p.3

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

18 GEOLOGY

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other purposes where a pure silex, is needed.
Though usually too soft to be used as a
building stone, it is occasionally quarried for
that purpose. One quarry near Darlington
in La Fayette county affords a beautiful
brown stone like that from Lake Superior.
The colors are often very bright and arranged
in spots and stripes in a picturesque manner.

Like some other rocks it often assumes
strange forms in consequence of unequal
weathering, affording "wonder-rocks" for
the admirers of wild, rocky scenery.

VII. Buff and Blue Limestones.

These with the next (Galena Limestone)
belong to the lower Silurian age, and to the
Trenton period. They rest directly upon the
St. Peter's Sandstone and are conformable to
it, thus showing apparently a continuous formation;
there being little or no evidence of
any interval between the two. They are
found along the hillsides in the lead-region
but exist in greatest force in Rock County,
and thence northward through Jefferson,
Dodge Fond du Lac, Winnebago, Outagamie,
Brown and Oconto; this being their Eastern-most
limit in Wisconsin. They have together
a thickness of about one hundred and
twenty feet; and are the first to show an
abundant display of animal life. These remains
are all of marine origin showing a deposition
upon the bottom of an ancient Sea.
The existence of plant-life in its lowest forms
is shown, not only by the remains of fucoidal
stems, but by a layer of highly bituminous,
shaly limestone, often so well saturated
with bitumen as to burn with a blaze.

VIII. Galena Limestone.

This rock, belonging to the Trenton period,
is found chiefly in the lead-region in this
and the adjoining States of Iowa and Illinois.
It has not been recognized among the rocks
at the East or at the West of us. It is so
names because it is well developed at Galena
in Illinois, and because it is the rock that
chiefly bears the lead ore (galenite) found in
that district. It has a thickness of about two
hundred and fifty feet. Like all other limestones
heretofore found in this state it is highly
Magnesian (dolomite) and was at first
named the Upper Magnesian Limestone.

Veins of lead, zinc, and copper occur in
great numbers, extending downwards often
into the limestones below but never into the
St. Peters' sandstone. These mines are extensively
worked for lead and zinc.

At the surface, the Galena Limestone becomes
softened, and colored yellow by exposure
to the weather and by the percolation of
water. The yellow particles are washed by
rains into the lower grounds where they constitute
the surface soil from which abundant
crops are raised; thus uniting a mining and
an agricultural interest in this rock. The
Galena Limestone is distinguished by the occurrence
of a peculiar fossil of an obscure nature
having eccentric rows of cells, arranged
much like the ornamental work put upon the
back of a watch. These are aggregated into
a broad disc having the general appearance
of the receptacle of the sun flower. From

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this circumstance it is called Receptaculites.

The rock extends into the eastern part of
Rock County, and the central and northern
portions of Jefferson, but here no lead veins
have yet been found.

IX. Cincinnati Group.

Resting immediately upon the Galena
Limestone is found a series of highly fossiliferous
shales with intercolations of limestone,
heretofore called the Hudson River Group
but now generally known under the name
adopted above; the same rocks with the same
fossils being found at Cincinnati, Ohio, where
they are well developed, and where they
have been thoroughly studied. They have a
thickness, in Wisconsin, of about one hundred
feet. They occur on the slopes of the
several mounds in the lead-region, and at
various other places. It is only where these
soft shales have been protected from denudation,
by the limestone above, that they
have been preserved for our present inspection;
when removed, by natural causes the
unsupported limestone falls often in large
masses.

These shales in some localities have an external
resemblance to the shales of the coal-bearing
rocks, and they have been explored
in the expectation of finding coal. The slightest
knowledge of Paleontology shows that
they were laid down at a period long before
the existence of those peculiar conditions
which gave origin to coal.

X. Clinton Group.

The limestones and shales belonging to
this group form a prominent escarpment in
Dodge, Fond du Lac, Calumet and Brown
counties. They are scarcely to be distinguished
from the Niagara Group above.
The great deposit of oolitic iron ore at Iron
Ridge in the first named county, belongs to
this group, and makes its study one of great
practical importance. Ore of the same character
has been found in the same connection
eastward to New York and southward to Tennessee.
Trace of it are found at various
other places in this state but not yet in such
quantity as to justify the investment of capital
for its extraction.

A large amount of the Iron Ridge ore is
brought to Milwaukee to be mixed with ores
from Lake Superior, by which means iron of
special qualities may be produced.

It is also smelted at the mine and at Mayville,
near by. In quantity it is here, practically,
inexhaustible.

XI. Niagara Limestone.

The rock over which the waters are precipitated
at Niagara in New York, has been
traced through Canada and Upper Michigan
to the islands obstructing [obstructing] the channel between
Lake Michigan and Green Bay; and thence
along the west shore of that lake into Illinois.
It is known throughout this great extent by
lithological characters and by the occurrence
of certain fossils. It is quarried for
building purposes at many places.

Like all the other limestones of Wisconsin
this contains a large percentage of carbonate
of magnesia, being therefore a Dolomite

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rather than a limestone. Some small streams,
falling over these rocks, have worn out channels,
forming miniature representations of the
great falls at Niagara.

XII. Racine Limestone.

The rock found, and quite extensively
quarried near Racine in this state has been
named the Racine Limestone, though perhaps
it should be regarded as the upper part of
the Niagara Group.

It makes quicklime of an excellent quality.
When much exposed to the weather it becomes
yellow and soft. It is characterized
by some peculiar fossils called crinoids, not
found in the layers below.

XIII. Salina Group.

At a single locality on Mud Creek near
Milwaukee is found a very thin-bedded limestone
which has been referred to this Group.
It contains no fossils. Fragments found in
the drift indicate a much greater extent of
this rock than can be ascertained in any
other way. Its stratigraphical relations,
though obscured by drift, are with the Racine
limestone below, and the Devonian limestone
of the Milwaukee river above. Its association,
in other states, with salt and gypsum
makes the study of this formation one of considerable
importance. Only small fragments
of gypsum, have been found; and salt is an
ingredient in most of the waters that have
been analyzed.

XIV. Devonian Limestone.

The newest of the rock-formations proper,
found in Wisconsin, are certain limestones
along the lake shore north of Milwaukee and
in the bed of the river above Humboldt,
which contain traces of fish remains and certain
other fossils showing that they belong to
the Devonian age -- the age of fishes. The investigations
of the state survey, now in progress
will, doubtless, throw some light upon
the character and extent of this group of
rocks. Some of the layers appear to be of
the same age as the water-limestones of Louisville [Louisville?]
Ky., and may have the same useful
quality.

The existence of the Black Shale in the
Drift at Milwaukee shows that we have ascended
almost to the horizon of that highly
bituminous rock.

XV. Drift.

Under this head we may include all those
deposits of clay, sand, gravel and boulders
that are spread out uncomfortably upon all
the before mentioned rocks. They cover
equally the newest and the oldest of the series
and are therefore the last results of that
long contained succession of event that were
instrumental in producing that part of our
great continent included in Wisconsin.

Evidence derived from the existence, in
other countries, of extended series of rocks between
the Devonian and the Drift, shows
that an immense interval of time was interposed,
during all which Wisconsin was dry
land -- subject to the abrading influences of
rains and frosts.

Drift shows itself in three distinct forms;