448 STUDIES FOR STUDENTS 



This conclusion concerning the age of the Fort Union forma- 

 tion has been supported by the Dawsons, as shown by the fol- 

 lowing : "Dr. G. [M.] Dawson and the writer [Sir William 

 Dawson] have, ever since 1875, maintained the lower Eocene 

 age of our [Canadian] Laramie, and of the Fort Union group 

 ■of the northwestern United States " z 



THE PUERCO FORMATION. 



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." 2 The rocks of this formation consist of "sand- 

 stones and gray and green marls." 3 The formation is thus 

 characterized by Wortman: 4 "The thickness of the beds is 

 roughly estimated at 800 to 1000 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 Annual Report U. S. Geol. 

 Surv., Part II, p. 347). "The lower fossil-bearing strata occur 

 in two layers, the lowermost of which lies within 10 or 15 feet 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." 5 



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." 6 The upper hori- 

 zon is richer in fossils than the lower. "The genera Chirox 

 and Pantolambda appear to belong exclusively to the upper 

 .beds." 7 



1 Quoted by Knowlton, Bull. V, Geol. Soc. Amer., p. 589, 1894. 



2 Clark : U. S. Geol. Surv., Bull. No. 83, p. 138, 1891. 



3 Clark : loc. cit, p. 138. 



* Quoted by Osborn, Bull. Amer. Mus. Nat. Hist., Vol. VII, p. 1, 1895. 

 6 Wortman : quoted by Osborn, loc. cit., p. 2. 

 6 Ibid. 1 Ibid. 



