Page:Popular Science Monthly Volume 20.djvu/248

236 have spoken of it as an island in the ice age. I have fully stated elsewhere the facts proving the presence of the ice-sheet there. They are, briefly, the usual glaciated appearance of the ledges, striæ, 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 angular fragments by the long-continued action of frost, insomuch that the embossment and striation have been nearly obliterated, and the fragments so covered by lichens that extraordinary care is required to discover any dissimilarity among them. Washington is the highest peak southeast from the St. Lawrence Valley; hence, if that has been glaciated by ice moving from the northwest, then every part of New England 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. The distance from these highlands to the base of the Rocky Mountains along which the ice is known to have moved southwesterly is about fifteen hundred miles, requiring, on the basis of Professor Dana'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 motion; and in connection with it mention such possibilities of ice-accumulation as to suggest a method of relief from the difficulty of understanding 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 susceptible to the action of gravity, and the motion is greatest where the most heat is manifested, or upon the south side of the glacier.