246 GEOLOGY OF THE DENVER BASIX. 



In the Livingston area of Montana the known tacts speak for an order 

 of events similar to that of Middle Park. 



The Arapahoe epoch. — As pointed out in tlir preceding section, the Arap- 

 ahoe epoch of sedimentation represents only a part of the time of erosion 

 which followed the pre- Arapahoe uplift. When further identifications and 

 correlations of formations have been made, it may be possible to extend the 

 scope of this epoch to cover the entire period of erosion and contempora- 

 neous sedimentation. 



In mam ways the Arapahoe epoch was probably much more impor- 

 tant than the Denver, though its deposits are less widely identified at 

 present. It is plain that far from shore-lines the sandstones and shales of 

 the Arapahoe might readily be lithologically indistinguishable from those of 

 the Laramie, and if it shall be proven that the pre- Arapahoe uplift extended 

 through Wyoming into Montana, the deposits of the Arapahoe epoch are 

 to lie sought for in these States in the upper portions of the great sections 

 which have been referred to the Laramie, provided these sections are not 

 incomplete through removal of the Laramie before the Arapahoe deposition 

 began. 



The influence of the great pre-Arapahoe uplift upon life existing at the 

 close of the Laramie is unfortunately not yet known to a degree allowing 

 much discussion. The fossil plants found must be intermediate in character 

 between those of the Laramie and Denver, and allied to both. The verte- 

 brates of the Arapahoe are highly modified and specialized types, but when 

 and how they acquired their remarkable characters is not known. If it 

 be true that the Ceratopsidae were not modified by the climatic and other 

 changes of the pre-Arapahoe interval it may well be wondered what 

 caused their sudden extermination. 



pre-Denver volcanic eruptions. — The numerous deposits of andesitic tuff, sand- 

 tone, and conglomerate, which have been described, testify to ernormous 

 outpourings of andesitic lavas all over Colorado and extensively in Mon- 

 tana, and at nearly the same time. It will be evident to all that this is a 

 very remarkable and important event in the volcanic history of the Rocky 

 Mountains. But in judging of this epoch as an element in the general 

 history, it is probable that the chief data available for forming that judgment 

 can lie fully appreciated only by penologists. 



