INTRODUCTION. 



SECTIOl^ I. 



THE TERTIARY FORMATIONS OF THE CENTRAL REGION 



OF THE UNITED STATES. 



The principal Tertiary formations of the region between the Mississippi 

 River and the Sieira Nevada are the following, as mainly determined by Dr. 

 Hayden : The Puerco, the Wasatch, the Bridger, the Uinta, the White River, 

 the Loup Fork, and the Equus beds. Several of these are again distinctly 

 subdivided, and in a few instances such divisions have been regarded by 

 authors as of equal importance with those above mentioned; as, for instance, 

 the Green River portion of the Wasatch. But the evidence of vertebrate 

 paleontology is not as yet clearly favorable to further primary subdivision 

 than is indicated by the above names. I will briefly describe the character 

 and distribution of these formations before entering on the description of 

 the fossils which they contain. 



The general history of the succession of the Tertiary Lakes of the 

 interior of the North American continent and their deposits has been devel- 

 oped by the labors of various geologists, prominent among whom must be 

 mentioned Hayden, Newberry, and King. It may be synoptically stated 

 as follows : 



The Laramie-Cretaceous period witnessed a great difference in the 

 topography of the opposite sides of the Rocky Mountain range. To the 

 east were extensive bodies of brackish and nearly fresh water, with Umited 

 ocean communication, studded with islands and bordered by forests. On 



