AGE OF THE AEAPAHOE AND DENVEE. 247 



It is a striking feature of all the deposits of andesitic material that 

 they show a great range of andesitic types, making it probable that a very 

 long series T>f eruptions occurred, in the course of which marked changes in 

 the composition of the volcanic products took place To the petrologisl this 

 variation from basic to acidic extremes within the andesitic group means 

 a long period during which chemical differentiation went on, producing 

 magmas of widely different constitution. The variation of lavas in the 

 Denver beds is much greater than that shown in many of the largest known 

 volcanoes, such as .Etna and the Hawaiian Islands. 



The andesitic eruptions of Colorado can not he regarded as mere 

 interruptions of Arapahoe sedimentation, tor the reason that in Middle 

 Park, <>n the Animas River, and in the Elk Mountains, the beds of volcanic 

 material rest directly on the Laramie, or on an eroded surface, not on any 

 equivalent of the Arapahoe. It is quite possible that no sedimentation took 

 place in these regions from the close of the Laramie epoch until the time 

 of subsidence which led to the Denver and equivalent deposits. To one 

 appreciating the enormous amount of molten material extravasated in 

 this period it must suggest itself that the subsidence which so generally 

 followed the eruptions was in some measure a result of the enormous 

 outpouring. 



The Denver epoch. — The importance of the Denver epoch is to he measured 

 by the thickness and character of its sediments and by its fossils. The 

 strata show the epoch to have been one of subsidence in several localities 

 and to an extent making it probable that large continental areas were 

 involved. In the Denver Basin the remaining beds of this epoch are 1,400 

 feet in thickness; the Ruby beds of the West Elk Mountains are 2, 0(H) feet 

 thick; and Marvine assigns 6,400 feet of strata to his "Lignitic" and 

 "doleritic breccia." These thicknesses are equal to or exceed those of the 

 Laramie in the same districts. 



The materials forming the Denver beds are softer than those of the 

 Laramie sandstones, and much less abrasion, and hence probably a shorter 

 time, is represented in the accumulation of Denver sandstones than in the 

 ease of texturally similar rocks of the Laramie. Hut the time represented 

 by the Denver ami equivalent formations is one of much importance. 



