610 E. BLACKWELDER ORIGIN OF BIGHORN DOLOMITE OF WYOMING 



GEOLOGIC AGE 



Early students 7 of the Wyoming geologic formations referred the Big- 

 horn dolomite to the Xiagaran epoch on the basis of a few corals, such 

 as Holy sites catenulatus, which were then believed to indicate Silurian 



age. 



In connection with the work of Darton in north-central Wyoming 

 Ulrich 8 has studied the Bighorn fauna, meager though it is, and has con- 

 cluded that it is all of Ordovician age, according to current usage by 

 which the Bichmond is assigned to the Ordovician period, but that several 

 of the eastern horizons, including the Lower Trenton and the Bichmond, 

 are represented. Other collections from the Wind Biver, Gros Ventre, 

 and Teton ranges have lately been submitted by me to Mr. Ulrich and all 

 have been referred to the Ordovician. 



On the other hand, I recently found in the Gros Ventre Bange, in rocks 

 which I confidently refer to the Bighorn, a small collection of fossils con- 

 taining not only the usual corals and fragmentary brachiopods and mol- 

 lusks, but also several species of trilobites. This faunule is referred to 

 the Bichmond fauna by Mr. Ulrich, but in the estimation of both Dr. 

 Kindle and Dr. Stuart Weller it is of Xiagaran age. In view of this 

 apparent conflict of evidence and the disagreement among the students 

 of the faunas, the age limits of the Bighorn must for the present be con- 

 sidered unsettled. 



(ti^xeral Characteristics 



The massive ledges and the huge talus blocks which in most case^ have 

 fallen from them generally afford plenty of opportunity for examining 

 the character of the Bighorn dolomite. A first inspection is sufficient to 

 impress one with the unusual roughness of its weathered surface, a fea- 

 ture which has been repeatedly described by Darton and others. In suc- 

 cessive beds there is considerable variety in the pattern, as an inspection 

 of plates 29-33 will serve to show. One of the commonest phases in 

 eastern Wyoming shows a network of raised welts about half an inch wide 

 (see plate 33 and illustrations in Darton's papers). Many other ledges 

 are covered with.roughly conical projections which have waxy dark brown 

 points, contrasting with the dense cream-colored matrix surrounding 

 them. Again, the surface may be covered with branching stalklike fea- 



7 Orestes St. John : Hayden Survey. Ann. Kept. vol. xi. 1ST7, and vol. xii, 1878 

 J. B. Comstock : Expedition to Rocky Mountains and Yellowstone Park D S Armv 

 Engineers, 1873. ' 



• N. H. Darton : Geology" of the Bighorn Mountains of Wvoming. TJ S Geological 

 Survey, Professional Paper No. 51, 1906, pp. 28-29. 



