ST. PETERS SANDSTONE. 285 



Insoluble in acids, 6.17 per cent., or nearly the entire amount of 

 silica and alumina. 



Messrs. Blish & Barlow manufacture a hydraulic lime from rock 

 belonging to this formation, found in the X. "W. J of Sec. 26, town of 

 Poygan, Winnebago county. 



The following analysis by Prof. "W. "W. Daniells, shows it to be an 

 impure dolomite: 



Carbonate of lime 49 . 747 



Carbonate of magnesia 38 . 189 



Insoluble in acid 9.442 



Sesquioxide of iron and alumina 1 . 587 



Water.. .... 1.190 



The insoluble residue consisted of silica, 5.803, and alumina, 3.639. 

 The manufactured product is used in the vicinity, and in the neigh- 

 boring cities. 



Some of the argillo-arenaceous dolomites, associated with the shale 

 belt of this formation, have a composition approaching very near that 

 of some of the well known cement rocks of the country, as will be 

 seen by reference to the analysis of Layer 6, at Oconto Falls, previ- 

 ously given, which was made with this fact in view, and which de- 

 serves consideration, though the stratum at that point is unfavorably 

 situated. 



ST. PETERS SANDSTONE. 



Upon the billowy surface of the Lower Magnesian limestone, filling 

 up its troughs, and in most cases surmounting the crests of its prom- 

 inences, lies the St. Peters sandstone. This formation has usually 

 been described as a very uniform deposit of purely quartzose, inco- 

 herent, pebbleless, non-fossiliferous sandstone. All these characteris- 

 tics fail in eastern Wisconsin. 



Owing to tne unequal surface of the Lower Magnesian limestone pre- 

 viously described, its thickness, instead of being remarkably uniform, 

 is precisely the opposite. It is known to vary within the district un- 

 der description from two hundred and twelve feet down to a single 

 layer of sand grains. It ranges from zero to one hundred feet or 

 more within a quarter of a mile, in one case, at least, and changes 

 in a similar rapid and remarkable manner at many other points. In 

 the instances of its entire disappearance, its thickness is not sufficient 

 to overtop the prominences of the lower formation, and the Trenton 

 limestone above rests directly upon the Lower Magnesian limestone. 



