FORMATIONS OF THE NACIMIENTO GROUP 703 



relationships in the hght of new evidence; to present the first detailed 

 geologic map of the upper Rio Puerco region; to discuss the physio- 

 graphic changes which have taken place in that district between the 

 close of the Cretaceous and the beginning of the Wasatch (Eocene) ; 

 to present a hst of fossil vertebrates with photographs of certain 

 species apparently new to science; and to furnish a bibliography 

 relating to the subjects under discussion. 



HISTORY OF THE PUERCO 



The Puerco formation was first described by Professor E. D. 

 Cope in the Annual Report of the Chief of Engineers to the Secretary 

 of War for the year 1875. His report deals with the geology of that 

 part of northwestern New Mexico examined by him during the 

 field season of 1874 when he discovered the Eocene deposits of what 

 has since been called the San Juan Basin, from the river of that name 

 which crosses it. This basin is in the extreme northwest corner of 

 New Mexico and southwest corner of Colorado (see Fig. i). On 

 this map, the area inclosed by heavy black lines represents that 

 examined by the writer in 1907 and is a miniature of the larger map 

 presented herewith. The area described by Cope in connection with 

 his Puerco formation is mainly along the east side of this district. 



Inasmuch as the original description of Cope is of prime importance 

 in the present discussion it is well here to quote his remarks on the 

 Eocene Plateau or that portion of the San Juan Basin just west of 

 the Sierra Nacimiento Mountains. In the following quotation, the 

 names of formations referred to by Cope as correlated by the writer 

 with those of the present time, are placed in brackets. - 



EOCENE PLATEAUS 



West of the hog-back of Cretaceous No. 3 ["Laramie"] at an interval of per- 

 haps two miles, at a point just north of the Gallinas Mountain, a sandstone bluff 

 [Wasatch] presents a bold escarpment to the northeast. This is the angle of a 

 mass of rock whose eastern face extends southward parallel to the mountain - 

 axis, and whose strata dip first 15° and then 10° south, and soon disappear beneath 

 a similar mass. This series [Wasatch] also presents an escarpment to the north- 

 east, and its beds also dip 10° south, nearly opposite the canon of the Gallinas. 

 This fagade rises to from 600 to 900 feet elevation, and is cleft to the base by a 

 deep gorge, the Canoncita de las Vegas. I traversed this fissure, passing entirely 

 through to the elevated country to the westward. Six miles from its mouth -is a 

 large pool, fed by a spring known as the Mare's Spring. The canon is narrow, 



