SOURCES OF DEBTEE MATERIAL. 203 



pebbles appear exceeding .'! inches in diameter. A study of the finer sands 

 and clays shows that they arc largely made up of grains which are simply 

 remnants <»t' pebbles and bowlders of massive andesites, of the types shown 

 in a i\>\\- layers by distinct pebbles. Some layers are doubtless composed 

 of angular grains thai may lie ashes, or the debris of rapidly cooled lavas 

 plunged into water; hut it becomes more and inure evident, through 

 intimate acquaintance with these peculiar strata, that the 900 feet of beds 

 represent a manifold greater mass of lava which has been destroyed in 

 furnishing this material. How great these masses were it is useless to 

 discuss beyond the plain conclusion that they must have been fully sufficient 

 to conceal for a long- period the Archean and sedimentary rocks adjacent 

 t<> the Denver sea. Had the quartzose granites, gneisses, ami other hard 

 rocks of the continental area been exposed during the earlier stages of 

 sedimentation they would certainly have been represented at least by quartz 

 sand in the sandy or conglomeritic layers, and especially in the deposits 

 nearest the shoredine. The gradual appearance of Archean materials 

 and of the older sedimentary rocks upward in the Denver beds of Green 

 Mountain also points clearly to a return to the conditions prevailing in the 

 Arapahoe epoch. 



Since the first description of the Denver beds, at which time the 

 covering of the Archean rocks by vast floods of andesite was advocated, 

 there have been several discoveries that seem to confirm the theory under 

 discussion in a striking manner. The one with most direct bearing upon 

 the case is the ascertaining that in Middle Park, almost directly opposite 

 the Denver Basin, at the western base of the Colorado Range, there exists 

 a sedimentary formation, resting unconformably upon the Cretaceous, 

 which is directly comparable with the Denver formation in lithologic 

 character and in its fossil plants. This formation was represented upon 

 the Hayden maps as "doleritic breccia*' in its lower part, where it con- 

 sists of andesitic breccia, conglomerate, and sandstone, and as "Lignitic" 

 (Laramie) in the upper part, where Archean debris gradually comes to 

 predominance. This series of beds exceeds 2,000 feet in thickness. 1 This 



1 "The post-Laramie deposits of Middle Park, Colorado," by Whitman Cross: Proc. Colo. Sci. 

 Soc., Vol. Ill, 1891. 



