348 THE CAMBRIAN. [bull. 81. 



to form a very narrow band surrounding the "Archean" area. Though 

 essentially a sandstone formation it has some variety of composition. 

 Usually it carries a conglomerate at the base, but this is sometimes ex- 

 changed for a dense quartzite, and in many places there are interstrati- 

 fied beds of quartzite. In a few localities the formation is quite calca- 

 reous and in a great number it contains peculiar greensand deposits. 

 Its thickness is quite uniform, ranging generally from 200 to 250 feet, 

 but attaining to 300 feet on the north branch of Eed water Creek. Com- 

 plete sections from the "Archean w to the Carboniferous are rarely met 

 with, but one measured in the eastern canon of Spring Creek by Mr. 

 Jenney is as follows : l 



Archean. 



Feet. 



(1) Argillaceous slates, dipping 60° west 



Potsdam, 



(2) Brownish yellow conglomerate, with quartz pebbles, resting unconformably 



on 1 and clipping 25° northeast 25 



(3) Reddish brown sandstone, thinly bedded at base and alternating with soft, 



shaly sandstones, containing large fucoids, Lingulepis, and fragments of 

 trilobites 200 



Carboniferous. 



(4) Reddish brown, or pinkish calcareous sandstone, thinly bedded, containing 



Spirifera camerata, cyathophylloid corals, and crinoid columns 20 



(5) Limestone: Spirifera camerata, Productus, etc 335 



Many details are given of the local phenomena presented by the sedi- 

 mentation and mode of occurrence of this formation ; extended com- 

 parisons are made between it and the correlated sandstones about the 

 Big Horn and Wind Kiver Mountains of Wyoming, the sandstones of 

 the Mississippi Valley, and the typical Potsdam saudstone about the 

 Adiroudacks of New York. 



The following is a list of the fossils found in the sandstone and 

 identified or described by Prof. K. P. Whitfield: 2 



Palseochorda prima. Lingulepis dakotensis. 



Palaeophycus occidentalis. Obolus ? pectenoides. 



Palaeophycus. Obolella polita. 

 Scolithus. nana. 



Lingulepis pinuaformis. Crepicephalus centralis. 



cuneolus. planus. 



perattenuatus. 



Prof. F. E. Carpenter describes the u Potsdam" in the Black Hills as 

 follows : 



The present site of the uplift existed as a slowly sinking island in the Potsdam 

 ocean, probably the last surviving land of a vast area eroded and submerged by 



1 New ton, Henry : Geology. Report on the Black Hills of Dakota. TJ. S. Geog. and Geol. Survey 



of the Kooky Mountain region, J. W. Powell in charge, 1880, p. 88. 

 *Op.cit.,p. 107. 



