«..v,w,.i GEOLOGY ROCKS EAST OF FRONT RANGE LIGNITIC. 107 



So much for the general character and extent of the testimony thus 

 far presented from the marine life found in or near the base of these 

 beds. 



Terrestrial vertebrate remains, however, occur under neither so equiv- 

 ocal nor restricted circumstances, but such as have been fonnd range 

 higher in the formation, and are considered as of decided Cretaceous 

 types, and, judging from them alone, the formation would be considered 

 as Cretaceous.* 



It must be supiDosed, then, that either a Cretaceous fauna extended 

 forward into the Eocene period, and existed contemporaneously with an 

 Eocene flora, or else that a flora, wonderfully prophetic of Eocene times, 

 anticipated its age, and flourished in the Cretaceous period to the ex- 

 clusion of all Cretaceous xjlant-forms. Which of these views is correct 

 is still the undecided point. Though the latter might seem the more 

 probable view, if judged by the rapidity of faunal changes as com- 

 jjared with floral changes in the more immediate past, yet not only the 

 great amount, but the remarkable unanimity of the evidence of fossil 

 botany, as interpreted by Lesquereux, would indicate the former. New- 

 berry's interpretation of the facts from fossil botany, however, again 

 leans toward the latter view. In either case the fact remains that here 

 the ijhysical and other conditions were such that one of the great king- 

 doms of life, in its progress of development, either lost or gained uidoq 

 the other, thus destroying relations and associations which existed 

 between them in those regions from which were derived the first ideas 

 of the life boundaries of geological time, causing here apparent anom- 

 alies. 



Much of the confusion and discrepancy has, in my opinion, arisen 

 from regarding difi'erent horizons as one and the same thing. It must 

 be distinctly understood that this group as it exists east of the mount- 

 ains in Colorado is very difi'erent from and must not be confounded with 

 the horizon in which much of the Utah and New Mexican lignite occurs, 

 and which belongs undoubtedly to the Lower Cretaceous; and, further, 

 that the extended explorations of Hayden and others would seem to 

 prove almost conclusively that the Colorado lignitic group is the direct 

 southern stratigraphical equivalent of the Fort Union group of the 

 Upper Missouri,! which is considered generally to be no older than the 

 Eocene, while Newberry asserts it to be Miocene.f When all the facts 

 are known they may develop some new ideas as to geological transi- 

 tions. 



But only the consideration of large areas of country, as Hayden, Les- 

 quereux, Meek, Cope, Leidy, Marsh, and others have already done^ will 

 solve the problem. Here we have to do only with the aspects of the forma- 

 tion, whatever may be its age, as it occurs within our district, and for 

 the present we will speak of the formation as the lignitic. § 



The nature of the rocky fold found along the mountain front, and of 

 the subsequent erosion, has been such that in passing eastward over 



* See Cope, Trans. Amer. Philos. So. 1869, pp. 40, 98, 243 ; also BuUetiu No. 1 of tlie 

 U. S. Geol. and Geog. Surv. of the Terr., 1874, p. 10, and Bulletin No. 2, p. 7. 



t Equivalence founded on similar fossil vertebrates has recently been suggested by 

 Cope. (See Bulletin No. 2 of the U. S. Geol. and Geog. Surv. of the Terr., p. 7.) 



t Hayden's annual re^iort, 1870, pp. 95, 96. 



$ The most complete r(^s!(?n^ of the evidence bearing upon the age of the lignite forma- 

 tion has probably been given by Professor Lesquereux in Hayden's last report, 1872, 

 pp. 339-350. See also Newberry, in Amer. Jour. Science, April, 1874, (III, vol. vii., p. 

 90,) and Lesquereux, Ibid., June, 1874, (III, vol. vii. No. 42, p. 546.) See also CoYud, 

 Bulletin No. 2 of U. S. Geol. and Geog. Surv. of the Territories ; Meek in Haydeu's 

 report, 1872, p. 401, &c. 



