SURFACE GEOLOGY. 211 
In the lapse of countless ages, all the mountain peaks and 
chains of the Laurentian continents have been removed and 
carried into the sea, and this has been done by rivers of 
water and rivers of ice. That these mountains once. existed 
there can be no reasonable doubt, for their truncated bases 
remain as witnesses, and it is scarcely less certain that gla- 
ciers have flowed down their slopes of sufficient magnitude 
and reach to deeply score the plain which encircled them. 
It will be noticed that all the great lakes of the continent 
hold certain relations to the curving belt of Laurentian high- 
lands. 
Some of them are embraced in the foldings of the Eozoie 
rocks, and fill synclinal troughs; but most of the series, 
from Great Bear Lake to Lake Ontario, exhibit the same 
geological and physical structure, are basins of excavation 
in the paleozoic plain that flanks in a parallel belt the Laur- 
entian area. Few of us have any conception of the enor- 
mous general and local erosion which that plain has suffered. 
Those who will take the trouble to examine the section 
across Lake Ontario, from the Alleghanies to the Laurentian 
hills of Canada, and compare it with the other sections in the 
Lake Winnepeg district, radial to the Laurentian arch, given 
by Mr. Hind in his report on the Assiniboin country, will be 
sure to find the comparison interesting and suggestive; sug- 
gestive especially of a community of structure and history, 
and of an inseparable connection between the lake phe- 
nomena and the topographical features of the Laurentian 
highlands flanked by the paleozoie plain. 
In estimating the influences that might have affected the 
number and magnitude of glaciers on the sides of the Lau- 
rentian mountains, it should not be forgotten that the Cre- 
taceous sea swept the western shore of the Paleozoie and 
Laurentian continent from the Gulf of Mexico to the Arctic 
Ocean ; and whether we consider this sea as a broad expanse 
of water simply dotted with islands, or a strait traversed by 
a tropical current, we have in either case conditions peculi- 
