228 DRIFT DEPOSITS AND ANCIENT EXTENSION 
over and around our present lake basins, then became converted into 
a fresh-water sea. This probably found its outlet 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 retro- 
cession 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 in- 
tervening 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 fresh-water 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 incontestibly the 
former expansion and union of our lakes, or, in other words, the 
presence in this part of Western America, of a widely-extended fresh- 
water sea, covering an enormous area. A curious circumstance, 
and one of great significance in its bearings 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 fresh-water lake of this character, 
gradually falling from a high level, necessarily involyes the additional 
assumption of an eastern barrier, extending at one period between 
the lake-waters and the Atlantic. This view was maintained by some 
