SURFACE GEOLOGY. 73 
debris of the land was deposited in strata that subsequently rose to the 
surface, and formed a broad, low margin to the central mountain belt; 
just as the Cretaceous and Tertiary strata flank the Alleghanies in the 
Southern States. 
In the lapse of countless ages, all the mountain peaks and chains of 
the Laurentian continent 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 trunc- 
ated bases remain as witnesses, and it is scarcely less certain that 
glaciers have flowed down their slopes, of sufficient magnitude and reach 
to deeply score the plain which encircled them. 
Tt will be noticed that all the great lakes of the continent hold certain 
relations to the curving belt of Laurentian highlands. 
Some of them are embraced in the foldings of the Eozoic rocks, and fill 
synclinal troughs; but most of the series, from Great Bear Lake to Lake 
Ontario, exhibit the same geological and physical structure, and are 
basins of excavation in the Paleozoic plain that flanks, in a parallel belt, 
the Laurentian area. Few of us have any conception of the enormous 
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 Assinniboin country, will 
be sure to find the comparison interesting and suggestive; suggestive 
especially of a community of structure and history, and of an insepara- 
ble connection between the lake phenomena and the topographical 
features of the Laurentian highlands, flanked by the the Paleozoic plain. 
In estimating the influences that might have affected the number and 
magnitude of glaciers on the sides of the Laurentian mountains, it 
should not be forgotten that the Cretaceous sea swept the western shore 
of the Paleozoic 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 peculiarly favorable to the for- 
mation of great glacial masses of ice, 2. ¢., a broad evaporating surface of 
warm water swept by westerly winds, that carried all suspended moisture 
immediately on to a mountain belt, which served as a sufficient con- 
denser. 
This, at least, may be positively asserted in regard to the agency of 
ice in the excavation of the lake basins, that their bottoms and sides, 
wherever exposed to observation, if composed of resistant materials, bear 
