154 ROLLIN T. CHAM BERLIN 



that it is not known that the older sedimentary formations were 

 conformable and once extended continuously over the now uplifted 

 area, but states that in any case the Cretaceous sedimentary 

 rocks are conformable throughout, and in order that pebbles of 

 sandstone from the Dakota and the still older Red Beds could be 

 incorporated in the conglomerate there must have been differential 

 upHft and erosion of more than 4,000 feet. Though making this 

 very safe and modest estimate, Lee has expressed his belief that 

 the actual erosion was probably very much greater than 4,000 

 feet. An estimate upon the Dawson arkose would probably be of 

 similar import. 



Above the Arapahoe in the Denver Basin is the Denver for- 

 mation, whose composition is of much significance. According 

 to Cross the material of the Denver beds, which must have come 

 from the highlands to the west, may be classed as debris partly 

 from the Archean, partly from sedimentary rocks, and partly from 

 andesitic eruptive rocks. ^ The Arapahoe strata contain little or 

 no fragmental material derived from eruptive rocks, while on the 

 contrary the lowest 900 feet of the overlying Denver beds are 

 composed largely of andesitic materials and contain very little 

 granitic or sedimentary debris. Cross says: " The andesitic masses 

 which furnished the materials for the lower part of the Denver 

 sediments were so situated as to effectively prevent the access of 

 all Archean and sedimentary debris to the sea of that epoch. 

 That is to say, the Archean and sedimentary rocks in the moun- 

 tainous area drained by the tributaries of the Denver sea must 

 have been covered by andesitic lava flows so that no material 

 other than the eruptive debris could appear in the Denver sedi- 

 ments, from this, the prominent source, until erosion had laid 

 bare, here and there, small areas of granite, of gneiss, or of sand- 

 stone. " 



More direct evidence of these floods of andesite, the postulated 

 source of much of the material of the Denver sediments, has been 

 found in Middle Park, where there is a thick formation of andesitic 

 breccias and tuffs with conglomerate and sandstone resting uncon- 

 formably upon the Cretaceous, and which is comparable to the 



' Whitman Cross, op. cit., pp. 199-206. 



