Davis.] 22 [May 3, 



flowing along cracks made in the original ice-sheet by the uplift of the 

 Alps. 



Belt l compared lake-basins to pot-holes on a large scale ; Shaler 2 em- 

 phasized the action of subglacial water supplied by local excess of heat 

 conducted from the earth's interior, and by pressure-melting. 



A closer examination of the question leads us to divide the 

 evidence pro and contra, into the following classes. Evidence 

 derived from — 



A. The action of glaciers in general ; 



B. The amount and distribution of glacial drift ; 



C. A comparison of the topography of glaciated and nongla- 

 ciated regions ; 



D. A supposed necessity for glacial erosion; that is from 

 ignorance of any means except this action to produce certain 

 observed effects. 



A. 1. Observations beneath Glaciers. It was for a long time 

 unnoticed or denied that glaciers eroded the surface beneath 

 them ; but observers have since then made their way a little 

 distance under certain Swiss glaciers and have clearly shown that 

 the ice wears and scratches the rocks below by rubbing sand and 

 stones against them. 



They generally report also open spaces between ice and rock 

 except at certain projecting points of support, and infer from 

 this that the glacier wears most on the convex parts of its base ; 

 that it does not fit closely down on its bed, and therefore does 

 not materially deepen its valley ; this work being left to the sub- 

 glacial streams. 



Agassiz describes his difficulty in convincing his friend, Studer, of the 

 glacial origin of rock-striations until he showed them to him freshly made 

 under the ice. (Etudes sur les Glaciers, 189.) 



Desor, Soc. Helv. Actes, 1860, 133, and Gebirgsbau der Alpen, 1865, 116; 

 Niles, these Proceedings, xv, 1873, 378-381 ; Bonney, Geol. Mag., in, 1876, 

 198, describe and reason thus from unoccupied hollows beneath the ice. 



But direct observations of this kind cannot be carried far 

 against the erosive power of the old glaciers, as the conditions in 

 which one can now see the under surface of the ice must be very 

 different from those that obtained when its mass was much 



1 Geol. Soc. Journ., xx, 1864, 464. 



2 These Proceedings, x, 1866, 359 ; xvm, 1876, 126. 



