1882.] 23 [Davis 



greater, and when outer air and observers were excluded. Erosion 

 was certainly greatest on rocky knobs, but the old glaciers often 

 fitted closely into the hollows of the ground, especially into hoi 

 lows opening to the approach of the ice, and wore them smoother 

 and deeper. 



De Mortillet thus criticizes Desor. (Milano, Soc. Ttal. Atti, V, 1863.) 



Newberry describes scratches on the under surfaces of projecting ledges. 

 (Geol. Ohio, ii, 77.) Similar scratches may be seen at Catskill, N, Y. 



A. 2. Advance of Glaciers. Glaciers have been observed to 

 advance over a surface of loose material without causing great 

 disturbance in it, certainly without ploughing it up, and hence 

 it has been argued that there never could be great glacial erosion. 

 This however goes to the conservative extreme and neglects the 

 elements of time and weight. Surely the friction on the bottom 

 gravels of the Valley of Chamounix, when the ice rose to the 

 upper line of the scored rocks, would be effective in amount and 

 duration to an extent not fairly indicated by the short-lived action 

 of a small modern glacier. 1 Moreover, examples of an opposite, 

 destructive effect have been seen as well and are also entitled 

 to attention. It would seem therefore that glaciers in advancing 

 over new ground sometimes do and sometimes do not tear and 

 plough it up ; a smooth gravel surface appears to be less liable 

 to disturbance than a rough one or than loamy soil ; but it may 

 be noted that examples of slight erosion are entitled to especial 

 attention, for they are not what one would expect. 



J. de Charpentier (Essai sur les Glaciers, Lausanne, 1841, 41) records 

 the advance of the Glacier du Tour eighty feet over a gravelly surface without 

 destroying it, but the swampy soil of a field beyond was overturned ; also of 

 the Glacier du Trient in 1818, when the ice insinuated itself between bed- 

 rock and soil, overturning the latter and the trees growing in it. Further 

 examples by Favre (Recherches Geol. i, 201) ; and Credner (Deutsch. 

 Geol. Gesell. Zft., xxxn, 1880, 75, with figures.) 



A. 3. Retreat of Glaciers. An enrphasis that seems unwar- 

 ranted has been laid on the occurrence of drift deposits sometimes 

 disclosed by glacial retreat. The preservation of such detritus 

 is claimed as evidence that the ice could not have worn it away, 

 but this is not conclusive ; it shows simply that the ice had not 



1 Ramsay, Phil. Mag. xxviii, 1864, 304, 



