152 GEOLOGY OF THE DENVEB BASIN. 



determined. The position of the formation beneath the plains, east of its 

 upturned western edge, is either horizontal or slightly undulatory. The 

 total thickness of the Arapahoe as originally laid down is indeterminable, 

 either because of recent erosion or by reason of the removal of much of 

 its material prior to the deposition of the overlying formations, or from the 

 uneven floor presented by the Laramie. 



STRATIGRAPHY. 



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 

 da\ . 400 to 600 feet thick. 



THE LOWER DIVISION. 



The sandstones and conglomerates constituting the lower division of 

 the formation are especially well developed along the upturned portion 

 next to the foothills. They are here generally about 150 feet thick and 

 appear at intervals either as low projecting combs of rock or as streaks of 

 weathered-out, pebbly debris upon the surface along their, line of outcrop. 



The lower 40 feet is prevailingly a coarse conglomerate, consisting of 

 material derived not only from every formation of lower horizon in the 

 Denver Held, but also from other formations beyond. In this material are 

 white sandstones belonging to the Laramie and to the Dakota; fragments 

 of coal and pebbles of silicitied wood from the Laramie; clay-ironstones, 

 which might have come from the Laramie or from lower divisions of the 

 Cretaceous; limestone pebbles from both Niobrara and Jura; numerous 

 and most characteristic pebbles of the hard, cherty conglomerate at the base 

 of the Dakota; red sandstones, which might have come from the Jura, and 

 others that certainly represent the Triassic Red beds; and, lastly, silicified 

 limestone pebbles from the Carboniferous, some containing excellent speci- 

 mens of Beaumontia. Besides the debris of recognized sedimentary origin, 

 vari-colored jaspers, flints, agates, and silicitied woods exist in profusion. 

 SUicificatioD. is, indeed, a marked peculiarity of the coarser material of 

 the Arapahoe conglomerates; this process seems, however, to have affected 



only the pebbles, the sandy matrix of the conglomerate and the sandstones 



proper showing no evidences of it. The conglomerate is somewhat 



