ANCIENT GLACIERS. 91 



not much less than a thousand feet. " Northward the 

 angle of the slope diminished, and the glacier surface ap- 

 proximated to a great level plain. The distance between 

 the high southwestern peaks of the Catskills and Pocono 

 Knob in Pennsylvania is sixty miles. The difference in 

 the elevation of the glacier could not have exceeded a 

 thousand feet," * that is, the slope of the surface was 

 about seventeen feet to the mile. 



Professor Dana estimates the thickness of the ice in 

 southern Connecticut to have been between fifteen hun- 

 dred and two thousand feet. Attempts to calculate the 

 thickness of the ice farther north, except from actual dis- 

 covery of glacial action on the summits of the mountains, 

 are based upon uncertain data with reference to the slope 

 necessary to secure glacial movement. In the Alps the 

 lowest mean slopes down which glaciers move are about 

 two hundred and fifty feet to a mile ; but in Greenland, 

 Jensen found the slope of the Frederickshaab Glacier to 

 be only seventy-five feet to the mile, while Helland found 

 that of the Jakobshavn Glacier to be only forty-five feet. 



It is doubtful if even that amount is necessary to se- 

 cure a continental movement of ice, since, as already re- 

 marked, it is unsafe to draw inferences concerning the 

 movements of large masses of ice from those of smaller 

 masses in more constricted areas. We have seen, from 

 the glacial deposits on the top of Mount Washington, 

 that over the northern part of New England the ice was 

 more than a mile in depth. We have no direct evidence 

 of the depth of the stream which surrounded the Adiron- 

 dack Mountains. Nor, on the other hand, are we certain 

 that the Catskills were not completely enveloped in ice, 

 though most observers, reasoning from negative evidence, 

 have supposed that to be the case. But from the facts 

 stated concerning the boulders along the glacial boundary 



* American Journal of Science, vol. exxv, 1883, p. 339 et seq. 



