108 E. Desor on the Drift of Lake Superior. : ® 
ever have been left by a violent agency. Finally, many of them, 
in spite of contradictory assertions, have rather sharp mee 
suming that, during 5 
the period of the . 
o 
: Z 1B $ 
groovings, the waters 2 8 
iJ 
stood nearly at the. a 
sunk during the sec- ‘T, ThunderCape. 1. R., Isle Royale. D., Divit 
ond period to the  ing-ridge betwnmme Take Superior and Lake Michigan. 
nois. This ridge, abounding in eruptive and metamorphic vet 
is therefore to be considered henceforth as the true birth-place 
the boulders scattered over the Western States, and we need 2? 
The drift epoch may be considered as closed with the trans 
portation of the boulders. The waters, after having thus reach 
their highest level—during which the transport of the boulders av® 
 reiage was accomplished—again subsided. With this subsi 
ence commences the era of the alluvium. We ha e no reasod : 
re 
