124 Cincinnati Society of Natural History. 



At another point, also near Ripon, and the evident base of 

 the Trenton, there is an intimate mixture of sandstone and 

 calcareous material. In this occur large specimens of Ortho- 

 ceras, which are of Trenton rather than of St. Peter's age. 



At Poraeroy's quarry, near River Falls, Wisconsin, at the 

 top of a high hill, is an exposure of the Trenton limestone in 

 heavy layers, and containing quantities of brachiopods. Be- 

 neath this exposure is an outcrop of the St. Peter's, with a 

 vertical height of about fourteen feet. The sand is dazzlingly 

 white, easily disintegrating on exposure. It forms a con- 

 spicuous feature in the landscape, the white line of outcrop 

 appearing in numerous places along the face of the hill. No 

 fossils were observed. 



On the bluffs east of Prairie du Chien the sandstone occurs 

 above the Lower Magnesian limestone. It is mostly covered, 

 and has an estimated thickness of eighty feet. . In the bed of 

 a stream that comes down the hill, several gigantic steps have 

 been formed, each from five to eight feet high and with 

 " treads " two or three feet wide. No fossils were observed. 



Professor Chamberlin has also noted passage bed as seen in 

 Rock County, Wisconsin. He says "At several points in 

 Rock County the passage of the St. Peter's to the formation 

 above is attended by an alteration of sandstone and calcareous 

 rock. The sandstone just below the calcareous bed is marked 

 with fucoidal impressions, and the base of the calcareous 

 layer contains abundant Scolithus tubes. The calcareous bed 

 is of a greenish-gray cast, containing a large percentage of 

 insoluble, argillo-arenaceous material, in addition to the 

 evident quartzose grains that are more or less freely scattered 

 through portions of it. This has not been observed to attain 

 a thickness of more than four or five feet. The upper 

 portion is usually shaly, and appears at some points to have 

 been eroded before the deposition of the stratum of sandstone 

 above. This latter is thin and mixed with argillaceous 

 material on which sometimes supervenes a thin seam of car- 

 bonaceous matter followed by the fossiliferous Trenton lime- 

 stone. At the most northern point at which the junction was 

 seen, the sand mingles freely with the calcareous layers of 

 the Trenton, for several feet above their base. At most other 

 points the usual abrupt transition was observed."* 



*Geol. of Wisconsin, Vol. II, p. 287. 



