538 CENOZOIC TIME. 



which are given in the American Journal of Science, volume v., 1873, the height on the 

 northern border of New England, north and northwest of the White Mountains, was, on 

 this basis, 8,000 feet; and, if ten feet a mile is sufficient to give motion, the height over the 

 Canada watershed (570 miles from Mount Washington), was at least 13,000 feet. As this 

 watershed has an average height of 1,500 feet, the thickness of the ice, to make the 

 .height, would have been 11,500 feet, unless the watershed were above its present level; 

 and, in view of the enormous thickness thus required, we may reasonably infer that it 

 was more elevated than now, at least some hundreds of feet. The movement of the 

 glacier, across the St. Lawrence valley over New England, iiroves that, about the mouth 

 of the St. Lawrence, the ice stood higher than over the watershed; and this was owing 

 to two causes: (1) the greater amount of precipitation (as now true), near the seashore, 

 and (2) the higher latitude, and hence the greater cold. At the same time, the course 

 of the glacier over New England was determined largely by the fact that to the south- 

 east lay the ocean, affording a place of discharge for the ice-stream. 



The absence of glacial phenomena from the slopes of the Rocky Mountains, within 

 the United States, between the meridians of 98° and 108°, is probably a consequence of 

 the small amount of precipitated moisture over that region (now only 20 inches a year), 

 and also of the high summer temperature. 



4. Abrasion: Erosion: Gathering of Material for Transportation. — 

 The glacier, with a thickness of several thousand feet, must have had 

 great abrading power, owing to (1) its weight, (2) its motion, and (3) 

 the stones in its under surface. 



The weight of the glacier, equivalent to 2,000 pounds to the square 

 inch where 4,500 feet thick, would have pressed the bottom ice, where- 

 ever the weight was felt, into all depressions and crevices in and among 

 the rocky hills, and even into the earthy material that decomposition had 

 made over the hillsides. It would have filled, to its bottom, Lake Erie, 

 now but 80 feet deep ; and so also Long Island Sound, now 150 to 

 180 feet deep ; and it is probable that, if the ice were but a thousand 

 feet thick, it would have gone to the bottoms of Lakes Huron and 

 Michigan, supposing them to have had their present depth. 



As the glacier slowly moved, it would have torn off the tops and sides 

 of ledges, and have taken the stones into its mass, for transportation 

 southward. Thus it was ever abrading, and ever gathering material 

 for distribution. The stones and earth were taken up by the lower 

 part of the glacier, where in contact with the hill-tops and ledges, and 

 hence they occupied, for the most part, the lower 500 or 1,000 feet. 

 In connection with the onward movement, and in consequence of it, 

 there was intestine motion throughout the whole ice-mass, and espe- 

 cially in this lower portion ; and this would have ground the stones 

 against one another, rounded their edges, caused scratches in their 

 surfaces, and made, through the mutual grinding, the earth of the 

 bowlder clay, as well as sand and pebbles for sand and pebble beds. 

 The glaciers of Greenland, which are parts of the old Continental, 

 afford examples of all these operations. 



Moreover, since the snows of the commencing Glacial period fell over 

 a continent of great forests, the forests were in the bottom of the first 



