434: On the Drift Deposits of Western Canada. 
uprise of the land appears to have taken place, and a vast area, 
extending over and around our present lake-basins, then became 
converted into a freshwater sea. This probably found its out- 
let to the ocean through what is now the broad valley of the 
Mississippi. Its waters stood at a great elevation above the 
waters of our present lakes, and were gradually lowered to these 
levels by physical changes in the surrounding country, and more 
especially by the depression of a higher region lying to the east. 
During this gradual fall and retrocession of the great lake-waters, 
the upper layers of the Drift were re-sorted, mixed with newer 
sediments, and thrown up here and there into secondary ridges ; 
and the remarkable terraces which form so salient a feature in 
the general aspect of our lake shores and intervening districts, 
were then in chief part produced. The escarped faces of these 
Drift terraces, it should be observed, always front the present lake- 
basins, and thus look in some places towards the north, and in 
others towards the south, &c., according to the direction of the 
nearest shores. This would necessarily arise if they were pro- 
duced, as here imagined, by a gradual lowering of the waters, 
with intervening periods of repose. The shells of freshwater 
mollusca buried in the modified Drift, at various levels above 
the existing lake-waters, and in localities so far apart—for these 
shells have been found throughout the region south of the lakes, 
in addition to the localities mentioned in this paper—prove in- 
contestably the former expansion and union of our lakes, or, in 
other words, the presence in this part of Western America, of a 
widely-extended freshwater sea covering an enormous area. A 
curious circumstance, and one of great significance in its bear- 
ings on this question, is the fact that all the inclined layers of 
modified Drift (to the east, at least, of Lake Superior) appear to 
slope towards the west or south. A remarkable instance of this, 
hitherto, it is believed, unnoticed, may be seen near the mouth 
of the Niagara river, at Lewiston. At this spot, oblique layers 
of modified Drift, in beds made up of coarse gravel and pebbles, 
point nearly due south, and thus bear witness to the fact that 
the current which occasioned the inclined stratification must 
have set directly up the gorge, or against the direction of the 
present stream. 
The assumption of an immense freshwater lake of this cha- 
racter gradually falling from a high level, necessarily involves 
the additional assumption of an eastern barrier, extending at 
one period between the lake-waters and the Atlantic. This view 
was maintained bysome of the earlier investigators of our geology, 
and notably by Mr. Roy, in his well-known paper on the ter- 
races of Lake Ontario, communicated to the Geological Society 
of London in 1837. The difficulty of finding a satisfactory loca- 
