736 JAMES H. GARDNER 



the Nacimiento group. Unfortunately, no flora has been discovered 

 from either of these formations. The great break in the character of 

 the fossil vertebrates in passing from one formation to the other, or to 

 the underlying "Laramie" and overlying Wasatch, has long been a 

 puzzle. The beds were thought by Cope to he conformably on the 

 "Laramie" and in turn to be overlain conformably by the Wasatch. 

 This idea was borne out by later authors on the subject and collectors 

 in the field. Not until the work of the writer in 1907, however, 

 had there been any attempt at mapping the formations. It was in 

 connection with tracing the formation boundaries and a study of the 

 physiographic record that the unconformities became evident. 

 Lithologically the Torrejon is not sufhciently distinct from the Puerco 

 to permit of its being readily mapped in the field, the separation being 

 made on fossil evidence. 



In this paper the topography, structure, stratigraphy, and physio- 

 graphic record of the Puerco district have been described for the 

 first time with the exception of a brief reference by the writer in one 

 of the bulletins of the U.S. Geological Survey.^" 



The faunas of the Nacimiento group are unique. The upper 

 fauna, the Torrejon, is known in only one limited area of North 

 America outside of the type-locality. This is east of the Crazy 

 Mountains in Montana. The lower fauna, the Puerco, is not known 

 to occur in America except in the San Juan Basin of New Mexico. 

 Outside of North America, fossils closely related to those of the 

 Nacimiento group have been found in certain districts of Europe 

 and South America, the European localities being confined to France 

 and Belgium, where fossil mammals are known in the early Eocene 

 corresponding closely to those of the Torrejon formation, while 

 fossil mammals more nearly related to those of the Puerco formation 

 are found in Patagonia. 



BIBLIOGRAPHY 



1. Newberry, J. S., 1859. "Report of the Exploring Expedition from Santa 

 Fe, New Mexico, to Junction of Grand and Green Rivers of the Great Colo- 

 rado of the West, under Command of Capt. J. N. Macomb," Geological 

 report by Newberry, printed Washington Government Printing Office, 1876. 



2. Cope, E. D., 1873. "On the Mutual Relations of the Cretaceous and Ter- 

 tiary Formations of the West," ph Annual Report, U.S. Geological and 

 Geographical Survey of the Territories, 431-44. 



