THE LOWER SILURIAN ROCKS. 545 



section has also an increased thickness, reaching 45 feet, hut other- 

 wise it shows the characters before noted. Northward, along the west 

 side of Sank county, both layers continue well marked as far as the 

 Bamboo river. Still farther north the Madison, beds thicken to 60 

 feet, are made up of fine-grained, red and white, saccharoidal sand- 

 stone, closely resembling the St. Peters, and have at top one or two 

 feet of cherty quartzite-like material. 



To the list of fossils of the lower sandet3ne series given by Hall, 

 but little has been added by the present survey, as far as Central Wis- 

 consin is concerned. It has already been said that his general group- 

 ing, of upper, middle, and lower species, appears to hold true as re- 

 gards the order, but that his lower species must really be assigned to 

 the middle of the series, since its thickness is about twice as great as 

 Mr. Hall supposed. Fossils are not plenty in the Central Wisconsin 

 Potsdam. In the ordinary non-calcareous rock they occur as mere 

 ferruginous coatings on the loose sand, trilobite fragments being the 

 most common. In the upper shaly layers of the Mendota beds, very 

 large impressions of Dicellocephalus Minnesotensis are quite abund- 

 ant. The pygidium is most frequently found, some specimens meas- 

 uring as much as- six inches across. The same fossil, however, is 

 found in the loose friable sandstones that lie upon the quartzites of 

 the Baraboo ranges, and not improbably has a considerable vertical 

 range, since it is quoted by Hall from the Lower Magnesian of Min- 

 nesota. One new fossil of some interest has been added to Hall's 

 list. This is a very large new species of the genus Palc&acmea, which 

 was originally established by Hall and Whit-field, in 1867, to cover a 

 "conical, patelliform, univalve shell," which occurs in the Potsdam 

 sandstone of New York. The Wisconsin species is twice as large as 

 that from New York, and is found in a very hard quartzite, which 

 occurs interstratified with loose, friable sandstone on the Trempealeau 

 river, in Jackson county, in the middle portion of the Potsdam series. 



The economic contents of the Lower sandstone come under the 

 heads of building stones, glass sand, and iron ores. These are des- 

 cribed here in general only, all details, exact locations, etc., being 

 given in subsequent pages. 



The Madi&on sandstone, in the vicinity of Madison, yields a buif- 

 colored calcareous sandstone which is largely quarried and used for 

 building in that vicinity. This rock contains about ten per cent, of 

 the carbonates of lime and magnesia, is easily cut, and obtainable in 

 large blocks. It has a very pleasing appearance in the fresh state, 

 but has some tendency to darken and become blotched under the ac- 

 tion of the weather. The same rock is opened upon at Middleton, 

 Wis. SUK. 35. 



