154 GEOLOGY OF THE DENVER BASIN. 



of the Laramie. South of the Denver field the gray color is varied by 

 bright vrds and yellows, in this respect a resemblance to the Monument 

 ( !reek formation arising. 



LIFE. 



The only animal remains yet found in the Arapahoe beds are the bones 

 of vertebrates of new and. remarkable types. These occur in the conglom- 

 erate along the foothills and in the basal sandstones and overlying clays 

 beneath the prairies. In the conglomerate but few have been found, and 

 these are more or less worn; in the clavs they are abundant and their 

 articulations, edges, and muscular insertions are sharp and clearly defined, 

 and their cancellous tissue is in complete preservation. Their chemical 

 composition differs but little from that of bone in general, excepting' ill the 

 foreign matters washed into their hollows and interstices. They are found 

 at all horizons in the formation, and occur buried in the clays or sandstones 

 or partially weathered out upon the surface of the prairies. The detailed 

 description will be found in another chapter. 



Plant remains occur in considerable abundance and are discussed by 

 both Messrs. Cross and Knowlton in Section III of this chapter and in 

 Chapter VII. 



DISTINGUISHING CHARACTERISTICS. 



The Arapahoe formation is distinguished from the Laramie by the 

 sand) nature of its clays, by the comparative paucity of 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. From the Monument Creek, 

 owing to the similarity of their materials, it is indistinguishable — unless 

 by its fossils — when, through the absence of the Denver formation, the 

 two come ill contact. 



STRATIGRAPHICAL RELATIONS. 



Within the area of the Denver Basin the Arapahoe formation rests 

 tmconformably upon the Laramie, although along its upturned western 

 edge the break is recognized only through change in sedimentation. Upon 

 the Arapahoe rests unconformably the Denver; upon the Denver forma- 



