THE MIDDLE PALEOZOIC STRATIGRAPHY OF THE 

 CENTRAL ROCKY MOUNTAIN REGION 



C. W. TOMLINSON 



University of Chicago 



PART II 



STRATIGRAPHY — Continued 

 UPPER CAMBRIAN AND EARLY ORDOVICIAN 



Members o and i. — -Throughout western Wyoming, and at least 

 as far north as Livingston, Montana, the thin-bedded upper part 

 of the Gallatin formation rests upon a very massive cliff-making 

 dolomite (Member i), ranging up to 400 feet or more in thickness. 

 Near Three Forks this member constitutes the "mottled limestone" 

 (middle division of the Gallatin formation) of Peale, 1 which was 

 correlated by Weed 2 with the Pilgrim limestone of the Little Belt 

 Mountains. It corresponds roughly in position and in general 

 character to the Hasmark formation, which was differentiated by 

 Emmons and Calkins 3 in the Philipsburg quadrangle, Montana, and 

 tentatively correlated by them with the Pilgrim. 



Underlying this massive member in Wyoming and Montana is 

 a zone of weaker strata (Member o) in which green shales predomi- 

 nate, interstratified with thin beds of dolomite and flat-pebble 

 limestone conglomerate. This belt includes the "Obolella shales" 

 of Peale 4 in the Three Forks quadrangle, and at least the upper 

 part of the Gros Ventre shale of Blackwelder 5 in western Wyoming; 

 and is probably represented in the Park shale of Weed 6 in Montana. 



1 A. C. Peale, "Description of the Three Forks (Montana) Sheet," Geol. Atlas 

 U.S., Folio 24 (1896). 



2 W. H. Weed, "Geology of the Little Belt Mountains, Montana," U.S. Geol. 

 Survey, 20th Ann. Rept., Part 3 (1900), PI. 40, opp. p. 284; p. 286. 



3 W. H. Emmons and F. C. Calkins, "Geology and Ore Deposits of the Philips- 

 burg Quadrangle, Montana," U.S. Geol. Survey, Prof. Paper 78 (1913), pp. 57 _ 59, °3- 



4 Op. cit. 5 Eliot Blackwelder, unpublished manuscript. 6 Op. tit., p. 286. 



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