1 14 THE GLACIAL LAKE AGASSIZ. 



limit of the usual di-ift deposits, strife, and ice-woni ledges about 1,000 feet 

 below the top of Mount Washington ; and of his son, Prof. Charles H. 

 Hitchcock, who in 1875 found glacially transported bowlders on the very 

 summit of this mountain, 6,293 feet above the sea.^ The ice-sheet, there- 

 fore, at one time overtopped even this highest peak of the eastern portion 

 of its area. Very rare bowlders and small fragments of gneiss foreign to 

 Mount Washington, which in its ujiper part is andalusite mica, schist and 

 gneiss, occur above the limit of the ordinary di'ift action, as similar foreign 

 rock fragments are found very scantily on the high portion of Katahdin to 

 within 600 or 600 feet below its highest point. But on Mount Washington 

 the di'ift fragments are scattered thus scantily quite to its summit, near 

 which Professor Hitchcock has obtained two bowlders, each weighing about 

 90 pounds. One of these is in the museum of Dartmouth College, and the 

 other in that of the Boston Society of Natural History. These bowlders 

 were transported by a glacial current moving from northwest to southeast, 

 and in the distance of probably 15 miles from their parent ledges to the 

 top of the mountain they were carried upward about 5,000 feet. 



Before this discovery, while it was believed that Mount Washington 

 and adjacent portions of the same range rose above the ice-sheet at its time 

 of greatest thickness. Prof James D. Dana had computed, from the slope 

 of the ice surface thus known, and from the courses of striation and trans- 

 portation of bowlders in Canada, that the elevation of the surface of the 

 ice-sheet oyer the northern border of New England was about 8,000 feet, 

 and over the Canadian watershed between the St. Lawrence and Hudson 

 Bay 13,000 feet above the present sea-level, giving to the ice an average 

 thickness of about 5,000 feet in the region of the White Mountains, 6,500 

 feet on the international boundary, and not less than 12,U00 feet on the 

 Laurentide highlands." It still appears to be true that the upper limit of 

 the ice-sheet was about 1,000 feet below the summit of Mount Washington 

 during the greater part of the Ice age, and that Dana's estimates of the 

 thickness of the ice farther north are very probable. There seem to be 

 good reasons for believing that the land at length sank beneath this heavy 



'Geology of New Hampshire, Vol. Ill, 1878. 



2 Am. Jour. Sci. (3), Vol. V, pj). 198-211, March, 1873. 



