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

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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 older rocks and re- appears 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 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

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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 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.

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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 silicious, containing much sand, chert, drusy quartz, and Even layers of sandstone. One of these, in Dane County, seems to correspond in thickness and relative position with the Jordan sandstone as described by Prof. N. H. Winchell in a recent report upon the geology, of

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Minnesota. 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

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mines; hopes which have not yet 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

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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, gray, 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 a 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, Dodge, Fond du Lac, Winnebago, Outagamie and Shawano ; EastEastward of this line it is concealed by overlying rocks. In some localities it affords a pure white sand suitable for the manufacture of glass, and for other purposes where a pure silex?] is needed.

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