FORMATIONS OF THE NACIMIENTO GROUP 727 



The Nacimiento beds probably once extended far beyond the 

 present confines of the Eocene in the San Juan Basin; possibly south- 

 ward toward the Gulf of Mexico and westward into Arizona and 

 Utah. The beds are confined to this basin at present because it 

 owes its origin to structure which was produced certainly in part, 

 and perhaps largely, after the close of Nacimiento time. Subse- 

 quent erosion has removed the formations of this group probably 

 over extensive bordering regions while within the basin they are 

 preserved by a thick covering of Wasatch. The Nacimiento Moun- 

 tains, which limit the basin on the east, were surely elevated after the 

 close of Nacimiento time, since the strata of that group are tilted 

 at high angles and even overturned along its flanks. These moun- 

 tains are of laccolithic origin and both the Puerco and Torrejon 

 undoubtedly at one time continued beyond them. 



FOSSILS 



The fossils thus far found in the beds of the Nacimiento group 

 by Baldwin and subsequent collectors are chiefly vertebrates and 

 in a large measure representatives of the Mammalia. There are 

 several genera and species among the Reptilia and fragments of 

 bones of undetermined specimens of Aves. Of the Mollusca, four 

 fresh-water forms from the Puerco, near the town of Nacimiento, 

 were sent to Cope and described by Dr. C. A. White in 1886.^3 

 Among these forms. White doubtfully determined one species as 

 Unio rectoides which he had found in the base of the Wasatch near 

 Wales, Utah. Unfortunately the Nacimiento has furnished no fossil 

 leaves, the only indications of flora being found in the form of 

 silicified limbs, trunks, and stumps of dicotyledonous and palm trees. 



The following remarks relative to the fossil mammals of the Naci- 

 miento group are quoted from Professor H. F. Osborn:^^ 



POLYMASTODON ZONE (PUERCO PORMATIOn) 



Small archaic mammals evolving from Cretaceous, Jurassic, and Triassic 

 ancestors. Multituberculata, which originated in the Triassic, 3 families. Two 

 orders of archaic ungulates — (i) Amblyopoda-Periptychidae, (2) Condylarthra- 

 Phenacodontidae. Archaic Camivora-Creodonta, 3 families: (i) Oxyclaenidae, 

 (2) Mesonychidae-Triisodontinae, (3) Arctocyonidae {Claenodon protogonoides). 

 Edentata-Taeniodonta, with enameled teeth, 2 families: (i) Stylinodontidae, 

 (2) Conoryetidae. 



