32 GEOLOGY OF THE DENVER BASIN. 



are an indication that strong long-shore currents setting northward pre- 

 vailed in the Arapahoe Lake, as they did in the Dakota Ocean. 



The material distinctly traceable to sedimentary beds is throughout 

 of subordinate amount in the beds, the bulk of the sediments being derived 

 from the abrasion of the crystalline rocks here classed as Archean. It 

 is noticeable, however, that none of the andesitic debris, which form so 

 important a part of the succeeding series, are found in these beds. 



Vertebrate remains are found in both the conglomerates and the clays, 

 more abundant and better preserved, however, in the latter. They are 

 classed among the Ceratops fauna, which is also characteristic of the 

 Denver beds. The forms found are, however, in general more fragment- 

 ary and less well preserved than those obtained from the latter. 



POST-ARAPAHOE MOVEMENT. 



Between the deposition of the Arapahoe and Denver beds a consider- 

 able time-interval occurred, during which, as the record of the rocks shows, 

 the Arapahoe Lake was drained and the sediments deposited in its bottom 

 were considerably eroded. The movement which caused the drainage of 

 the lake was, as far as present indications go, rather local in its effects, and 

 produced no important deformation of the lake beds already deposited. In 

 the mountain region, however, it was accompanied by outbreaks of ande- 

 sitic lava, which must have completely covered the crystalline rocks in the 

 drainage area tributary to the lake basin. This movement was succeeded 

 after a considerable lapse of time by a depression sufficient to allow of the 

 formation of a second lake in the Denver Basin, and probably of others in 

 the Middle Park region to the west of the mountains and in other parts of 

 the Rocky Mountains. 



The nature of the depression which produced .such lakes without 

 admitting marine waters to any extent within the areas affected is not 

 readilv conceivable, vet its effects are shown to have been widespread 

 by the considerable thicknesses of fresh- water beds, consisting largely of 

 eruptive debris, which are found overlying the Laramie in various portions 

 of the Rocky Mountains, and which are manifestly more recent than the 

 Laramie, yet older than any Eocene deposits hitherto recognized. From 



