268 W. Cross — Denver Tertiary Formation. 



II. The Age of the Denver Formation. 



Having given a description of the Denver beds and of their 

 occurrence, there arises the problem as to their geological age, 

 — a problem involving in its own solution that of several others 

 of far greater importance. The evidences to be given touch- 

 ing the age of these beds are very conflicting and are plainly 

 not to be brought into harmony until certain elements of that 

 evidence have been subjected to renewed examination by com- 

 petent hands. It is hoped that the necessity for this revision 

 will be plain from the following discussion. 



There are certain facts of primary importance which have 

 been brought out during this investigation, and these facts 

 must be regarded in all future discussions. From these facts 

 certain apparently logical deductions may be drawn, which 

 should be accepted unless the logic is proven faulty. 



a. Stratigraphical evidence. 



The Denver beds lie between two distinct Formations and 

 the relations to each are definitely known. The upper Forma- 

 tion is of recognized Tertiary age, — the Monument Creek ; the 

 lower is the newly recognized Willow Creek Formation, which 

 will be briefly described by some extracts from the article by 

 Mr. Eldridge, which has already been cited. 



Description of Willow Creek beds. — " The Formation next 

 succeeding the Laramie in geological order and unconformably 

 resting upon it is the lower of the three Tertiaries that occur 

 in the Denver field, for which the name " Willow Creek " is 

 here suggested, from the locality in the southern part of the 

 field, from one to three miles southeast of the mouth of the 

 Platte Canon, where it has its greatest and most typical devel- 

 opment. It is composed of a basal member of conglomerate 

 or gritty sandstone, according to its distance from the foothills, 

 with an overlying zone of gray, argillaceous or arenaceous 

 shales containing lenticular masses of hard, quartzose sandstone, 

 with an occasional ironstone. Where confined between under - 

 and-over-lying groups, it has a thickness varying between 600 

 and 1200 feet. 



" The conglomerate at its base has a thickness over the 

 greater portion of the field of about 200 feet, though this may 

 become the bulk of the formation, as in its type locality, or 

 may decrease to the merest edge as at its northern limit, along 

 the Platte River, near Brighton. It is extremely character- 

 istic, containing as it does pebbles derived not only from every 

 formation that lies below it in the Denver field, but also from 

 others lying far beyond, especially the Carboniferous, of which 

 the debris affords some excellent specimens of Beaumontia, 



