182 THE CERATOPSIA. 



THE HELL CREEK, MONTANA, LOCALITY. 



This locality" lies in the northern part of Dawson County, Mont., along the canyon of Hell 

 Creek, a tributary flowing northward into the Missouri River about 30 miles above the mouth 

 of the Milk River and 150 miles east of the Judith River localities. The country has an altitude 

 up to 3,000 feet above sea level and consists of grassy table-lands with occasional flat-topped 

 buttes and, in places along the stream courses, wild dissected badlands. 



There seems to be no continuous bone-bearing layer, but occasional localities where speci- 

 mens, mainly fragmentary, may be found, some in joint clay, some in unconsolidated sandstone, 

 and again in concretions so typical of the Laramie formation. 



The American Museum party of 1902 found the remains of thirteen or more skulls, pre- 

 sumably of Triceratops, but all had weathered out and disintegrated but one, which was intact 

 except for the nasal horn core, the nasals, and the distal portions of the supraorbital horn 

 cores, which had weathered away. This specimen (see p. 185, fig. 26) was found on the extreme 

 point of the divide separating Hell Creek from a tributary which entered it from the west 

 about 15 miles from the Missouri River. It was about 35 feet from the bed of the canyon and 

 lay in its natural position. The precise horizon was not ascertained. 



Few of the Ceratopsia found in this region were in concretions, although the party unearthed 

 portions of the skeleton of an enormous theropod dinosaur ( Tyranosaurus rex Osborn) , which 

 was contained in several bluish calcareous concretions of extremely hard, homogeneous texture. 

 The skull, No. 970 of the American Museum, is doubtless referable to Triceratops serratus Marsh. 



In addition, the party secured portions of the skeleton of another specimen at a point 

 about a mile south of the place where the first was found and an equal distance away from Hell 

 Creek Canyon. This was also Triceratops, but the species has not been determined. It was 

 embedded in joint clay, at the base of a large table butte, the upper portion of which had been 

 baked to a terra cotta from the accidental burning out of the lignite seams. 



Two splendid skulls, one probably referable to Triceratops Irevicornus, were found near 

 Hell Creek, in 1904, by Mr. W. H. Utterback, of the Carnegie Museum in Pittsburg. The 

 T. Irevicornus skull lay in soft sand and was in perfect condition. 6 



DENVER LOCALITIES. c 



Ceratopsia remains have been found in the vicinity of Denver, Colo., in beds known as the 

 Arapahoe and Denver, considered to be of post-Laramie age. 



The Arapahoe, the older of the two formations, occupies the site of an ancient lake of 

 considerable extent. 



Along the northern and northwestern edges the formation now appears only as a thin horizontal sheet, or in scattered 

 outliers upon the uneven surface of the underlying Laramie. Along the western outcrop, where the strata are highly inclined 

 and confined between underlying and overlying terranes, the formation is 600 to 800 feet thick. * * * The total thickness 

 of the Arapahoe as originally laid down is undeterminable. 



The Arapahoe is divisible into two well-marked series of beds; a lower, of sandstones and conglomerates, 50 to 200 feet 

 thick, and an upper, of clay, 400 to 600 feet thick. d 



The lower member is composed of debris derived from the underlying Carboniferous, the 

 Triassic, the Jurassic, and from the lower divisions of the Cretaceous, up to and including the 

 Laramie. The shales of the overlying member of the Arapahoe are light gray and arenaceous 

 and contain a few ironstones similar to those of the Laramie. 



The vertebrate remains "occur in the conglomerate along the foothills and in the basal 

 sandstones and overlying clays beneath the prairies." The few specimens from the conglom- 

 erate are worn, while the abundant remains in the clays are finely preserved. 



The Arapahoe formation is distinguished from the Laramie by the sandy nature of its clays, by the comparative paucity 

 of its ironstones, by the generally brighter colors, and by the vertebrate remains. From the overlying Denver the Arapahoe 

 is readily distinguished by the eruptive nature of the material composing the former, e 



a Lull, R. S., Bull. Am. Mus. Nat. Hist., vol. 19, Art. XXX, Dec, 1903. See also this monograph, p. 185. 



6 Ann. Rept. Carnegie Museum for 1905, p. 24, figure facing p. 64. 



c Emmons, Cross, and Eldridge, Geology of the Denver Basin: Mon. U. S. Geol. Survey, vol. 27, pp. 150-254. 



d Eldridge, op. cit., pp. 151-152. 



' Eldridge, op. cit., p. 154. 



