22 



LARAMIE FLORA OF THE DENVER BASIN. 



It is thus a remnant of a once larger formation 

 that has been reduced by erosion. Concerning 

 its stratigraphic relations Richardson said: 



Everywhere the Castle Rock conglojjierate rests on an 

 undulating surface of the underlying Dawson arkose, and 

 there is an abrupt change in texture of tlie material from 

 the medium or fine grained arkose of the upper part of the 

 Dawson to the coarse Castle Rock conglomerate. 



Richardson procured additional vertebrate 

 remains from the Castle Rock conglomerate, 

 which were identified by J. W. Gidley, of the 

 United States National Museum, as Titanothe- 

 rium, and the correlation of these beds with the 

 Chadron formation, the lower formation of the 

 Oligocene Wliite River group, was thus con- 

 firmed. 



More interest attaches to the Dawson ar- 

 kose — that is, the lower division of the "Mon- 

 ument Creek group" — for its exact age had 

 long been in doubt. It has a maximum thick- 

 ness of about 2,000 feet on the west, toward 

 its source in the mountains, and is thinner 

 toward the east. Its appearance was de- 

 scribed by Richardson as follows: 



The formation is a complex aggregate of varicolored and 

 varitextured conglomerate, .sandstone, shale, and clay, de- 

 rived from the rocks of the Front Range and deposited 

 under a variety of continental conditions. They are me- 

 dium to coarse textured arkosic grits, composed chiefly of 

 quartz and feldspar derived from the Pikes Peak granite 

 and associated rocks. * * » Rhyolitic rocks * * * 

 were extravasated on an iineven surface of arkose in the 

 extreme upper part of the Dawson. * * * The strati- 

 graphic relations of the Dawson arkose to the Denver and 

 Arapahoe formations * * * are generally concealed 

 by a cover of Quaternary deposits, so that actual conditionw 

 are obscure. It is not claimed for the recent work that 

 final correlations have been established, but nevertheles 

 previously unsuspected relationships are indicated. 



Approaching the geologically mapped part of the 

 Denver Basin from the south, where detailed work had 

 not pre\-iousIy been done, it was found that the lower part 

 of the Dawson arkose seems to pass along the strike into 

 the Arapahoe and Denver formations; that the Dawson 

 and .Arapahoe can not be separated lithologically, even at 

 the type locality of the Arapahoe, on the bluffs of Willow 

 Creek; and that the Denver and Dawson apparently merge 

 into each other, or interdigitate, layers of arkose typical of 

 the Dawson being found intercalated in andesitic Denver 

 material. These conditions indicate that the .\rapahoe 

 and Denver are equivalent to the lower part of the Dawson 

 arkose. The marked difference in lithology between the 

 andesitic Denver and the arkosic Dawson may be 

 accounted for by the geographic distribution of the rocks 

 which supplied the sediments. 



The paleontologic support of the age deter- 

 mination of the Dawson arkose consists of a 

 number of collections of fossil plants, which 



are pronotmced to be of undoubted Denver 

 age, and a single bone of a mammal, which was 

 identified by Gidley as that of a creodont and 

 of which he said : ' ' From our present knowledge 

 of the oreodonts such a type could not be older 

 than Wasatch." .^Vlthough no dinosaurs were 

 found by Richardson in the Dawson arkose, he 

 referred to their occurrence, as reported by 

 Marsh, in Monument Park. 8 miles north of 

 Colorado Springs, in beds that must b?long to 

 the Dawson. Ceratopsian dinosaurs have been 

 found in the Arapahoe and Denver formations, 

 but not thus far in the Laramie of the Denver 

 Basin. 



In 1912, however, Lee" visited the locality 

 where the creodont bone above mentioned was 

 found (Jimmy Camp Creek, 9 miles east of 

 Colorado Springs) and there collected the 

 remains of turtles and of dinosaurs. The tur- 

 tles were examined by O. P. Hay and the dino- 

 saui-s by C. W. Gilmore. Mr. Gilmore reported 

 on these bones as follows: 



The collection consists of fragmentary ceratopsian bones 

 none of which is sufficiently characteristic to determine the 

 genus to which it belongs, and one ungual phalanx is 

 doubtfully regarded as belonging to the dinosaur Tmcho- 

 don. Two small shell fragments show the presence of 

 hard and soft shell turtles. Of these Dr. Hay says: "The 

 soft shell resembles those of the Lance formation; the other 

 resembles those of the Wasatch but is too fragmentary to 

 be certain of its affinities." 



In commenting on the above statements Lee 

 wrote: 



We have then at this locality, near Colorado Springs, 

 in beds that lie unconformably on the Laramie, a flora 

 that correlates these beds with undoubted Eocene on 

 the one hand, and on the other with the Denver and 

 Arapahoe formations; a mammal of a type not known here- 

 tofore to be older than Wasatch; a turtle whose nearest 

 known allies are in the Wasatch fauna; another that 

 resembles those of the Lance fauna; and dinosaurs that 

 have been somewhat generally regarded as indicative 

 of Cretaceous age but which occur in associations that 

 convince some geologists that they are Tertiary. Con- 

 sidering this association of fossils in connection with the 

 great unconformity, it seems wholly irrational longer to 

 regard a formation as Cretaceous merely because it contains 

 dinosaurs. 



We may now return to the more direct con- 

 sideration of the Laramie of the Colorado 

 Springs region. In 1908 M. I. Goldman " 

 spent some months in the investigation of the 



" Lee, W. T., Recent discovery of dinosaurs in the Tertiary: Am. 

 Jour. Sci., 4th ser., vol. 3.5, pp. 531-534, 1913. 



"TheColorado Springs coalfield, Colo.: U. S. Geol. Survey Bull 381, 

 pp. 317-:i4n, 1910. 



