Davis.] 24 [May 3, 



worn it away in the possibly short time it had been there. If it 

 were found that a non-glaciated surface existed below this detritus, 

 the observation would be decisive, but I am not aware that this 

 has been done. 



The rocky surface abandoned by the ice is by no means 

 a smooth trough, but is of uneven, irregular form. If its irregu- 

 larities were of broad curvature, they might be considered the 

 remains of large masses of rock, now nearly destroyed ; but they 

 are not ; the shapes rapidly change from hollow to mound, and 

 such small unevenness can result only from the broad glacier 

 having but little rubbed down a surface originally roughened by 

 the more detailed action of atmospheric weathering. Kere then 

 where the ice has acted longest, it has failed to destroy all the 

 smaller preglacial forms of the surface. (See C. 2, 5, 6.) 



L. Agassiz gave a chapter to " Taction des glaciers sur leur fond " in his 

 Etudes sur les Glaciers, Neuchatel, 1840, and concluded that while they 

 might be supposed to act powerfully on the rocks over which they move, 

 the only effects noted were those of rounding and polishing ; he considered 

 the possibility of their deepening their channel and forming rock-basins 

 for lakes, but rejected it because such basins are not found in the.valleys 

 from which glaciers have recently retreated. 



E. Whymper gives good examples under this heading. (Scrambles in the 

 Alps, 1871, 324.) He suggests that the final flat form of the roclie moutonnee 

 be called nivelee ; such are rare in the Alps, but he found them in Greenland 

 where the ice has presumably worked longer. 



A. 4. Transporters not Eroders. What little drift occurs on 

 these recently uncovered surfaces is very probably of modern 

 arrival at the point where it is found, and may very possibly 

 have largely come by crevasses from the top of the ice. For these 

 surfaces are as a rule of nearly bare rock ; the ice has been at 

 work there so long that it has ages ago cleared away all the loose 

 preglacial fragments, and since then has been working j)ersever- 

 ingly, but with little avail as has just been shown, to rub down 

 the solid rock. Hence it is probable that after its early activity 

 in sweeping away what it found ready loosened, the ice did not 

 supply itself with much more detritus, and its further erosion 

 was slow. 



Alex. Miiller holds that glaciers, like rivers, carry away detritus pro- 

 vided for them by surface weathering ; while the rock surface is snow- 

 bound or ice-covered its waste will be slight. (Ueber Thalbildung durch 

 Gletscher, Pogg. Ann. CLU, 1874, 476-482.) 



