LOWER SILURIAN OR CAMBRO-SILURIAN AGE.


above effective action, and the agitated material would thus again be left
in more
quite waters, and would settle back into repose. Thus there would be an alterna-
tion of relatively forcible and quiet action. During the former the coarser
ma-
terial might be rolled and triturated and the finer stirred into suspension,
while
during the latter, both fine and coarse might, in large measure, settle back
together, the fine insinuating itself between the coarse, tending to bind
it together,
and renAder it less easily disturbed by subsequent wave-action.
Such an alternating action would appear to offer us some aid in conceiving
of
the method of formation of extensive sheets of brecciated limestone. A large
portion of the Galena formation is of this character. In the Niagara limestone,
a single brecciated stratum may be traced 100 miles along the strike of the
forma-
tion, and probably 50 miles or more back from its original margin. Much the
larger part of the Silurian limestone of Wisconsin is of a rough irregular
texture, which implies that the original material was heterogeneous. A system-
atic alternation between vigorous and quiet action like that now described
would
seem to aid greatly in supplying the needed conditions.
Variatwons in the Formation.   The foregoing characters relate to
to the Galena limestone as found distributed over the low arch of
the south-central part of the State. As traced northeastward ob-
liquely down the slope of the arch, and into the Michigan strati-
graphical basin, over what may be conceived to have been a broad,
deepening gulf, the deposit gradually changes through the introduc-
tion of clayey material, and a progressive increase in the fineness
and compactness of the limestone material - changres beautifully
consonant with the sub-marine conditions indicated. With this
modification in the nature of the deposit, there was a corresponding
change in the character and preservation of life forms.  The ampler
fauna of the Trenton beds below extended upward, and mingled
with Receptaculites Oweni, Mu rchisorbia manjor, and Lingula qaadrata,
which characterize the Galena proper, thus beautifully illustrating
the palaontological principle that a fauna, apparently cut off in one
region, may be flourishing luxuriantly in an adjacent one.
This merging of one rock into another -though the transition
here is not great -well illustrates the dependence of deposits upon
local marine conditions. It further shows the superiority of strati-
graphical over paleontological evidence in determining the true
correlation of formations where the stratigraphical method is appli-
cable, for this northeastern equivalent of the Galena limestone has
been heretofore uniformly referred either to the Trenton horizon
below, or to the Hudson River above. The same fact will be further
illustrated if the argument of Mr. C. B. Walcott, maintaining that
this is the western equivalent of the Utica shale, shall be sustained
by future investigation.
Metallic Contents. This formation, in the southwestern part of
the State, is rich in lead and zinc deposits. The source and method


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