480 GEOLOGY OF THE DENVER BASIN. 



northeastern Colorado, at Chalk Bluffs and various other localities, and 

 here interesting discoveries of vertebrate fossils have been made. 



Southeast of Denver, the Brontotherium beds appear to be covered by 

 later Tertiary deposits, which may include equivalents of the Oreodon 

 series, although these have not been observed in place much south of the 

 South Platte River. The greater part of these overlying strata are of 

 Pliocene age. 



PLXOHIPPUS BE1>S. 



The most important horizon in the Pliocene is that of the Pliohippus 

 beds, which have a great extension both north and south of Denver, often 

 remaining as table-lands far out on the plains, or as isolated buttes where 

 rhc\ have not been entirely removed by erosion. The great Arkansas 

 divide, especially south of the Smoky Hill River, is mainly composed of 

 these strata, as ascertained by the writer in 1870 and 1871, by personal 

 exploration at various points in eastern Colorado and western Kansas. 

 Northeast of Denver these beds form high bluffs above the Miocene, and 

 in Nebraska and Dakota this is their usual position. Pliocene vertebrate 

 fossils have been found in the vicinity of Denver and are quite abundant 

 at various points in the adjacent regions. The Quaternary and Recent 

 deposits above the Pliocene also contain interesting vertebrate remains, 

 which need not be discussed here. 



These various horizons, marked by characteristic vertebrate fossils, 

 have been determined and named by the writer mainly upon evidence 

 secured during his own explorations in the Rocky Mountain region, begin- 

 ning in 1868. In many of the strata examined no other characteristic fossils 

 except vertebrates were to lie found, and this fact should give the present 

 determinations additional value. 



PART II. 



JURASSIC VERTEBRATE FOSSILS. 



After this brief review of the series of geological horizons, Mesozoic 

 and Cenozoic, represented in the region of Denver, and their chief charac- 

 teristics, it remains to discuss the vertebrate life successively entombed as 

 the various strata were deposited. The extinct animals thus preserved are 

 of great interest in themselves, as they give conclusive evidence of many 



