210 GEOLOGY OF THE DENVER BASIN. 



various materials identified by him in the beds, but a brief recapitulation 

 of the facts may not be out of place. ■ 



Aside from the debris of granite and gneiss, which must be expected 

 in coarse sediments adjacent to a continent largelymade up of those rocks, 

 the Arapahoe conglomerate contains pebbles from various Mesozoic forma- 

 tions. Pieces of coal and friable sandstone were found in a few places. 

 These may be considered as derived from the Laramie, which also seems 

 the probable source for pebbles of fossil wood. Clay ironstone and certain 

 dense earthy limestones are most plausibly from the Colorado and Montana 

 Cretaceous, while hard, almost quartzitic, white sandstones may be referred 

 to the Dakota. That the latter contributed to the Arapahoe sediments is 

 most conclusively established by the pebbles of the very characteristic fine- 

 grained conglomerate at its base. Red sandstone and certain limestone 

 pebbles of the Arapahoe are probably derived from Jurassic strata, as they 

 are not known to occur in any higher horizons. There are also present 

 pebbles apparently of the Triassi'c sandstones and of cherty masses 

 containing * Jarboniferous Beaumontia. 



In distinction from the Denver beds the Arapahoe strata contain no 

 pebbles of volcanic rocks, and by their constituents above mentioned they 

 are markedly different from any other horizon below them in the foothill 

 section. This lithologic character is evidence of important stratigraphic 

 relations to be further considered in a succeeding section. 



Volcanic materials of the Denver beds. TllC sliai'p distinction tO be draWU IX'tWeeU 



the lithologic characters of the Denver and earlier sediments has been 

 fully described and emphasized. While the volcanic types represented in 

 the pebbles of the Denver beds all belong to the andesites, as tar as 

 observed, it is to lie pointed out that the variety within that very large 

 group is great, and indicates a source of supply which must have been 

 long in accumulation. Whether from a single great volcano or from sev- 

 eral different centers these various andesite lavas may safely be considered 

 as the products of a very long period of volcanic activity which did not 

 begin until after the (dose of the Arapahoe epoch. The basaltic magmas 

 of the Denver epoch, preserved in Table Mountain, may belong to the 

 close of this cycle of eruptions. 



