p.2

[continuation of article by I. A. Lapham, LL.D., State Geologist.]

GEOLOGY 17

[first column]

Owen's Report; and may hereafter be applied
to purposes of ornament and use.

No limestone has yet been found associated
with the Archaean Rocks in Wisconsin.

Artesian wells have shown that these ancient
rocks probably underlie the whole
state; they slope very gradually towards the
south and west; and abruptly towards the
north and east.

III. Copper-bearing Rocks.

These rocks, consisting of "trap," conglomerate
and sandstone, extend from the
copper mining district in Michigan into Ashland,
Bayfield and Douglass counties in Wisconsin,
where numerous small veins of copper
have been found. Their position is intermediate
between the Huronian and the
sandstone of the Potsdam period; hence
not only the granytic and the iron-bearing
rocks but also these copper-bearing rocks are
to be assigned to a pre-Silurian age; all having
been deposited, metamorphosed, and tilted
into their present, often highly inclined
position, before the deposition of the older
Silurian rocks.

As one of the results of the state survey
now in progress we have the important fact
that the synclinal valley or trough between
Keweenaw Point and Isle Royale, occupied
by the waters of Lake Superior, is continued
inland in Wisconsin, both the north and the
south-dipping strata being there found. This
portion of the trough is filled, not with the
water of the lake, but with Potsdam Sandstone
in horizontal layers, covered with red,
marly drift. White river and the upper
waters of the St. Croix occupy this synclinal
valley; the first running eastward to Lake
Superior the other westward and into the
Mississippi.

On the accompanying section an attempt
is made to represent the relative ages and
geological position of these formations and
also the synclinal here spoken of. This section
is partly ideal, and is intended to represent
the general facts, rather than the local
details which would be out of place here.

[black and white illustration]
L. Superior
Penokie Iron Ridge

Section Across the State. [pencil note] Complete this title

It is quite apparent [apparent] that a very long time
must have elapsed, between the formation of
these Archaean rocks (including under that
name the copper-bearing as well as the iron-bearing
series,) and that of the Potsdam
sandstone resting upon them. During this
time deposits of great thickness may have
been accumulating in other parts of the ancient
world, but none here. Dry land only
existed, and the denuding agencies, then
doubtless more active than now, were already
at work preparing the surface for the reception
of the Potsdam sandstone, when the

[second column]

proper time for its deposition should arrive.

IV. Potsdam Sandstone.

Though this rock is called a sandstone it
includes layers of shale and of shaly limestone.
We find here the first certain traces
of animal and vegetable life known in Wisconsin.
Among the fossils, found chiefly in
the calcareous layers is a large Trilobite, an
animale of the same class as the lobster, and a
Lingula, very much like those living in our
own times.

Wave-marks upon the sandstone show that
the winds and the waves were then at their
work of wearing down and rebuilding continents.
The sandstone everywhere rests upon
the upturned edges of the Archaean rocks or
is abutted against their sloping sides. At
the junction of this sandstone with the trap
rocks of the copper-bearing series it is often
disturbed and broken into irregular fragments;
but these cases admit of explanation
not inconsistent with the supposition of the
older date of the trap. Perhaps they may be
due to the expansion caused by changes of
temperature, as is often seen in ice on the
smaller lakes.

It will be seen by reference to the map that
the sandstone occupies a very considerable
district in the central parts of the state with
branches extending eastward to the Menomonee
river (of Green Bay) and westward to
the St. Croix, having a V shape, the arms
embracing the Archaean rocks. Following
up the course of the St. Croix the sandstone
curves around the elder rocks and reappears
on the south shore of Lake Superior. It has
a thickness of about five hundred feet where
fully developed. Further explorations will,
doubtless, show that it exists in outliers within
the district now represented as Archaean;
and artesian wells have shown its presence
beneath the newer rocks in various places.

Potsdam sandstone is not often sufficiently
indurated to constitute good building stone.
It is easily crumbled by frost and rain; and
swallows have no difficulty in making their
nests in it, in many places. The brown

[black and white illustration]
Level of the Sea

sandstone from Lake Superior is the most
notable exception to this statement. The
disintegrating and wearing agencies have cut
channels of great width and depth through
this rock, along the course of the larger
rivers, reaching in many cases down to the
crystalline rocks. By the unequal effects of
these agencies the rock is often left in sharp,
bold cliffs and isolated standing rocks, presenting
many strange forms, much to the
delight of the photographers. One of these
in Sauk County is called "The nigger-head"
on account of its resemblance to the head of a

[third column]

negro. Its porous nature renders it peculiarly
a water-bearing rock, and there are, doubtless,
many places where the water could be
brought to the surface, as at Sparta, by
artesian wells.

V. Lower Magnesian Limestone.

This rock which is the equivalent of the
Calciferous sandstone of New York, occurs
in large isolated patches resting immediately
upon the Potsdam sandstone, often capping
the tops of hills whose sides are made up of
sandstone. If we follow the southern boundary
of the Potsdam sandstone as shown
upon the map, all the way from the Menomonee
(of Green Bay) to the St. Croix, we
shall have the position of the lower Magnesian
Limestone.

To the south and east it passes under the
next rock. It is highly siliceous, containing
much sand, chert, drusy quartz, and even
layers of sandstone. It is often difficult to
trace the exact boundary between this rock
and the Potsdam sandstone below.

It has a thickness of about two hundred feet.
Some ores of lead and copper have been
found in it, giving rise to hopes of finding
valuable mines; hopes which have not been
realized. In many places it affords a valuable
and beautiful building stone; and it has
been extensively quarried for that purpose in
some favorable localities, especially near
Prairie du Chien. The sandy soils of the
state are largely made up of particles of this
and other limestones, thus yielding a soil of
great fertility and easily worked.

VI. St. Peter's Sandstone.

Above the last named rock is another
sandstone about one hundred feet in thickness,
very uniform in texture, of various
colors, white, grey, yellow, brown and red;
often forming prominent cliffs which are
gradually crumbling into sand. It is the same
that has been called the upper Sandstone to
distinguish it from the Potsdam which was
called the lower Sandstone. It underlies the
lead-bearing rocks in the south-west part of
the state, and its determination therefore becomes

[black and white illustration]
Iron Ridge
Potash Kettles
L. Mich

a matter [matter] of much practical importance.
being the bed-rock in and below which no
workable veins of lead or zinc ores have been
found. Many river valleys have been cut
down through the superincumbent limestones
into this soft stratum of sandstone. It is
traced from near Beloit northward through
the counties of Rock, Jefferson [Jefferson], Dodge, Fond
du Lac, Winnebago, Outagamie and Shawano;
Eastward of this line it is concealed by overlying
rocks.

In some localities it affords [affords] a pure white sand
suitable for the manufacture of glass, and for