W. Cross — Denver Tertiary Formation. 261 



Art. XXIX. — The Denver Tertiary Formation;* by 

 Whitman Cross. 



Introduction. 



It is the desire of the writer to present in the following 

 article a succinct account of a newly recognized Tertiary 

 Formation, which, while of very limited geographical extent, 

 yet possesses characteristics of special importance in several 

 directions. The points of interest to be brought out may be 

 grouped as follows : 



1. The Formation in question occupies a portion of the area 

 about the city of Denver, Colorado, hitherto assigned to the 

 Laramie Cretaceous. 



2. The conglomerates and sandstones of the Formation are 

 chiefly made up of materials derived from a great variety of 

 andesitic lavas of whose outpouring and destruction alike there 

 is no other record now known. 



8. The celebrated fossil-plant beds of Table Mountain, at 

 Golden, belong to the Denver Formation, — hence the taxo- 

 nomic value which has been given to this rich flora must be 

 considered subject to revision. 



4. The vertebrate remains are of individual importance and 

 also present some very remarkable associations, which are 

 apparently in direct conflict with all past observations. 



It must be assumed in this notice that the reader is already 

 more or less familiar with the geological structure of the belt 

 where the stratified rocks of the Great Plains abut against the 

 Archaean foothills of the Bocky Mountains. Especially in Col- 

 orado has this band been repeatedly studied and described in 

 well-known publications, by members of the Hayden and of 

 other Government surveys. 



The principal feature of the region in question is a sharp 

 folding of the sedimentary rocks, in general parallel to the line 

 of contact with the Archaean, so that the larger streams issuing 

 from the mountains expose in their banks more or less exten- 

 sive sections of vertical or of steeply dipping strata, which 

 soon assume a horizontal position under the plains. 



At Golden, twelve miles due west of Denver, the section is, 

 from local causes, exceptionally thin, so that the horizontal 

 beds of Table Mountain, protected by a basalt sheet, approach 

 to within four thousand feet of the Archaean foothills. Mid- 

 way in this interval stand the vertical coal beds at the base of 



* Published with the permission of the Director of the U. S. Geological 



Survey. 



