DEVONIAN AGE, OR AGE OF FISHES.


coming more and more jagged, and now, after the last stratum had
been added, and the whole had been lifted from the waters that
gave them birth, there ensued a prolonged era during which the
history was simply that of surface-erosion. The face of the State
became creased with the wrinkles of age. The edges of her rocky
wrappings became ragged with the wear of time. The remaining
Devonian periods, the great Carboniferous age, the Mesozoic era,
and the earlier Mesozoic periods passed, leaving no other record than
that of denudation.
It was only in the glacial period that deposition was again resumed
and then in a form greatly diverse from that with which we have
now grown familiar. In the interval, Wisconsin was a witness of,
rather than a participant in, the history of rock-forniation and life-
preservation. For the sake of continuity and conformity to legal
requirements, a brief sketch of the salient characteristics of these
ages will be given, but before entering upon this, it may be profit-
able to take a sweeping backward look across the Paleozoic ages,
and catch and group in distant perspective their grander outlines.
SUMMARY OBSERVATIONS ON TIHE WISCONSIN PALEOZOIC SERIES.
Groups of Deposits. Glancing comprehensively over the marine
fossiliferous formations of Wisconsin, we observe that they gather
into five main groups, reposing in succession upon the great crystal-
line formations. The Potsdam sandstone and Lower Magnesian
FIG. 64.


GEsNERALIZED GEOLOGICAL SECTION Of the formations or te inw-ur--, W--'--
- -
relations of the Wisconsin series to the fuller system of the Interior Basin.
limestone constitute the first group. The St. Peters sandstone, the
Trenton and Galena limestones and the mixed shales and limestones
of the Hudson River series, constitute the second assemblage. The
Clinton Iron ore deposits and the Great Niagara limestone, with its
several subdivisions, constitute the third division, which is only com-


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