THE 



AMERICAN NATURALIST. 



Vol. xvi. — MARCH, 1882. — No. 3. 



THE TERTIARY FORMATIONS OF THE CENTRAL 

 REGION OF THE UNITED STATES. 



THE principal Tertiary formations of the region between the 

 Mississippi river and the Sierra Nevada are the following, as 

 mainly determined by Dr. Hayden : The Puerco, the Wasatch, the 

 Bridger, the Uinta, the White River, the Truckee, the Loup Fork 

 and the Equus beds. Several of these are again distinctly subdi- 

 vided, 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 palaeontology is not as yet clearly favor- 

 able to further primary subdivision than is indicated by the above 

 names. In the following pages I will briefly describe the charac- 

 ter and distribution of these formations. 



The general history of the succession of the Tertiary lakes of 

 the interior of the North American continent and their deposits 

 has been developed by the labors of various geologists, promi- 

 nent 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 limited ocean communication, studded with 

 islands and bordered by forests. On the west side of the range 

 was a broad continent, composed of mostly marine Mesozoic 

 rocks, whose boundaries are not yet well ascertained. Towards 

 the close of the Laramie, the bed of the great eastern sea began 

 to emerge from the waters, and the continent of the western side 

 of the great range descended. The relations of the two regions 



