Box 15, Folder 8: Geological Survey, Draft Report 1874

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so as to show their practical value; and it is believed that as the country becomes better known, richer and more Easily worked deposits may be found. The completion of the Wisconsin Central rail=road will bring this now almost inaccessible region into notice.

Roofing slates are quarried in these rocks in Michigan and suitable localities may be found in Wisconsin.

Huronian rocks are found in Ashland, Bayfield, Oconto and Jackson Counties; also, in the form of quartzytes in Sauk Columbia and Marquette. No doubt other localities will be found when these older rocks have been more thoroughly studied. It is believed that they rest non=conformably upon the granytic rocks, as represented on the accompanying section; but this fact has not yet been confirmed by observation within this state.

The quartzytes in Sauk County show ripple marks upon surfaces now standing almost vertically; thus showing that they were once horizontal sand = beaches, that have been hardened and titled up to their present position. They were also at a very early period broken up into boulders and pebbles

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that were again cemented into layers of breccia and conglomerate. Thus boulders were formed long before the glacial or ice=period. By the disintegration of these Conglomerates, boulders are reproduced that originated in Archaean times.

Kaolin or Porcelain clay is found in several localities, resulting from the disintegration of the granyte and the decomposition of feldspar. Some Efforts have been made to use this clay in making fire brick for lining furnaces, which seem to ^have been successful.

Pipe clay similar to that, so celebrated among the red men, from the Coteau de Prairie Minnesota, has been found in Barron County upon lands belonging to the. Cornell University of New York. It was first discovered and described by Mr. A. Randall in 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.

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III Copper= bearing Rocks.

These rocks consisting of "trap," conglomerate and sandstone Extend from the copper mining district in Michigan into Ashland Bayfield and Douglas 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

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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.

It is quite 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 proper time for its deposition should arrive.

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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 animal 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 it often seen in ice on the smaller lakes.

It will be seen by reference to the map that the sandstone occupies

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