80 
ICE-PERIOD IN AMERICA. 
from the Arctic regions to about the fortieth 
degree of northern latitude, the glacier-marks 
trending from north to south, with occasional 
slight inclinations to the east or west, accord¬ 
ing to the minor inequalities of the surface. 
There is, however, no decided modification of 
their general trend in consequence of the range 
of hills intersecting them at right angles for 
nearly the whole width of the continent be¬ 
tween latitudes forty-six and fifty ; indeed, the 
Canadian, or, as they are sometimes called, 
the Laurentian Hills, did not form a more 
powerful barrier to the onward progress of the 
immense fields of ice covering the continent 
than did the small hummocks, or roches mou- 
tonnees , in the Swiss valleys to the advance of 
the Alpine glaciers. In fact, these low hills 
may be considered as a succession of roches 
moutonnees trending in a continuous ridge 
from east to west, over which the masses of 
northern ice have moved unimpeded to the 
latitude of the Ohio. 
Owing to the absence of high mountain- 
ranges over this vast expanse of land, the 
glacial phenomena of America are not grouped 
about special centres of dispersion, radiating 
