CONTENTS 381 



Page 



Greenhorn limestone 431 



Carlile shale 431 



Niobrara formation 432 



Pierre shale. 433 



Resume . 434 



Cambrian 434 



Ordovician 434 



Silurian-Devonian 435 



Lower Carboniferous 435 



Upper Carboniferous and Red beds , 436 



Sundance formation 441 



Morrison formation , 442 



Lakota- Dakota series 443 



Benton group 444 



Niobrara formation 446 



Pierre 447 



Fox Hills 447 



Laramie 448 



Introduction 



For the past six years I have been engaged in detailed studies of the 

 geology of the Black hills and a portion of the Bighorn mountains. 

 Considerable attention has also been given to the sedimentary formations 

 exposed along the Rocky Mountain front range in Wyoming and Colo- 

 rado. Some of the results of the work in the Black hills have been pre- 

 sented in previous papers and informal accounts have also been given 

 of observations in the Bighorn and Rocky Mountain regions. In the 

 present paper I propose to give a brief presentation of the salient strati- 

 graphic features of the formations in the different areas and point out 

 the regional variations which the formations present. 



I think all geologists will be greatly impressed by the wonderful uni- 

 formity in most of the formations throughout the region to which this 

 paper relates, a uniformity which indicates widespread unity of condi- 

 tions in a succession of deposits presenting great variation in character. 

 There are relatively thin sheets of sandstone which underlie many thou- 

 sand square miles, apparently at constant horizon, limestones and shales 

 of still wider extension, red beds marking a period of very special condi- 

 tions of wide extent and long duration, and later the great mantle of Ter- 

 tiary clays and sandstones covering the Great Plains. The interpretation 

 of the geologic history of the great series of formations can only be out- 

 lined in a general way with our present information, especially as we 



