E... Desor on the Drift of Lake Superior. ws - 
however, to suppose that the subsidence was sudden. Every- 
thing leads us to believe, on the contrary, that it was gradual, 
and that the same agencies continued to operate to a certain de- 
gre Is we may infer that beaches were formed, sand bars 
built up, and boulders transported, in the same manner as before, 
although at lower levels. Meanwhile, the former and higher 
beaches receded more and more from the shores ; the bars, shoals, 
and submarine banks appeared as ridges or cesars above the plains 
recently laid dry, whilst new ones were forming at lower levels; 
Whenever the water, in its receding movement, stood for a 
sufficient. time at the same level to allow the new shore to be 
acted upon and undermined by the waves, these bluffs and ter- 
races were formed, as shown before. 
Ow, since terraces and ridges occur frequently along the 
great lakes, they may be considered as a conclusive proof that 
the subsidence was really gradual. In this respect, terraces and 
ridges, although composed of drift materials, belong properly to 
the alluvial period, as well as the denudations along the channels 
through which the waters are supposed to have been discharged. 
I shall therefore examine them with more detail in my report 
upon the alluvium. Thus far, we are not aware of any striking 
8eological event—such as the elevation of a mountain chain— 
having taken place between the two epochs of the drift and allu- 
— It might therefore be asked if there is sufficient reason 
ti Separate them. There are, indeed, some geologists who ques- 
On the: propriety of such distinction. My chief motive in 
then it is derived from all of the drift phenomena, rather 
0 from any single event. ane 5 
the he drift is the last phasis of any importance through which 
Wien passed before it became fitted for the habitation of man. 
a if it not for these deposits, a great portion of this continent, 
Wace ue the district embraced in this report, would have been a 
aste of naked and barren rocks, covered partially with heaps of 
‘and or rough detrital materials. Through the long continued 
i of water, these materials have not only been reduced and 
tsed, but also mingled in such proportions as to afford a most 
perepriate soil for vegetable and animal life. When, afterwards, 
Sinan of the continent caused the waters to recede within their 
rlaing wuits, they left behind them those wide, drift-covered 
fhe esti a of 
ac intelligent, and prosperous nation. = 
Justified in considering the period, when the waters, after 
W and grand era which has 
man, and of which the alluvial 
