252 Canadian Record of Science. 
to consider are: Do these strata presently form the floor of 
the lake, or have they within the lake area been removed 
by some vast erosive force acting at a recent period? In 
other words, is the lake the result of a synclinal depression 
or of erosion, or both? Again, is the apparent parallelism 
in the outcrops of the formations due to successive, gradual, 
permanent elevations of the land from the Laurentian 
period onward, each elevation stretching farther south than 
its predecessor, or is it due to a great erosive force which 
exposed in succession the upturned edges of the different 
strata, and as a farther result produced Lake Ontario? 
In Michigan, again, the Carboniferous area which there 
at one time was the centre of depression, is even more con- 
spicuous in its relations to both the surrounding geological 
features and the adjacent lakes. Here, on every side, there 
is a regular series of formations whose outcrops, after 
making every allowance for estimations, appear each in 
proper geological succession within the other, and in 
Michigan, form, as it were, irregularly concentric areas 
around the Carboniferous. Again, the contours of the 
shores of Lakes Michigan, Huron and St. Clair, and of Lake 
Krie at its western end, present the same idea of arrange- 
ment around the same central area. The interesting ques- 
tions arising are: Were these formations originally laid 
down here with this more or less concentric arrangement 
which in Michigan they presently possess, or have they in 
recent or earlier times been the subject of some denuding 
force, which has given them this peculiar arrangement, and 
which probably has also aided in the creation or enlarge- 
ment of the adjacent lakes? Again, as certain of these for- 
mations were evidently originally more or less continuous 
across the area now occupied by Lakes Huron and Michi- 
gan, has some vast erosive force created these lakes by re- 
moving the strata where they occupied the lake area, or do 
the strata underlie the waters of these lakes as a result of 
a depression, or, are there here the effects of both denuda- 
tion and depressiou ? 
The central area of Michigan was, as far onward as the 
