542 GEOLOGY OP CENTRAL WISCONSIN. 



rocks is usually a quite friable, fine to coarse-grained, brownish rock, 

 containing pebbles from the rock below. 



It is not at all impossible, as already indicated, that the anomalous 

 occurrences about the Baraboo quartzite ranges, and in Adams county, 

 may mean that the Lower sandstone really consists of two series, the 

 one, including the ordinary calcareous sandstone that comes beneath 

 the Mendota, and an unknown thickness below, resting upon the 

 eroded surface of the other. Dr. Owen gives sections from the St. 

 Croix region, showing the Lower Magnesian occupying positions 

 lower than the Potsdam in the immediate vicinity, which may indi- 

 cate the same thing. These occurrences on the St. Croix are also 

 described by Dr. Percival in some detail, he considering them best 

 explained by the existentie of several alternations of limestone and 

 sandstone. Still more strongly confirming the idea, are the occur- 

 rences in the vicinity of the Archaean patches at Berlin, in Green 

 Lake county, and Portland, in Dodge county, as described to me by 

 Professor Chamberlin. In the former case, a mound of quartz- 

 porphyry projects into the horizon of the Lower Magnesian, but is 

 flanked by sandstone containing the fossils regarded by Hall as be- 

 longing to the middle Potsdam. In the latter case, several distinct 

 mounds of Archsean quartzite lie in the horizon of the St. Peters 

 sandstone, which shows on the margin of the low ground in which 

 the mounds occur. Flanking the quartzite, however, are layers of a 

 bowlder-conglomerate carrying IScolithus, which is usually regarded 

 as restricted to the lower sandstone. It is quite evident that even if 

 the lower sandstone really does include two formations so distinct in 

 age as these facts seem to suggest, such a division of the series would 

 be quite difficult to demonstrate, on account of the great lithological 

 similarity between the two divisions, whilst, if proved, the separation 

 of the two in mapping would be even more difficult. 



The beds of passage between the Potsdam and Lower Magnesian 

 series include, as already said, two well marked beds, 60 to 90 feet in 

 combined thickness — the Mendota limestone and the Madison sand- 

 stone — which, from their prominence in Central Wisconsin, are 

 worthy of separate mention. For the most part these layers come to 

 tlie surface only on the flanks of the higher levels occupied by the 

 Lower Magnesian, so that they present on the map only narrow bands 

 bordering the areas of the last named formation. In that part of the 

 Catfish valley, however, which lies between the southern shores of 

 Lakes Monona and Kegonsa, they are at the surface over a wide area, 

 whilst in some parts of Columbia county the belt occupied by them 

 sometimes reaches two or three miles in width. Both beds are to be 



