756 INDEX TO THE STEATIGKAPHY OF NORTH AMERICA. 



The Animas beds, as this post-Laramie formation may be caUed, are to be regarded as a 

 most direct equivalent of the Denver beds, identical in pecuHar lithologic character, lying 

 between typical Laramie and Puerco, and containing fossils which, so far as known, indicate 

 a similar fauna and flora. 



The Tertiary of the northwestern part of the San Juan Basin is outlined in a 

 recent map accompanying a report on the coals of the region, in which M. K. Shaler ^^ 

 thus states the relations of the Tertiary and Cretaceous: 



The Animas beds outcrop only in a narrow belt along the north border of the field, thinning 

 rapidly both east and west from Animas River. If present at all in the central part of the 

 area, they are hidden by the Puerco marl, which north of Pueblo Alto apparently overlaps the 

 Cretaceous rocks. Unconsolidated white sand, unquestionably of Puerco age, here lies in 

 contact with an irregular surface of Laramie coal, immediately overlying the basal sandstone 

 member of that formation. At this place the outcropping Laramie is only about'200 feet thick. 



The upper and lower limits of the Puerco marl are difficult to determine in the area of flat- 

 lying beds near Rio Chaco. There is nearly everywhere a marked gradation between these 

 rocks and the Laramie, although the main body of the Puerco is easily distinguished by the 

 presence in it, of fossil mammal remains and silicified wood. 



The available notes on the stratigraphy of the Puerco and Torrejon have been 

 well summarized by James Hervey Smith : ''" 



The Puerco formation is located in northwestern New Mexico at the headwaters of Puerco 

 River, from which the formation takes its name, and where it "reaches a thickness of outcrop 

 of about 850 feet."'' The rocks of this formation consist of "sandstones and gray and green 

 marls."" The formation is thus characterized by Wortman: '' "The thickness of the beds is 

 roughly estimated at 800 to 1 ,000 feet, and as far as can be observed they lie conformably upon 

 the Laramie." 



The fossils occur at two horizons which are separated by barren strata 700 to 800 feet 

 thick (not 30 feet as erroneously quoted by Dall in the Eighteenth Ann. Rept. U. S. Geol. 

 Survey, pt. 2, p. 347). "The lower fossil-bearing strata occur in two layers, the lowermost of 

 which lies within 10 or 15 feat of the base of the formation. This is succeeded after an interval 

 of about 30 feet by a second stratum in which fossils are found. * * * Both of these strata 

 are red clay, and at no place did we find them more than a few feet in thickness." "^ 



This horizon "is especially and sharply distinguished by the occurrence of the remains of 

 Polymastodon, which appear to be entirely absent from the upper horizon." '^ The upper 

 horizon is richer in fossils -than the lower. " The genera Chirox and Pantolambda appear to 

 belong exclusively to the upper beds." ' 



Wortman believes that the upper fossiliferous horizon contains several layers, and that 

 their vertical range is somewhat greater than that of the lower horizon. Matthew states that 

 the "Upper and Lower Puerco beds do not contain a single species in common, and only three 

 or four genera pass through. The two faunas are entirely distinct. Dr. Wortman proposes 

 . to call the upper beds the Torrejon formation, retaining the name Puerco for the lower beds." "^ 

 Scott Qorrelates the Puerco with the Cernaysien of Europe.* 



Osborn ^^^* briefly describes the Puerco, 500 feet (Polymastodon zone), as 

 basal Eocene, characterized by small archaic mammals; and the Torrejon, 300 feet 

 (Pantolambda zone), as the equivalent of a part of the Fort Union formation of 

 Montana, also still characterized by descendants of Mesozoic types. 



a Clark, W. B., Bull. U. S. Geol. Survey No. 83, 1891, p. 138. 



6 Quoted by Osbom, H. F., Bull. Am. Mus. Nat. Hist., vol. 7, 1895, p. 1. 



" Wortman, quoted by Osborn, op. cit., p. 2. 



<i Science, new ser., vol. 6, 1897, p. 852. 



« Scott, W. B., Science, new ser., vol. 2, 1895, p. 499. 



