THE POTSDAM. 81 



formation is one of the most remarkable features of American geology. 

 It consists generally of a coarse sandstone, usually reddish in color, <ind 

 often containing beds of conglomerate. In certain regions it is shaly, in 

 a few places it embraces beds of limestone, and in the Far West it often 

 contains beds of quartzite. Where the underlying Acadian formation is 

 absent, the Potsdam rests unconformably on the upturned edges of the 

 Archaean. Outcropping in immense beds in northern Wisconsin, it skirts 

 the Archaean area on the south, and flatdcing the Canadian and Adirondack 

 Mountains, it is found in great thickness along the entire line of the Appa- 

 lachians. Though it is exposed nowhere in the great continental basin 

 between the Alleghanies and the Rocky Mountains, it has been recognized 

 by its lithological characters in the deep borings at Columbus, Ohio, and 

 Saint Louis, Mo., and in central Texas it has been investigated by Dr. 

 Shumard. 



The extension of the Potsdam to the region of the Rockv Mountains 

 was first recognized by Mr. Meek from the fossils collected and observations 

 made by Dr. Hayden during Warren's reconnaissance of the Black Hills. 

 The formation is there very fossiliferous, containing many of the charac- 

 teristic genera of the Potsdam as found in the east — Lingiila^ Lingidepis, 

 OboIeUa, and JJi/oIitcs, with trilobites, fucoids, etc., and from their study the 

 existence of the Potsdam in tlie Far West was first announced by Meek 

 and Hayden in the Proceedings of the Philadelphia Academy of Natural 

 Sciences for March, 1858. Since the time of Warren's reconnaissance the 

 upturned edges of the Potsdam have been found in many places along the 

 flanks of the Rocky Mountains, and though for long distances apparently 

 absent, it is probably in most cases merely concealed by overlapping strata 

 of more recent date. In no place, however, in its western development has 

 it been found so abundantly fossiliferous as in the area of the Black Hills. 

 Commonly in the study of Rocky Mountain geology its presence is recog- 

 nized only by its lithological character and its stratigraphical relations. 



In the Black Hills the Potsdam is the lowest member of the fossilif- 

 erous series, and in its numerous and excellent exposures its character and 

 relations were easily determined and studied. It everywhere rests uncon- 

 formably upon the upturned edges of the underlying Archaean slates and 



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