200 GEOLOGY OF THE DENVER BASIN. 



some extent by vegetation, and that they were incapable, under any condi- 

 tions that can be considered probable, of furnishing rock materials which 

 could have more than a local influence upon the deposits in the sea. Hence 

 in some subsequent paragraphs the western land area is for the time being 

 treated as the only source for the coarser materials, at least, which it is 

 necessary to consider. 



The materials may be classed as the debris of Archean, of sedimentary, 

 and of eruptive rocks, and they will be taken up in this order. 



ARCHEAN DEBRIS. 



The sands, pebbles, and 1 >owlders derived directly from Archean sources, 

 which are found in the Denver formation, are similar in kind to those of the 

 underlying Arapahoe strata, and came undoubtedly from the land areas to 

 the westward. But such materials are practically limited to the upper part 

 of the series, and when they appear the large size of the bowlders and the 

 marked angular form of the sand grains in the cementing matrix become 

 characteristic features. 



SEDIMENTARY ROCKS. 



Mingled with the Archean debris of the upper portions of the formation 

 are small pebbles of distinctly recognizable sandstones and limestones, di- 

 rectly comparable to the material of beds which may be seen in the adjacent 

 upturned sections of Mesozoic strata. In company with these are many peb- 

 bles or bowlders of a conglomerate, quickly and surely recognizable as de- 

 rived from a certain layer at the base of the Dakota Cretaceous. All of these 

 types are also found less abundantly in the conglomerate layers of the Arap- 

 ahoe formation. It has been shown during the discussion of the preceding 

 formations, that the western shore-line of each of them must be considered 

 as having been near the base of the present foothills. Hence when debris 

 from the Dakota or Jura is found in either the Arapahoe or Denver conglom- 

 erates, it must be assumed that the included fragments came from adjacent 

 exposures of the already somewhat folded strata of Mesozoic age. We do 

 not know of any other source of such materials, and do not need to seek any. 



In the Denver strata of the plains a subordinate amount of quartz and 

 feldspar originally derived from Archean sources is a feature of horizons 



