Davis.] 28 [May 3, 



results are of no value, because the assumed conditions are not natural 

 conditions, and in this the work unfortunately resembles many of the 

 attempts to apply mathematics to geology. 



A. 9. Ice, more than Rock, is Eroded. It must be granted that 

 the ice itself will suffer most when pressed heavily on its bed, and 

 in spite of its long action will fail to produce much erosive change. 



W. H. Niles found by direct observation that the bottom ice moves fas- 

 ter than the stones below it, and that a groove is worn in the ice as it 

 passes on. (These Proceedings, xix, 1878, 332.) 



K. Zoppritz looks on glaciers as very limited agents of erosion, and 

 thinks that the ice rather than the rocks below will be eroded. (Der gegen- 

 wartige Stand der Geophysik ; Wagner's Geogr. Jahrb. vin, 1880, 74.) 

 A. Gurlt comes to the same conclusion. (See under Fjords, C. 1.) 



A. 10. Preservation of Scratches. A vigorous opponent of 

 glacial erosion brings forward the frequently observed double set 

 of scratches on a single rock surface to show that, far from being 

 able to cut valleys and fjords, a glacier cannot even rub out its 

 own marks. 



Th. Kjerulf, in an excellent article, entitled Die Eiszeit, (Virchow u. 

 Holtzendorff. Samml. wiss. Vortrage, Berlin, xin, 1878, 74 ; also, Geol. 

 Norwegen, Bonn. 1880, 39), considers this the strongest argument against 

 glacial erosion. 



If it could be fully proven that the divergent scratches were 

 made at widely separated times, and that the rock surface was 

 through all the interval subjected to glacial rubbing without pro- 

 tection from the onward travelling ground-moraine, this conclu- 

 sion would be well based ; but this is not proven, and the argu- 

 ment turns better in the other direction. Just because the older 

 scratches survive, they cannot be very much older than the newer 

 ones, and are therefore useless in giving a clue to any very early 

 direction of ice motion, unless Ave suppose they have been shel- 

 tered by a temporary covering of boulder-clay. It is more proba- 

 ble that nearly all the scratches now seen were made in a late 

 phase of the glacial epoch, near the margin of the retreating ice. 



A. 11. Self -limitation of Erosion. It has been claimed that 

 glacial erosion is self-limiting ; that as time goes on, the trough 

 is rubbed into a form of least resistance to ice motion, and then 

 further erosion is pratically stopped. But the attainment of this 

 form of least resistance may require great change in the valley 

 form, and even after its attainment it would be difficult to deny 



