W. Cross — Denver Tertiary Formation. 265 



thickness of the Willow Creek and Laramie Formations, end- 

 ing with the coal-bearing basal sandstone of the latter. From 

 this ravine the coal horizon and the characteristic conglomerate 

 of the Willow Creek beds may be traced far in either direction. 



These two important outcrops do not seem to have been seen 

 by any of those persons who have previously described this 

 district, yet they contain the keys to the stratigraphy, without 

 which the latter cannot be correctly interpreted. 



The numerous outcrops of South Table Mountain, which do 

 not need to be specified, supplement those of Green Mountain 

 to a great degree. Here the peculiar constitution of the Den- 

 ver beds may be conveniently studied in detail, though the 

 well-preserved fossil leaves which they contain have hitherto 

 received exclusive attention. 



Clear Creek has not cut quite deep enough at Golden to re- 

 veal the actual base of the Denver beds. This horizon there- 

 fore continues westward from Table Mountain until brought 

 to the surface by the great fold. It is only clearly shown at a 

 point west of the State Reform School, in an old railroad cut- 

 ting which also discloses the Willow Creek conglomerate. 



Mechanical constitution. — The section of the Denver strata 

 consists of two very distinct parts, both as regards texture and 

 composition. By careful measurements projected upon a pro- 

 file line surveyed across Green Mountain, the total thickness of 

 Denver beds there represented is estimated to be 1440 feet. 

 Of this series the upper 525 feet are mainly made up of very 

 coarse conglomerates while the lower 915 feet are as a rule fine- 

 grained strata. The coarse conglomerates and the upper half 

 of the finer-grained beds are preserved only in Green Moun- 

 tain. 



A detailed section of the Denver beds is of little general 

 value because variability in make-up is the preeminent charac- 

 teristic of the finer-grained division. Yellowish brown friable 

 sandstones prevail, with all manner of gradations into clays or 

 into conglomerates. The transitions appear both laterally and 

 vertically so that it becomes difficult or even impossible to 

 closely correlate strata of isolated exposures occurring at the 

 same general horizon. The conglomerates of the lower divi- 

 sion are especially liable to variation, and few of them appear 

 to have more than a local development, yet they are prominent 

 features of almost any extensive outcrop. Few pebbles in these 

 conglomerates exceed three inches in diameter. Cross-bedding 

 and local unconformity between beds of sharply differing con- 

 stitution are further features of common occurrence. 



The heavy conglomerates of the upper division will be re- 

 ferred to later on. 



Materials of the Denver strata. — The peculiar composition 



