99 



r Lesley. 



sheet, southward and south-westward into the Mississippi valley — without 

 affecting the previously constituted topography beneath the ice — of which 

 previously constituted topography the Lake Basins were an essential part 

 and grand feature. 



Meanwhile, a totally different system of drainage was carrying on its 

 work of transportation beneath the ice sheet, in an opposite direction, 

 northward (from Pennsylvania and Ohio) and eastward, through the lake 

 basins. But this low^er or sub-ice river system, deprived of direct alimen- 

 tation from rain, must have been inferior in volume and power to the 

 upper or surface-ice river system ; although it may have received hei'e and 

 there through the ice sheet considerable accessions of surface rain water. 



I do not wish to discuss here the line of Prof Spencer's great river, nor 

 the claim of Prof. Newberry to the discovery, years ago, of its debouche- 

 ment, via the Mohawk and Hudson valleys, into the ocean at New York, 

 except to remark that Prof Newberry does not seem to appreciate Prof 

 Spencer's chief difficulty. It is not that the rocks appear at Little Falls ; 

 but that his Ontario river ran in a bed more than 780 feet beneath the 

 present level of the lake, and therefore more than 900 feet below Little 

 Falls, and the demonstration of a buried, concealed, old river channel 

 nearly 1000 feet deep anywhere alongside of the Little Falls exposure 

 seems a rather hopeless task. But worse than that ; the Mohawk valley 

 east of Little Falls, is barred by rock ranges 300 or 400 feet high, through 

 which the Mohawk cuts a canon, where its bed is at least 900 feet above 

 the old river bed in the lake.* 



I wish to confine my remarks to the feeble erosive power of the Cana- 

 dian ice-sheet, as a particularly inefficient kind of glacier, and to the prob- 

 able possibility of a mathematical demonstration of the feeble erosive 

 power of any glacier, even in the most favorable circumstances. 



Taking one stone graving-tooi as- the differential of means ; — the en- 

 graving quality of that stone tool (under the conditions (a), (b), (c), (d) 

 above stated) as the differential o? power ;— and the destruction of bed-rock 

 by that stone-tool during its life as a tool, as the differential of effect pro- 

 duced, i. e. of erosion, — then, — to obtain a transcendental maximum, we 

 must multiply one stone-tool (in area) by the total width and total length 

 of the ice bottom ; i. e. we must stud the whole bottom of the glacier with 

 tools ; keep them all at work, each one for the whole length of time of its 

 descent from the upper to the lower end of the glacier ; — replace those 

 that are lost or spoiled by fresh ones ; — and jjepeat the operation during 

 the entire life of the glacier. 



It is evident that this transcendental maximum if it could be calculated, 

 would be of little value, in as much as it would almost infinitely exceed 

 the actual practical erosive power of any given glacier. 



But it would be the best starting point for a reasonable discussion of the 

 erosive power of glaciers ; and it seems to me, that if the calculation were 



*See my notes to Dr. Spencer's appendix, at the end of White's Report of 

 Progress, 2d. Geo], Sur. of Pa., Q. 4, 1881, p. 403. 



PROG. AMER. PHILOS. SOC. XX. 111. M. PRIMTED MARCH o, 1882. 



