GLACIAL ROCK SLIDING 487 
dips north. The only explanation that now offers is that at the time the rocks 
were subjected to the force which produced the undulations the upper part 
slipped over the lower and at this point partook of the elevation south, while 
the lower was affected only by the uplifting to the north. 
It is not strange that Professor Hall should have failed to divine 
the cause of the stratigraphical disturbance since Pleistocene geology 
was then an undeciphered branch of the science. It is somewhat 
remarkable, however, that modern geologists have not commented 
upon this interesting phenomenon of the glacial invasion—interesting 
because it illustrates the great pressure sometimes exerted, and because 
it has a distinct bearing on that much-controverted theory of ice- 
erosion. 
