance. 
Sometimes, indeed, the strata are completely inverted, 
and in other cases they have been squeezed for 
miles out of their original position. “The great 
mountain ranges,” says Geikie, “may be looked upon 
as the crests of the great waves into which the crust 
of the Earth has been thrown.” Sir James Hall 
illustrated the origin of folds very simply (Fig. 3 ) by 
