conclusions, in advance of the fullest available data, and partly to the
fact that
the existence of two such periods had not been generally recognized by American
geologists, although the doctrine of separate glacial periods had been entertained
by several in this country, following the lead of the Scotch school. The
only
American evidence then adduced, aside from theoretical presumptions, consisted
of supposed superpositions of newer upon older till, separated by supposed
inter-
glacial deposits -a class of evidence to be received with great caution,
since
temporary oscillations, or the shifting of sub-glacial streams, may produce
strik-
ingly analogous phenomena. Where the section exposed to observation chances
to be parallel to the glacial margin, or the course of a sub-glacial stream,
the
phenomena may seem to be much more prevalent than is really the case. A
further and more important ground of doubt arises from. the fact that certain
sub-aqueous deposits so closely resemble true Till, that they have been mis-
taken for it, and there is'perhaps no case of superposition of beds supposed
to represent two glacial periods that is not still open to these doubts.
Our present
firmness of conviction arises (1) from the discovery and working out of an
ex-
tended moraine, stretching across the whole of the glaciated area, and belonging
to a system of glacial movements which differ in many important respects
from
the earlier ones; and (2) from the differences of surface contour, due to
the
greater erosion of the earlier, as already indicated. We believe that this
line of
evidence, when developed in its fullness, will prove entirely demonstrative.
Only a small part of the results now gathered fall specifically within our
present
province as chronicler of the geological history of Wisconsin, but the total
result is, in some important measure, the outgrowth of investigations begun
in
this State.
When the ice a second time move-d down upon the land, it found
its pathway already smoothed by the preceding glacier, and strewn
with its debris.  The later ice followed the great channels of its
predecessor, though with some important variations owing to the
changed conditions of the two epochs, and other attendant circum-
stances.  The glacial movements in, and adjacent to, Wisconsin,
were of the following remarkable character:
-Lake Jichigan1 Glacier.  A great ice tongue thrust itself down
the basin of Lake Michigan, taking a form closely similar to that
of the lake, but broader and longer. Its western margin is now
marked by the Kettle Range, extending from Kewaunee county south-
ward, essentially parallel to the lake, through Manitowoc, Sheboygan,
Fond du Lac, Washinrgton, Waukesha, Jefferson, Walworth, Racine
and IKenosha counties, into Illinois, sweeping thence around the
south end of the lake, concentric with it, and northward to the
northern part of the Lower Peninsula of Michigan, being joined by
the marginal moraine of a similar lobe which occupied the basin of
Lake Huron and Saginaw -Bay. In other words, it had essentially
the contour of Lake Michigan expanded 20 to 40 miles on each side.
Green Baly Glacier. Another tongue of ice was thrust down
alongside this, having the Green-Bay-Rock-Rivr   valley for its axial


GENERAL GEOLOGY.


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