34 GEOLOGY OF THE DENVER BASIN. 



About midway in the period, or after 500 to 600 feet of beds had been 

 deposited, several successive flows of basaltic lava were poured out upon 

 the sea-bottom and rapidly covered by deposits of sand and tuff. This 

 eruptive action had no traceable connection with that which produced the 

 andesites, but proceeded from fissures in the strata of the plains. Although 

 most of the flows were poured out upon the surface of the sea-bottom, 

 some small sheets were evidently intruded between the strata already 

 deposited. The conclusion drawn by Mr. Cross from his most complete 

 and thorough study is that in the long period during which the lower por- 

 tion of these beds was being deposited the Archean and sedimentary rocks 

 in the area from which they were derived were entirely covered by flows 

 of andesitic lavas, but that toward the end of the' period these lavas had 

 been almost entirely worn away. Beds composed largely of coarse ande- 

 sitic material, resting unconformably upon upturned Cretaceous rocks, are 

 found in the Middle Park, on the western side of the Colorado Range, 

 which, as well from their contained plant remains as- from this similarity 

 of constitution, are evidently of the same period with the Denver beds. 

 Somewhat similar beds of andesitic de'bris are reported from the north- 

 eastern portion of the South Park. It seems probable, therefore, that the 

 andesitic flows must have covered the mountains lying between these 

 two depressions, but singularly enough no remnants of these flows have 

 yet been observed on them. It must be added, however, that as no system- 

 atic survey has yet been made of this interior region it is by no means 

 certain that some may not yet be found. 



The vertebrate remains found in both the Arapahoe and Denver beds 

 are considered by the paleontologists to whom they have been referred to 

 have Cretaceous rather than Tertiary affinities, and so high an authority as 

 Prof. O. C. Marsh is decidedlv of the opinion that, in spite of the evidence 

 of the two physical breaks and the long time-interval that must have 

 intervened, both Arapahoe and Denver formations are properly to be 

 considered a part of the Laramie. It is to be remarked, however, that in 

 the Denver Basin these vertebrate remains are not found in the coal-bearing 

 rocks here classed as Laramie; neither is there as yet any certain evidence 

 that this fauna existed prior to the Arapahoe and Denver periods. 



