MUSEUM OF COMPARATIVE ZOOLOGY. 213 
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 we 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 in 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 sufficiently 
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 
