FORMATIONS OF THE NACIMIENTO GROUP 735 



SUMMARY 



The beds comprising the Nacimiento group were discovered by 

 Professor E. D. Cope^ in 1874 on the head drainage of the Puerco 

 River in the San Juan Basin, northwest New Mexico. In 1875, 

 these beds were thought to have been identified on the opposite 

 side of the basin in New Mexico by Dr. W. H. Holmes.^ The same 

 year Dr. F. M. Endhch' working in the San Juan Basin north of 

 the New Mexico boundary hne correlated with the Puerco a series 

 of beds along the Animas River in southwest Colorado. These corre- 

 lations of the Puerco by Holmes and Endlich were based entirely 

 on lithologic resemblance and stratigraphic position. The beds 

 spoken of by Holmes^ in his Pinyon Mesa section may be either Puerco 

 or younger, while the beds described by Endlich^ and shown on 

 Hayden's Preliminary Map of Central Colorado are now known by 

 the writer to be identical in larger part with the Animas formation 

 of Dr. Whitman Cross.34 



The name Torrejon was proposed by Dr. J. L. Wortman in 1897 ^^ 

 for those beds previously known as the upper part of the Puerco, 

 but which contain species of fossil vertebrates totally different from 

 those of the lower beds, subsequently known as Puerco proper. The 

 discovery of the Torrejon fauna was due to Dr. Wortman's observa- 

 tions in the field in 1892 ^7 together with paleontologic records kept 

 by the American Museum of Natural History. 



The Puerco and Torrejon formations have not been identified 

 over wide areas. All the fossils collected from these formations 

 came from hmited districts in northwest New Mexico until recent 

 years. Neither of the two formations had been positively identified 

 beyond this region, nor similar fossils found elsewhere, until 1901. 

 That year Mr. Earl Douglass^^ discovered Torrejon vertebrates 

 near Fish Creek of the Musselshell River, Montana. The U.S. 

 National Museum has since made extensive collections of these fossils, 

 but no typical Puerco fauna has yet been discovered in North America 

 outside of the San Juan Basin in northwest New Mexico. As has 

 already been set forth, the "Laramie" and the Nacimiento of the 

 San Juan Basin were each followed by a stratigraphic break, involv- 

 ing a considerable erosion-interval and marked faunal change. 

 An unconformity of less importance separates the two formations of 



