202 GEOLOGY OF THE DENVER BASIN. 



With regard to the origin of the eruptive materials it must be stated 

 at the outset that no In/mm source com be assigned with any degree of plausi- 

 bility for any one of the itiiiiiii types represented in these strata. No andesite 



masses arc known in all the wide Archean area to the westward, as far as 

 the continental divide, though it is explicitly admitted that they may exist 

 in local development. Their presence, should they ever be found, would 

 only confirm the conclusions reached without knowledge of them. 



The Arapahoe strata contain no eruptive rocks. Above the clays of 

 the Arapahoe come beds deposited under similar conditions of sedimentation, 

 but which exhibit a complete change in composition. Both of these fresh- 

 water formations must have been of comparatively limited extent, and all 

 the evidence tends to show that the Denver sea was even more circum- 

 scribed than that of the earlier epoch. The source of the materials in the 

 Denver strata must then be sought for comparatively near at hand. The 

 fact that verv little Archean or sedimentary debris occurs in the lower 900 

 feet of the formation adjacent to the foothills not only supports this general 

 conclusion, that the source of the andesites must have been near at hand, 

 but practically determines the further forms of the solution to lie offered. 

 The andesitic masses which furnished the mutt rial* for the lower part <f flic 

 Denver sediments were so situated as to effectually pre rent the access if nil 

 Archean and sedimentary debris to the sea of that epoch. That is to say, the 

 Archean and sedimentary rocks in the mountainous 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 sediments, from this, the prominent source, until erosion had laid 

 bare, here and there, small areas of granite, of gneiss, or of sandstone. The 

 assumption of such a period of eruption for which there are no further 

 known proofs than the existence of these sediments, may seem at first 

 thought somewhat hazardous, but a careful consideration of the facts will 

 show that no less sweeping statement can satisfactorily meet the require- 

 ments of the case. 



In considering this question a factor of the utmost importance appears 

 in the fineness of the earlier sediments of the Denver formation. For 

 900 feet tile beds are sands, conglomerates, or sandy clays, in which t\-\v 



