GEOLOGY — WILLIS. 203 



have been led to conclusions which, though they command the confi- 

 dence of the best observers, yet challenge the principles of mechanics. 

 The problems have been studied qualitatively, but not quantitatively, 

 and it would not be surprising if the conclusions were found falla- 

 cious when tested with reference to the capacity of thin sheets of 

 rock to maintain their form and transmit thrust over great distances. 

 If we may draw an inference by extending to the Alps the general 

 conclusions based upon reconnaissance of other ranges of a similar 

 type in their immediate vicinity, I would suggest that, like the Apen- 

 nines, the Balkans, and the Karpathians, the Alps suffered erosion 

 and pronounced warping during the Pliocene, and that the solution 

 to some of the difficulties will be found in a proper recognition of 

 the sequence of erosion and warping. 



