WALCOTT.] WISCONSIN. 173 



left. 1 On the Upper Menomonee River the sandstone has a thickness 

 of 150 feet, and the entire thickness is estimated at 200 feet. 2 



In giving an account of his investigations in connection with the 

 geological survey under the direction of Messrs. Foster and Whitney, 

 Prof. J. Hall states: 



I have also satisfied myself that the sandstones of the Upper Mississippi are of the 

 same age as the Potsdam sandstones, and that the Lower Magnesian limestone of the 

 Western geologists is identical with the calciferous sandstoue of New York, the next 

 member of the series above the Potsdam sandstone. The thin bed of sandstone suc- 

 ceeding this rock can not be identified as the Potsdam sandstone by itself, but must 

 be regarded as a repetition of the arenaceous deposits below, which likewise alter- 

 nate with the calciferous sandstone near its base. 3 



" He thus corroborates the view of Dr. D. D. Owen that the lower sand- 

 stone of Wisconsin is the equivalent of the ^ew York Potsdam. 



In 1851 M. B. Desor called attention to the presence at St. Croix of 

 a fossiliferous sandstone 800 feet beneath that described by Professor 

 Hall, iu which he found fossils. 4 



In a paper on the paleontology of the lowest sandstone of the north- 

 west Dr. D. D. Owen states that he observed multitudes of Lingulas 

 and Orbiculas disseminated in strata abutting against the southwest 

 side of the trap range that crosses the St. Oroix River at the falls. 

 This was in 1847. During the remainder of this season and in that of 

 1848 he found finer-grained and more laminated soft sandstones, with 

 subordinated silico-calcareous layers, charged with Obolus, like those 

 which characterize the lowest sandstones of Russia. This was in a 

 section on the Mississippi, between the Falls of St. Anthony and the 

 mouth of the Wisconsin River. From the results of these observations 

 he finally developed beneath the Lower Magnesian limestone at least 

 six different trilobite beds, separated by from 10 to 150 feet of intervening 

 strata. 5 



In the final report on the geology of the Upper Mississippi Valley, the 

 same writer describes the lower sandstone as composed of light-colored 

 quartzose sandstone that forms the greater part of the formation, with 

 intercalations of magnesian limestone, especialty toward its upper part, 

 where it graduates into Formation No. 2, and at certain localities argil- 

 laceous, and other beds of a mixed character form a considerable por- 

 tion of its lower mass. 6 In a table of the elementary stratification of 

 the lowest Protozoic sandstones he places the Lake Superior ferrugi- 



1 Remarks upon the section from the falls of Wolf River, through Navarino to Lake Michigan. 

 Rept. on the geology of the Lake Superior land district, hy Foster and Whitney, part 2, 1851, p. 174. 



2 Whittlesey, Charles : The dip, hearing, and thickness of the Silurian groups. Rept. on the geology 

 of the Lake Superior land district, hy Foster and Whitney, part 2, 1851, p. 183. 



s (Geologioal investigations on Dramraond's Island and the north shores of Lakes Huron and Michi- 

 gan, etc.) Am. Acad. Proc, vol. 2, 1851, p. 254. 



4 1 Potsdam sandstone on the river St. Croix. J Boston Soc. Nat. Hist. Proc, vol. 3, 1851, p. 202. 



6 On the paleontology of the lowest sandstones of the Northwest. Am. Assoc. Proc, vol.5, 1851 

 pp. 169-172. 



6 Geol. Surv. of Wisconsin, Iowa, and Minnesota., and incidentally of a portion of Nehraska Territory, 

 Philadelphia, 1852, p. 48. 



