FORMATIONS OF THE NACIMIENTO GROUP 707 



the great basaltic plateau which, according to Lieutenant Wheeler, constitutes the 

 country south of the Rio Chaco for a great distance, one of little promise to the 

 agriculturist. The season being well advanced, October 22, I thought best to 

 commence the return march, which we accordingly did. 



The soapy marls, or, as they may be called, the Puerco marls, have their 

 principal development at this locality. I examined them throughout the forty 

 miles of outcrop which I observed for fossil remains, but succeeded ir finding 

 nothing but petrified wood. This is abundant in the region of the Gallinas, 

 and includes silicified fragments of dicotyledonous and palm trees. On the 

 Puerco, portions of trunks and limbs are strewn on the hills and ravines; in 

 some localities the mass of fragments indicating the place where a tree had 

 broken up. At one point east of the river I found the stump of a 

 dicotyledonous tree which measured five feet in diameter. 



As already remarked, the Puerco marls belong to the Eocene series in their 

 strict conformability to the superincumbent rocks of that age. They do not 

 appear to represent the Fort Union or Lignite beds of northern Colorado and the 

 North, as they differ in almost every respect. They contain no lignite nor coal, 

 although their occasional black color may be due to a small amount of carbona- 

 ceous matter. They have no resemblance to the Fort Union beds in mineral charac- 

 ter or fossils. I conclude, as a result of the investigation, that the latter formation 

 has no existence in this part of New Mexico. The presence of such quantities 

 of petrified wood gives weight to the probability that the Puerco marls are a 

 lacustrine formation. 



The geography o£ the greater part of the district referred to in the 

 above quotation is shown on the topographic geologic map presented 

 herewith. This map does not extend far enough northward by 

 about seven miles to include the Canyoncita de las Vegas mentioned 

 by Cope; however, that drainageway was identified in the geologic 

 study of the general district and is now shown on the new topographic 

 sheet of the Gallina quadrangle by the U.S. Geological Survey. 



As shown on the map, about five miles west of north of 

 Gallina the base of the Wasatch formation swings away from the 

 Cretaceous hog-backs forming the south boundary of the "extensive 

 horseshoe" referred to in the quotation from Cope where he obtained 

 many fossil remains of vertebrates. He mentions another horse- 

 shoe ten miles south of this also rich in fossil vertebrates; the southern 

 area is about four miles north of what is now known as La Jara P.O. 

 However, the horseshoe here is not formed by the extreme base of 

 the Wasatch as is the case farther north. In the La Jara district 

 the present writer obtained numerous vertebrate teeth and skeletal 

 remains. 



