158 GEOLOGY. 



In Colorado the amount of erosion between the epoch of the Lara- 

 mie proper and that of the Arapahoe formation is thought to have 

 been very great. Cross estimates it to have been 14,000 feet. 1 The 

 time involved must, therefore, have been long. Between the Arapa- 

 hoe and the Denver formations there is a lesser, though considerable 

 unconformity, and the interval represented by it witnessed the occur- 

 rence of igneous eruptions on an extensive scale. It was from the 

 lavas extruded at this time that the lower part of the Denver forma- 

 tion was derived. 



Traced eastward, the Denver beds pass beneath Miocene beds. 

 Stratigraphically, therefore, there is no reason why the Arapahoe and 

 Denver formations should not be referred to the Eocene. The fossil 

 plants of the Denver formation, of which something like 150 species 

 have been identified, are consistent with this interpretation. But 

 few species are common to the Denver and Laramie of Colorado, while 

 an equal proportion are common to the Denver and the Eocene of other 

 localities. The meager Arapahoe flora is more closely allied with the 

 Denver flora above than with the Laramie flora below. The inver- 

 tebrate fauna of the Denver beds is little known, and the identified 

 species are common to both Laramie and Eocene. The vertebrate 

 fauna has distinct Mesozoic affinities, and has been the chief reliance 

 in classing the Arapahoe and Denver formations with the Laramie. 

 If the presence of saurian fossils demonstrates the Cretaceous age of 

 the beds containing them, the Arapahoe and Denver beds are Creta- 

 ceous; but every other consideration seems to point rather to their 

 reference to the Early Tertiary. 2 After the deposition of the Laramie 

 below, and before the deposition of the Arapahoe and Denver beds, 

 there were great orographic changes, a long interval of erosion, and the 

 initiation of the protracted period of vulcanism which marked the 

 close of the Mesozoic. These physical changes were accompanied 

 by marked changes in vegetation, and these changes had been accom- 

 plished before the deposition of the Denver beds. The great physical 

 changes which inaugurated the changes in life appear to have taken 

 place before the Arapahoe formation was deposited. Their effects had 

 distinctly modified plant life by the time the Denver beds were de- 

 posited, but they appear to have had less effect on the vertebrate 



'Op. cit., p. 217. 



2 This whole question is well discussed by Cross and others in the monograph cited. 



