1882.] 49 [Davis. 



C. 4. Slope of Valley Sides. By far the strongest argument 

 against the preceding suppositions is the well established fact that 

 there is no marked change of slope on the sides of the Swiss 

 valleys, in passing from the upper nonglaciated surface, down 

 to the glaciated rocks. This does not show that the glaciers 

 had no effect, but merely that their power of erosion was not, 

 during the time they were in action, materially different from 

 that of ordinary weathering. For in post-glacial times these val- 

 leys have not lost their glaciated surface ; we see them as they 

 were left by the melting ice. And if, during the glacial period, 

 the valleys under the ice had increased in depth faster than 

 before or after, there should be an increase in the angle of slope 

 on the sides at the limit of glacial action ; but there is no such 

 increase. 



F. J. Kaufman considers glacial erosion as effective as other destructive 

 forces, and denies the preservative power of ice claimed by Rutimeyer and 

 others (A 4), because the surfaces that were covered during the longer 

 period of glacial occupation are now no higher than their surroundings 

 (1. c. 453). 



We may carry this a little farther, and show from it that the 

 Swiss valleys have not greatly changed since preglacial times. 



The valleys have scarcely deepened at all since the disappear- 

 ance of the ice, although exposed to weathering of the kind 

 that, during the glacial |)eriod, kept pace with the ice erosion. 

 Now supposing post-glacial is equal to glacial time, and post- 

 glacial weathering is equal to the weathering during the ice time, 

 the valleys cannot have changed form during the glacial period 

 any more than since that time. Probably neither of these sup- 

 positions is so far from the truth as to admit a very great change 

 in the conclusion deduced from them ; but admitting that former 

 weathering was even ten times as rapid as that of the present 

 time, and that the glacial period was as much as ten times longer 

 than the postglacial ; then the glacial change in the valleys should 

 equal one hundred times that which their general surface has 

 suffered since. But even this extreme change would not account 

 for the excavation of the Swiss lakes. 



F. Pfaff gives this argument. (Ueber die Bewegung und Wirkung der 

 Gletscher, Pogg. Ann. cxi, 1874, 325-336.) 



A". Heim writes " the slope of the valley sides (in the Alps) shows no 



PROCEEDINGS B. S. N. H. VOL. XXII. 4 DECEMBER, 1882. 



