36 Revieivs — Prof. N. S. Shaler — On Glaciers. 



lands on either side. Yet the ice must have been deep enough over 

 the summit to cause the current to disregard the obstacle, for in the 

 case of this and other hills the glacial strise pursue a straight course 

 up the northern and down the southern flanks. If the mountain had 

 only been buried for a slight depth below the ice, the glacier would 

 have been deflected to either side. The conclusion is therefore 

 drawn that the ice was more than a mile in depth. 



When, however, an explanation has to be given of the movement 

 of the ice-spread, great difSculties are met with; our author has, 

 however, the courage of his convictions. These difficulties are 

 enhanced by the circumstance that there is no slope down which the 

 ice could have flowed, in fact there is proof of a depression to the 

 north in glacial times. This depression has only been studied in 

 detail on the coast; "at its southern extremity it was only 20 ft. ; 

 at Boston it seems to have extended up to fifty or eighty feet ; on 

 the coast of Maine it was near 300 ft. In the valley of L. Cham- 

 plain it was as much as 350 ft. ; in Labrador it seems to have 

 attained 1000 ft., and in Greenland there are reasons for thinking it 

 amounted to over 2000 ft." Granting this subsidence, which is 

 candidly insisted on by the author, the difficulty of explaining the 

 motion of the ice-sheet by the action of gravity is well-nigh 

 insuperable. "The only way in which gravitation could be brought 

 to act under these circumstances would be by the heaping up of the 

 ice in the northern regions to so great a height that the slope of the 

 upper surface to the south would be as great as that which suffices 

 to impel ordinary glaciers down their declivities." Certainly the 

 thickness would have to be extraordinary to cause a movement of 

 the glacial sheet over say ten degrees of latitude even on level 

 ground, still more "up the slope which the depressed lands thus 

 offered to the ice." The author rejects this theory, for to give a 

 slope of over one degree from the shores of Hudson's Bay to New 

 York would require a thickness of many miles at the source of the 

 stream. Some other explanation has to be sought, for the iceberg 

 theory is deemed inadmissible. Yet drift material has been carried 

 from the Laurentian hills to Cincinnati, at least 500 miles ; and the 

 motion would seem to have been so consistently in one direction that 

 where the ice crossed oblique valleys, there is scarcely any deflection 

 of the stri^. The author puts forward another theory, according to 

 which the ice-sheet as a whole is not supposed to have moved 

 constantly southward. The ice came on gradually, its limit extend- 

 ing more southward year by year till the culmination was reached. 

 Only the immediate front might move, most of the ice added yearly 

 would go to increase the thickness, and proportionally so the 

 further north. Then the removal of the excess of ice would take 

 place by " pressure-melting." For a fuller exposition of the author's 

 views on this point, see pp. 146-7, 158-161. We here indicate only 

 the outline of the argument. Each atmosphere of pressure lowers 

 the melting-point of ice by -0025 of a degree Fahr. Therefore if the 

 ice was near the freezing-point, as it probably would be under a thick 

 sheet, from a certain amount of heat derived from friction, and from 



