62 MAN AND THE GLACIAL PERIOD. 



sides, among these boulders we readily recognised many of 

 granite, which must have come either from the Adiron- 

 dack Mountains, two hundred miles to the north, or from 

 the Canadian highlands, still farther away. 



Limiting our observations simply to the boulders, we 

 should indeed have been at liberty to suppose that they 

 had been transported across the valley of the Mohawk or 

 of the Great Lakes by floating ice during a period of sub- 

 mergence. But we were forbidden to resort to this hy- 

 pothesis by the abrupt marginal line, running east and 

 west, upon Pocono plateau, along which these northern 

 boulders ceased. South of this evident terminal moraine 

 there was no barrier, and there were no northern boulders. 

 On the theory of submergence, there was no reason for 

 the boundary-line so clearly manifested. Ice which had 

 floated so far would have floated farther. 



Still further, on going a few miles east of the Pocono 

 plateau, one descends into a parallel valley, lying between 

 Pocono Mountain and Blue Mountain, and one thousand 

 feet below their level. But our marginal southern bound- 

 ary of transported granite rocks did not extend much 

 farther south in the valley than it did on the plateau, 

 except where we could trace the action of a running 

 stream, evidently corresponding to the subglacial rivers 

 which pour forth from the front of every extensive glacier. 

 In these facts, therefore, we had a crucial test of the 

 glacial hypothesis, and, in view of them, could maintain, 

 against all objectors, the theory of the distant glacial 

 transportation of boulders, even over vast areas of the 

 North American continent. 



Since that experience, I have traced this limit of 

 southern boulders for thousands of miles across the con- 

 tinent, according to the delineation which may be seen in 

 the map in a later chapter. If necessary, I could indi- 

 cate hundreds of places where the proof of glacial trans- 

 portation is almost as clear as that on the Pocono plateau 



