SOURCES OF DENVER MATERIAL. 201 



which consist purely of eruptive materials in Table or Green mountains. 

 This admixture is very largely derived from the soft, friable beds of the 

 Arapahoe, which certainly formed the greater portion of the northern and 

 southern shores of the Denver sea. The observed facts sustaining this view 

 have been given in detail in a previous section of this chapter. 



The conclusions readied in the preceding sections make it necessary 

 to consider what materials might have Keen derived from the eastern shores, 

 which were certainly made up of Laramie strata in great part. Probably 

 the Laramie rucks' exposed were mainly clays or shales, but it would be a 

 difficult matter to prove that similar beds of the Denver series were made 

 up in any great degree of Laramie debris, yet such is perhaps the case for 

 some horizons. In the Denver series are strata scarcely distinguishable 

 from Laramie shales, and though microscopical examination will usually 

 reveal finely comminuted remains of eruptive rocks in these beds, yet the 

 greater part of the substance can simply be denominated clay, of an origin 

 so remote that it can not be determined. 



ERUPTIVE DEBRIS. 



The feature which lias been emphasized as preeminently characteristic 

 of the Denver formation is its composition in so great part of eruptive 

 rocks. These will be fully described in the petrographical chapter. They 

 are there designated, almost without exception, as andesites, though of 

 nearly every variety recognized within this family. The list includes mica, 

 hornblende, augite, and hypersthene rocks, and there is besides a great 

 range in silica and in the alkalies. It is quite possible that basaltic, types 

 may be present in some of the finer-grained layers of the formation, but 

 they have not vet been observed. 



Within the formation the eruptive materials range from bottom to top. 

 The lower half is almost wholly composed of them, and the line between 

 the Denver and Arapahoe deposits must be drawn, when the unconformity 

 is not plain, at the point where the eruptive material appears in the other- 

 wise very similar sediments. While eruptive pebbles and bowlders are 

 abundantly mingled with Archean and sedimentary rocks in the upper 

 horizons, a change in the variety of andesite here represented can be 

 pointed out. 



