326 H. F. REID ISOSTACY AND EARTH MOVEMENTS 



surface at a temperature not greatly above its melting point, could hardly 

 have suffered much cooling in the last few miles of its course, and there- 

 fore must have been in, at least, a very plastic condition when under con- 

 siderable pressure and at no extraordinary temperature. 



It does not seem legitimate to infer from Adams's experiments that 

 rock some miles below the surface can not yield to long continued and 

 not very great forces. 



Conclusion 



If we reject the principle of isostasy and its corollary, that mountains 

 are raised by the expansion of the underlying mass, we must believe that 

 they are raised by material being forced in under them from below; for 

 it has been shown that many ranges are not raised by simple compression. 

 This brings with it certain implications. It grants the possibility of sub- 

 terranean flow and weakens one of the objections to isostasy. As the 

 mountains of the present are in isostatic equilibrium with the rest of the 

 world, the region where they now are must, before the influx, have been 

 in defect by an amount at least equal to the mass of the mountains now 

 above their former level; and this defect must have existed in a region 

 where not only had thousands of feet of sediments been accumulated, but 

 into which still more matter had been forced by the earlier folding. No 

 such defect exists anywhere in the world at present. The plain of the 

 Ganges, which resembles closely the Appalachian trough shortly before 

 the folding, shows no defect of gravity that is not due to the local low 

 density of its deposits. Is it reasonable to believe that a region where 

 matter has been massed to such an extraordinary amount should be the 

 very region to be so extraordinarily in defect? And that, at the time of 

 elevation, exactly the right amount of matter to produce isostatic equilib- 

 rium should be forced into it? And, moreover, that there was a near-by 

 region with just the right excess of matter, which it gave up to the moun- 

 tain area by subterranean flow, remaining itself in equilibrium? 



If we refuse to apply the principle of isostasy to the past, we reject the 

 principle, so fruitful in the history of geology, that we must interpret the 

 past in the light of the present. This principle does not require us to 

 assume that conditions were always as they are now, but it does require 

 us to apply to the past the processes and principles now in force, and only 

 to modify them in the face of definite evidence. And what definite evi- 

 dence is there opposed to the general principle of isostasy ? I know of none. 



