THE EOCENE PERIOD 207 



begun by the time the Arapahoe and Livingston formations were 

 deposited. 



To the Early Eocene, the Telluride (or San Miguel x ) and Poison 

 Canyon 2 formations (Fig. 422) of Colorado are commonly assigned, 

 although their equivalence to the Arapahoe of the Denver basin has 

 been suggested. Locally, the Cretaceous had suffered as much as 

 7000 feet of erosion subsequent to the post-Laramie uplift before 

 the deposition of the Telluride formation; 3 but great as this is, it does 

 not exceed the post-Laramie erosion which is thought to have pre- 

 ceded the deposition of the Arapahoe formation (p. 158). The Tellu- 

 ride formation is conglomeratic, and has a maximum thickness 

 of about 1000 feet, while the Poison Canyon formation, of sandstone 

 and conglomerate, is said to attain a thickness of 2500 feet. The 

 assignment of these formations to the Eocene is based on stratigraphy, 

 for neither has yielded distinctive fossils. While both formations 

 have been described as lacustrine, it is not clear that this is their origin. 

 It is difficult indeed to conceive of lacustrine conditions which would 

 permit the accumulation of such thick and extensive beds of conglomerate. 



Another early Eocene formation (Puerco), nearly 1000 feet thick, 

 is found in northeastern New Mexico and the adjacent part of Colo- 

 rado. Its exact age has been the subject of much difference of opinion, 4 

 perhaps because the upper and lower parts of the formation have yielded 

 fossils of different ages. 



All the formations referred to the Fort Union stage of the Eocene, 

 as well as the Arapahoe, Denver, Livingston, Ohio, and Ruby forma- 

 tions, are to be looked upon as representing the transition from the 

 Mesozoic to the Cenozoic. 



The early Eocene sites of deposition were finally shifted. In 

 so far as the sedimentation had been in lakes, the basins may have 

 been filled or warped out of existence, and in so far as the sedimenta- 

 tion had taken place subaerially, the deformative movements of the 

 time, or the progress of the gradational work of the streams, or both, 



1 Purington, Telluride, Colo., Folio, U. S. Geol Surv. This formation formerly 

 called San Miguel, is now known as the Telluride. Bull. 182, U. S. Geol. Surv., p. 36. 



2 Hills, Science, N. S., Vol. XV, p. 417, 1902, and Spanish Peaks and Walsenburg, 

 Colo., Folio, U. S. Geol. Surv. 



3 21st Ann. Rept. U. S. Geol. Surv., Pt. II, p. 99. 



4 Osborn, Bull. Am. Mus. Nat. Hist., Vol. VII, p. 1, 1895. Wortman, Sci., N. S., 

 Vol. VII, p. 852, 1897, and Scott, Sci., N. S., Vol. II, p. 499, 1895. 



