ii 4 C. W. TOM LIN SON 



Devonian 



Pre- Jefferson Erosion 



Devonian Marine Invasions 



The Upper Devonian Muds 

 The Pre-Mississippian Interval of Emergence 



Depth of Erosion 



"Positive" and "Negative" Areas: Axes of Warping 



PART I 

 FOREWORD 



This paper is the result of three months' field work by the writer 

 in the central Rocky Mountain region during the summer of 191 5, 

 under the auspices of the University of Chicago. For assistance in 

 planning the field investigation the writer is indebted to Dr. Eliot 

 Blackwelder, of the University of Wisconsin, and to Dr. R. D. 

 Salisbury, of the University of Chicago; for aid in the identifica- 

 tion of fossils, to Dr. Stuart Weller, of the University of Chicago; 

 and for helpful suggestions and criticism of the material of the thesis, 

 to all of these gentlemen. 



The problem centered about several broad gaps in the existing 

 knowledge of the Ordovician, Silurian, and Devonian history of 

 western Wyoming and adjacent parts of neighboring states; for 

 example: (1) the " Jefferson limestone" of the Absaroka Range, 

 of Yellowstone National Park, and of southwestern Montana was 

 known to be in part Devonian, but the presence of Ordovician and 

 Silurian strata within this formation and its relation to the Bighorn 

 dolomite of central and north-central Wyoming were matters of 

 dispute; (2) hiatuses were suspected at the base and at the top of 

 the Ordovician- to-Devonian sequence of this region, and at more 

 than one horizon within that sequence, but no physical evidence of 

 any such hiatus ever had been cited; (3) the relation of the Wyo- 

 ming Ordovician and Upper Cambrian to the corresponding systems 

 in northeastern Utah had never been studied by careful strati- 

 graphic comparison; and (4) the relation of the Silurian to the 

 Ordovician in northeastern Utah was unknown. 



By first-hand studies, in one season, of ten complete, and in 

 most cases excellently exposed, sections at strategic localities 

 scattered throughout the area involved, the writer is enabled to 

 throw considerable new light upon all of these questions. 



