CEEATOPSIA LOCALITIES. 183 



Within the area of the Denver Basin the Arapahoe formation rests unconformably upon 

 the Laramie, although along its upturned, western edge the break is recognized only through 

 change in sedimentation. 



The Denver formation, which lies unconformably upon the Arapahoe, is very different 

 from the latter in the composition of its rocks, though textually they are similar. The materials 

 of which the Denver beds are composed may be classed as the debris of Archean, of sedimentary, 

 and of eruptive rocks. The Archean debris, which is confined to the upper portion of the 

 series, consists of bowlders, pebbles, and sands, and is similar to that of the underlying Arapahoe. 

 The material undoubtedly came from the Archean lands lying to the west. 



The sedimentary debris, which is mingled with the Archean in the upper portions of the 

 formation, consists of small sandstone and limestone pebbles derived from the upturned edges 

 of the Mesozoic strata. There are also conglomerate bowlders from the Dakota Cretaceous, 

 while on the plains there is some quartz and feldspar, originally from the Archean but derived 

 immediately from the soft Arapahoe beds. 



The eruptive debris is the most characteristic feature of the Denver formation, especially . 

 in the lower half, though it ranges to the summit of the beds. These eruptive materials 

 are almost entirely andesitic and imply a period of violent eruptions before the beginning of 

 Denver time. 



The total thickness of the Denver is estimated at 1,449 feet. 



Fragmentary remains, which doubtless represent several species of Ceratopsia, have been 

 found in the vicinity of Denver; from the Arapahoe is one identified as Ceratops montanus, 

 that is "the remains of the same reptile or one nearly allied to it." This identification seems 

 hardly possible, as Ceratops montanus is a Judith River type and is vastly older than the Arapahoe. 

 The fragmentary nature of the fossil precludes accurate determination. The type of Tricera- 

 tovs galeus is also from the Arapahoe, but Mr. Hatcher has rejected the species on the ground 

 of inadequate material. 



From the Denver two species are reported, one referred by Marsh provisionally to Tricera- 

 tops horridus, which maybe correct; the other, the type of Triceratops alticornis, by far the most 

 notable ceratopsian from this locality. This specimen was found by Mr. Cannon in the rocks 

 of the largest of the tributaries, which rises on the eastern slope of Green Mountain and enters 

 the Platte River near the Larimer Street Bridge, Denver. The exact locality was about 1 mile 

 from the mouth of the smaller stream. 



RELATIONSHIP OF THE BLACK BUTTES, CONVERSE COUNTY, AND DENVER BEDS. 



Stanton and Knowlton a thus summarize our knowledge concerning the relative age of 

 the "Ceratops beds:" 



Until a few years ago it was the custom to include in the Laramie all of the beds between the Fox Hills and Wasatch 

 formations. In the Denver region the detailed studies of Cross and Eldridge . . . have resulted in the recognition of the 

 Arapahoe (Willow Creek) and Denver beds separated from the Laramie and from each other by unconformities and distin- 

 guished hy marked Iithologic features. A revision of the fossil floras of that region has also shown that the Denver beds 

 •contain a flora composed of species a large proportion of which are not found in the underlying Laramie. . . . 



The Denver and Arapahoe beds have yielded representatives of a remarkable reptilian fauna consisting largely of horned 

 dinosaurs of the family Ceratopsidae. The presence of this family in the Ceratops beds of Converse County and probably at 

 Black Buttes has suggested the very reasonable query whether the beds containing them at these places also are not younger 

 than the true Laramie. The facts we have presented relative to the stratigraphy and paleontology of the Black Buttes 

 dinosaur horizon seem to us convincing that it is in the Laramie and near the base of that formation. It is less than 200 

 feet above the marine Cretaceous, and there is no evidence of a break nor of any abrupt Iithologic change. The character 

 of the flora and of the invertebrate fauna also, so far as the species have a distribution in recognized horizons elsewhere, 

 favors its reference to the Laramie. If the dinosaur bed of Black Buttes is not Laramie, then the Laramie is either absent 

 or is represented only by about 100 feet of sandstone. The overlying beds up to and including strata with a Fort Union 

 flora seem to form a continuous series that is indivisible either structurally or lithologically, and we can see no reason for 

 placing the top of the Laramie lower than the base of the lowest bed with a Fort Union flora. 



Closely similar conditions are seen in Converse County, the principal difference being a greater development of the beds. 

 The sandstones at the base, overlying the Fox Hills, are a few hundred feet thick, and the variable more argillaceous, higher 



a Bull. Geol. Soc. Am., vol. 8, p. 155. 



