Davis.] 50 [May 3, 



essential change above and below the limit of glaciation." (Mech. der 

 Gebirgsbildung, I, 252.) 



J. D. Whitney says, " no change of form can be observed at the former 

 line of ice. Aside from the morainic accumulations, there is nothing to 

 prove the former existence of the glacier except the smoothed, polished or 

 rounded surfaces of the rocks, which have no more to do with the general 

 outline of the cross section of the valley than the marks of the cabinet- 

 maker's sand-paper have to do with the shape and size of the article of 

 furniture whose surface he has gone over with that material." (Climatic 

 Changes of later Geological Times, 1880, 9.) 



The occurrence of rocky knobs in valleys as weighing against their gla- 

 cial origin, will be mentioned under C 5. 



C. 5. Lakes. The glacial origin of lakes has already been dis- 

 cussed in the paper to which this is supplementary, and here a 

 brief summary of the question will be sufficient. 1 



Ramsay first gave prominence to this theory in 1862. I believe that 

 if the limit of erosion given in his earlier paper three years before had been 

 retained, and no effort had been made to explain lakes larger than the 

 llyns of Wales and tarns of Scotland, his theory would have met with uni- 

 versal acceptance ; but while its extension to include the larger lakes of 

 Switzerland, and even the Great Lakes of this country, met a warm wel- 

 come from many, it encountered also the opposition I have here quoted. 

 It should be noted that Ramsay excluded valley-making from his theory, 

 and was careful to state that many lakes might be and had been formed in 

 other ways, but he referred the majority of lakes in the northern hemis- 

 phere to the excavating power cf ice. His argument was essentially that 

 of necessity (see D.) ; and he considered the erosive power of the old glaciers 

 to be near their ends or where they were thickest. (On the Glacial Origin 

 of Certain Lakes in Switzerland, etc., Geol. Soc. Journ., xviii, 1862, 185- 

 204.) 



J. Geikie states that glacial erosion was greatest where the ice was thick- 

 est, as is shown by the increased depth of the Scotch lakes near their heads ; 

 where the slope of the glacier's bed decreased, as is shown by the occurrence 

 of lakes along mountain borders ; and where the ice-stream was turned 

 aside, entirely or only in its lower layers, as is shown by his "deflection 

 basins." (Great Ice Age, 1877, 278-289.) 



Sir W. E. Logan describes the innumerable lakes of the Laurentian High- 

 lands as generally following the softer strata (often crystalline limestone), 

 and " it appears probable that one of the main erosive forces has been gla- 

 cial action." The Great Lake basins " are depressions not of geological 



1 See the Classification of Lake Basins. These Proceedings, xxi, 1882, 336-344. 

 In order that the two papers may be to a certain extent independent, some state- 

 ments are given in both. 



