268 A. Vaughan — Corrugation of the Earth's Surface. 



material will be, so to speak, squeezed out, and this will cause a 

 real transfer from under the sinking area to beneath the surrounding 

 regions. We may reasonably expect great compression, contortion, 

 and plication to accompany such a movement. Such transference 

 must cause real elevation of the surrounding regions in addition 

 to the relative elevation due to depression of the sinking districts. 



That this theory would be adequate to account for the present 

 height of mountains seems very probable from the following 

 considerations. The simple fact that a large area is depressed, and 

 therefore the sea-level brought nearer the centre of the earth, 

 implies the raising of the undepressed parts above it. Due to this 

 cause, we obtain the same maximum height as was found possible 

 under the theory referred to above. 



In the second place, actual transfer of material from under the 

 sinking area must add still more to the elevation caused by relative 

 displacement. 



Lastly, we have the important fact that the rate of contraction 

 beneath such areas is far in excess of the average under neighbouring 

 areas, and that this rate is maintained by the continual contraction 

 of fresh portions of the heated interior, brought into contact with 

 the cold outer layers by the squeezing out of intermediate material. 



The material thus pressed out from under the contracting shell-caps 

 and forced beneath the surrounding regions, will exert an additional 

 strain upon the overlying crust, which, as explained above, has 

 already been stretched by the contraction itself. In this process it 

 is quite conceivable that great metamorphism will result from the 

 movement under enormous pressure. But, so long as this pressure 

 is unrelieved, no igneous rocks could possibly be generated ; for, 

 though no doubt the compression will raise the rocks upon which 

 it acts to a temperature far above their melting point, any increase 

 of volume which melting would imply will be strenuously resisted. 



Two extreme cases must now be discussed : — 



Eirst : suppose the change from elevated to depressed areas to be 

 spread over a large space and to be uniformly slow. In this case, 

 the strain upon the overlying rocks will be spread over a very large 

 area, and there will be no lines of special weakness. Here, then, 

 we should not expect the crust to split right through at any point, 

 but rather that it would suffer an uniform extension by the extension 

 and development of local separation spaces, and in this way very 

 gradually relieve the state of tension. This gradual relief from 

 pressure might be sufficient to slowly melt the underlying materials, 

 which, when first brought into the position contemplated, must 

 have been under so great a pressure as to be raised above their 

 melting point. 



The slowness of the action would ensure the production of a holo- 

 crystalline igneous rock. 



Secondly : suppose the change from elevated to depressed areas 

 to be very rapid. Here we have special lines of weakness afforded 

 by sudden bends, and the probability seems to be that at such points 

 the crust would be unable to withstand the total strain and would 

 split right through. 



