POTSDAM SANDSTONE. 



267 



from whioli descends a steep slope covered with debns that doubtless conceals the Mad- 

 ison sandstone, below which there comes in a series of impm-e Umestone layers. These 

 are thin bedded, inclined to be shelly, of earthy fractm-e, soft and quite ar^aceous, the 

 almninous material forming shaly pai-tmgs. Below this follow a group of arenaceous 

 shales and shaly sandstones, chiefly of greenish and purple casts, whose structure is 

 somewhat changeable as traced horizontally. Oblique lamination is most beautifully 

 shown in some portions. Underlying these are heavy beds of calcareous sandstone, of 

 yellowish color, and rather uniform, firm texture, below which lie purple, u-on-stamed 

 arenaceous shales, succeeded by calcareous sandstone similar to that above. The expos- 

 ure reveals a low axis, the strata dipping both east and west from its center. 



Sugar Loaf, on the opposite side of the Bay, discovers essentially similar strata. Sur- 

 mounted by like Lower Magnesian ledges, its talus-covered slope of 60 feet is succeeded 

 by alternating layers of the Mendota group. 



Limited outcrops of the Potsdam strata occur at several points in Green Lake county, 

 which cannot here be specially described for want of space, but which the accompany- 

 ing maps will enable any one to identify, who may desire to do so. 



At Berlin, adjacent to the porphyritic ledges, a coarse silicious sandstone occurs, con- 

 taining, imbedded in it, fragments of the porphyrite, often of large size. These frag- 

 ments are sometimes well worn and rounded, but ofteijer angular. Fortunately these 

 beds are also fossUiferous, and the following species have been identified from the col- 

 lection made by Mr. F. H. King: Paleophycus, Sp. und., Orthis Pepiiia, Scolithus, Con- 

 ocephalites diadematus, C, minor, C, Gihhsi, n. sp., Dicellocephalus Misa. These show 

 that this sajidstone, conglomerate and breccia, is of the Potsdam age. The position at 

 which these occur in the western part of the state would indicate that these beds belong 

 to the middle portion of the formation, though the elevation as compared with that of 

 the Lower Magnesian hmestone, which occurs a httle to the east, is such as to lead to 

 the behef that it belongs to a higher horizon. 



The conglomerate and breccia were undoubtedly formed by the beating of the waves 

 against the adjoining porphyrite chffs, which formed a rocky island in the ancient ocean. 



From this point northward the Potsdam beds are almost whoUy concealed by drift, 

 except as a few feet are exposed here and tiiere at the base of the Lower Magnesian 

 ledges, that mark the western limit of that formation. Such exposures occur in the 

 towns of Winchester, Caledonia, Mukwa, Hortonia, Ellington, Cicero, Shawano and 

 Gillette, but nowhere is more than a few feet of the upper part of the formation shown. 



Near the " Big Bend " of the Oconto river, the bluffs on the south side are crowned 

 with impure limestone very meagerly exposed, below which the Potsdam sandstone oc- 

 casionally shows itself. About two nules below the bridge, the " Flat Rock " is formed 

 by beds of quartzose sandstone, stretching across the river and forming gentle rapids. 

 The rock is grayish white, mottled with yellow, and composed of well rounded grains of 

 transparent quartz of varying size, cemented with a Httle calcareous matter. The beds 

 dip gently to the southeast. The face of the layers, as they cross the river, is pitted with 

 " pot holes " not often exceeding the size of the homely utensil that gives them a name, 

 in some of which the gravel is stiU eddjong about, continuing the process of formation. 



At Little Falls on the Peshtigo river, a lower portion of the formation is presented, 

 consisting of white friable sandstone, composed of nearly uniform, weU rounded gi-ains 

 of quartz, with very httle cementing material. The upper beds are tliick and massive; 

 below these, the layers are thinner and softer, beneath wliich again are thicker beds. 

 The falls owe their origin t» this irregularity. 



Several miles down the river in Sec. 12, T. 31, B. 20 E., there is a low exposure con- 

 taining Scolithus tubes, and representing a higher horizon. 



The formation barely demonstrates its presence where it crosses the Menomonee river 

 into Michigan. 



