ISOSTASY AS A RESULT OF EARTH SHRINKAGE 209 



only vertical forces. Hayford's idea that undertow produces 

 folding^ meets with the same objection, that horizontal surface 

 forces are necessary to produce the shortening which must accom- 

 pany parallel folding. If it is these vertical forces that produce 

 folding, why is it the peneplain uphfts, which are perhaps the most 

 typical cases of vertical uplift, are generally not accompanied by 

 pronounced folding ? 



Since there is good geological evidence for shortening of the 

 earth's crust and good geodetic evidence for isostasy, it is fitting 

 that we should examine the possibility that there is no antipathy 

 between the two theories. It has been said that diastrophism due 

 entirely to isostatic forces would soon run down, but if we can 

 consider that earth shrinkage is a cause of perpetuation of the 

 inequahties of the earth's surface, the difiiculty is removed. It 

 remains then to explain how the wrinkling of the crust and the 

 various phenomena which geological history tells us have accom- 

 panied the changes of the earth's surface could have been produced 

 without the loss of that isostatic adjustment which geodesy leads 

 us to beheve has been a basic condition. The following observations 

 appear to be contradictory to the theory of isostasy. 



First, the squeezing in of material from the sides in the change 

 from geosynclines to mountain ranges makes it seem likely that the 

 mountain ranges would be a greater burden on the crust than the 

 geosynclines. 



Second, the elevation of plateaus from low lying plains appears 

 to suggest that during the process there were important changes in 

 the weighting of the crust with reference to the level of isostatic 

 adjustment. 



Thirdly, the development of peneplains and their subsequent 

 upHft gives the impression that the crust is strong enough to resist 

 for long periods without yielding to , the forces of isostatic equi- 

 librium. 



If diastrophism has been periodic, this also might indicate that 

 the crust was strong enough to withstand great burdens before 

 yielding. 



'J. F. Hayford, Science, New Series, Vol. XIII (1911), pp. 199-208. 



