BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 29, pp. 309-326, PLS. 18-19 JUNE 30, 1918 



AMSDEN FOEMATION OF THE EAST SLOPE OF THE WIND 

 EIVEE MOUNTAINS OF WYOMING AND ITS FAUNA - 



BY E. B. BRANSON AND D. K. GREGER 



{Presented in abstract before the Society December 29, 1916) 



CONTENTS 



Page 



Distribution of the Amsden 309 



Age of the Amsden 309 



Fossil horizons and locations 310 



Section and description of the Amsden 310 



Faunas and correlations .^ 312 



Descriptions of species 313 



Explanation of plates 325 



Distribution of the Amsden 



The Amsden formation was described by Darton^ in 1904 from its out- 

 crops along the Amsden Eiver on the east slope of the Big 'Horn Moun- 

 tains in northern W3^oming. Since then he has described and mapped it 

 along the AVind Eiver Mountains from Circle on the north to south of 

 Dallas on the south, in tlie Eattlesnake Mountains, and in the Owl Creek 

 Mountains, and Blackwelder writes of its occurrence in southern Mon- 

 tana. According to Blackwelder, "it can be followed with more or less 

 confidence clear across the State (Wyoming) from the Black Hills to 

 Idaho." ^ Only the Amsden of the east slope of the Wind Eiver Moun- 

 tains from Bull Lake soutliward is treated in this paper. 



x\ge op the Amsden 



Darton has called the upper Amsden Pennsylvanian, on the basis of 

 fossils collected from near Leo and Shirley, and the lower part probably 

 Mississippian. Blackv^elder-^ has found two faunas in the Gros Ventre 



* Revised manuscript received by the Secretary of the Society April 2, 1918. 

 1 Bull. Geol. Soc. Am., vol. 15, pp. 306-397. 

 -Am. .Tour. Sci., vol. 36, 4th ser., p. 175. 

 3 'Bull. Geol. Soc. Am., vol. 19, pp. 414-415. 



(309) 



