MUSEUM OF COMPARATIVE ZOOLOGY. 21 



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and other related metamorphosed rocks have not been planed away by 

 the glacier, but remain with the peculiar aspect which is commonly sup- 

 posed to be limited to the district south of the glaciated field. Again, in 

 the region immediately north of Kingston in Canada, a place situated 

 in what is supposed to have been the very heart of the great glacier, 

 the horizontal rocks of the Silurian Age retain their delicately incised 

 valleys, which were formed before the Ice Period, in a state of preserva- 

 tion almost as perfect as those formed in rocks of the same age and 

 character in Central Kentucky, in a district a hundred miles south of 

 the ice front. Here and there these valleys of the Kingston field are 

 somewhat embarrassed by accumulations of glacial waste, and at other 

 points the streams have made slightly deeper excavations in their old 

 paths, but on the whole the topography is substantially that which 

 existed before the advent of the glacier. 



It is evident that, if Ave assume the rate of glacial wearing to have 

 been rapid, and yet at the same time the amount of effective work to 

 have been small, we are at once compelled to believe that the duration 

 of the cutting action was but brief. Along the margin of the ice the 

 condition of the frontal accumulations of debris at a number of points on 

 this continent leads us to the conclusion that the southernmost part of 

 the field occupied by the ice was tenanted by the glacier for but a short 

 time. Thus m the central parts of New Jersey the morainal accumula- 

 tions are generally slight, while the margin of the field occupied by 

 the ice in the northernmost point in Kentucky, though the indications 

 which point to the presence of the sheet are unmistakable, shows no 

 frontal moraine whatever. If these peculiar instances of slight wear- 

 ing were limited to the margin of the glacier, we could sufficientlv 

 account for the facts by supposing that a sudden forward movement of 

 the glacier had occurred, during which the fringe of the ice sheet occu- 

 pied for a very brief time an area which the climatal conditions did not 

 permit it to remain in. Such temporary excursions of the ice, though 

 on a smaller scale, have been frequently observed at the lower extremity 

 of the Swiss glaciers. Owing to the existence of such slightly worn 

 areas as we have noted in the interior portions of the American gla- 

 ciated field, we cannot account for the facts in the manner just indicated. 

 It appears necessary to suppose the existence of some conditions which 

 would permit the glacier to rest over a surface, and at the same time 

 prevent its abrasive action on the bed rocks. 



Having been for some years engaged in preparing a series of maps 

 and reports on the surface geology of New England, I have been led to 



