236 THE POPULAR SCIENCE MONTHLY. 



have spoken of it as an island in the ice age.* I have fully stated else- 

 where f the facts proving the presence of the ice-sheet there. They 

 are, briefly, the usual glaciated appearance of the ledges, stria?, and 

 transported bowlders, two of them weighing nearly one hundred 

 pounds, and distinctly identical with ledges several miles distant. All 

 the surface rocks above five thousand feet have been riven into angu- 

 lar fragments by the long-continued action of frost, insomuch that the 

 embossment and striation have been nearly obliterated, and the frag- 

 ments so covered by lichens that extraordinary care is required to dis- 

 cover any dissimilarity among them. Washington is the highest peak 

 southeast from the St. Lawrence Valley ; hence, if that has been gla- 

 ciated by ice moving from the northwest, then every part of New Eng- 

 land has been covered by the same sheet. 



Assuming it well established that the center of dispersion for the 

 Eastern American drift was in the west part of Labrador, we are met 

 by the difficulty that the land beneath this central mer de glace is not 

 so much elevated as the New England mountains which have been 

 covered by the ice originating in those Laurentian highlands. As to 

 the facts, they are indisputable the glacier has moved over slopes 

 higher than the mountains at its source ; and it may be that we have 

 something new to learn from these facts about ice accumulation and 

 movement. Professor Dana has proposed the theory of a change of 

 level in the land since the ice period. Assuming the starting-point to 

 have been in the Laurentian highlands, if a descent of ten feet per 

 mile be allowed, the sheet must have been at least thirteen thousand 

 feet high to have allowed it to slide over Mount Washington ; or, the 

 land must have been four thousand five hundred feet higher than now. J 

 The distance from these highlands to the base of the Rocky Moun- 

 tains along which the ice is known to have moved southwesterly is 

 about fifteen hundred miles, requiring, on the basis of Professor Da- 

 na's data, an elevation of certainly four miles. A change of level to 

 that extent seems improbable. 



Recent writers have developed a molecular theory of glacier mo- 

 tion ; and in connection with it mention such possibilities of ice-accu- 

 mulation as to suggest a method of relief from the difficulty of under- 

 standing how the ice can move up an elevation. The mere weight of 

 the ice does not cause it to slide downward. According to Canon 

 Moseley, from thirty to forty times its weight is required to shear the 

 ice; the motion is proportional to the amount of heat present. The 

 melted ice is very Busceptible to the action of gravity, and the motion 

 is greatesl where the most heat is manifested, or upon the south side 

 of the glacier. 



' E. Hitchcock, "Proceedings of the American Association of Geologists," 1841, p. 

 182. L. Agassiz, "Geological Sketches," Second Series, p. 98. 

 t "Geology of New Hampshire," vol. iii, p. 203. 

 \ " American Journal of Science," III, vol. ii, p. 327. 



