THE AEAPAHOE FORMATION. [53 



unevenly developed, Locally assuming the nature of ;i uri t <>r even a coarse 

 sandstone. Rarely, however, are the pebbles wanting to such an extent 

 that they cease to be ;i means of distinguishing the horizon. 



The remaining portion of the lower division consists of alternating 

 hands ut' coarse sandstones, conglomerates similarto the one just described, 

 and a few thin, local bands of clay. The prevailing color of this division 

 within the Denver field is gray or white, but smith of the held reds and 

 yellows also appear. There is also present in the south a higher percentage 

 of materia] derived from formations outside the Denver held, notably 

 limestone fragments of Carboniferous age. 



The type locality for the foregoing series of beds is along the bluffs of 

 Willow Creek, .'! or 4 miles southeast of the entrance to the Platte < 'anvon. 

 In passing eastward to the prairie region, however, the conglomerates are 

 gradually replaced by sandstones, which still carry a few pebbles, here 

 unaltered and derived chiefly from the clays and ironstones of the Laramie. 

 In most cases such pebbles have doubtless been transported but compara- 

 tively short distances, though the materials of the matrix in which they are 

 embedded may have been brought from far distant and widely separated 

 sources. The sandstones of the prairies are generally irregular in thick- 

 ness, but rarely exceed 40 feet. 



The changes in sedimentation are clearly shown along the northern 

 edge of the formation. The coarse materials of the western side of the 

 deposit, derived from the Cretaceous and older formations along the neigh- 

 boring lake border, gradually give way, to the east, to the fine sediments 

 of deeper waters, in which there are but few pebbles except those derived 

 from the Laramie floor itself. Along this edge, too, the unconformity 

 between the Arapahoe and Laramie is clearly shown in the disappear- 

 ance of successive beds of clay and sand and in the outliers of Arapahoe 

 sandstone, which rests upon and against the slopes of the Laramie floor. 



THE UPPER DIVISION. 



The shales which constitute this portion of the Arapahoe are distinctly 

 arenaceous, are of light-gray color, and have a maximum thickness of 

 about 600 feet. They contain a few ironstones somewhat similar to those 



