1882.] 53 [Davis. 



to be in rock-basins have been shown to be only in obstructed 

 valleys, and others actually in rock-basins may have been formed 

 by dislocation or warping of the valley bottom. The amount of 

 erosion required to produce the larger lakes is greater than the 

 glaciers had time to effect, and it is very doubtful whether erosion 

 is as active at the bottom of a basin as on more prominent points 

 (A. 8). The erosion required to produce a lake where none 

 existed before is not measured by the depth of the lake alone, 

 but by the excess of the erosion there over that on the neighbor- 

 ing surface ; it is often impossible to account for this excess 

 for it is not always referable to the greater thickness of the ice 

 or softness of the rock. Many lakes claimed to result from 

 glacial action are near or within the marginal area of faint ero- 

 sion. Knobs of rock remain in the lakes and in the valleys above 

 them; it is impossible to explain their preservation if the basins 

 about them have been cut out by a broad eroding engine like a 

 glacier. 



The possibility of lake formation by warping or dislocation is advocated 

 by Riitimeyer (Ueber Thai- und Seebildung, Basel, 1869, 72), Heim (Mech- 

 anismus der Gebirgsbildung, 1878, I, 316), Lyell (Antiquity of Man, 1873, 

 358), Bonney (Lakes of the North-eastern Alps, etc. Geol. Soc. Journ., 

 xxix, 1873, 382-395), Hector (Geol. Mag., n, 1865, 377-378), Duke of 

 Argyll (Address, Geol. Soc. Journ., xxix, 1873, p. lxx; also xxiv, 1868, 

 255-273), Kjerulf (Geol. Norwegen, Bonn, 1880, 330), Whitney (Climatic 

 Changes of Later Geological Times, 1880, 16), and others. The common 

 occurrence of lakes of the barrier type has been emphasized in my previous 

 paper. When the examples that maybe thus fairly explained are deducted 

 from the list of the larger lakes, comparatively few remain, and the ar- 

 gument from necessity is greatly weakened. 



O. Heer contends that the valleys and lakes of Northern Switzerland had 

 much of their present form before the glacial period ; they are cut in up- 

 turned Miocene rocks, and the old gravels, that were washed from the ice 

 as it advanced, now unconformably overlie the eroded Miocene strata, fol- 

 lowing even insignificant irregularities of their surface. This could not 

 be the case if the ice had cut the valleys. (Die Urwelt der Schweiz, 1865, 

 516, 532, 581.) The valley cutting would then be of Pliocene date, and 

 this is confirmed by finding no Pliocene formations in Northern Switzer- 

 land. 



No sufficient reason has been given to show why the glaciers 

 on the Italian slope of the Alps should be suddenly endowed 

 near their ends with erosive power sufficient to cut out lakes one 



