THE NATURE AND INTERPRETATION OF GEODETIC EVIDENCE. 35 



by denudation, on lower lying tracts may lead to departures from 

 a condition of complete isostasy, but these will necessarily be in 

 the direction of an excess of compensation in the hills and a defect 

 in the plains ; it is not conceivable that any hypothesis belonging 

 to this class can account for the hills being under compensated 

 or in other words showing an excess of load. In this way, then, 

 we have a test which will distinguish between the two groups of 

 hypotheses; so long as the geodetic observations indicate that 

 the compensation of the hills is complete, or that the compensation 

 is in excess of the visible mass of the range, we are free to choose, 

 between the hypotheses, but if they indicate an unmistakeable 

 excess of mass in the hills, or a defect under the plains, after 

 allowance has been made for compensation, the whole of one group is 

 excluded. We are then restricted to the other, and can only choose 

 between those hypotheses which involve an addition to the mass 

 of the crust underlying the hills, whether this is brought about by 

 the compression of the crust or by an underground migration of 

 material from outside the limits of the range. 



[ 183 j 



