456 A. Hague— Age of the Igneous Rocks, etc. 



established, but there is no evidence of its continuance into the 

 Pleistocene. Slight outbursts of lava may have reached the 

 surface at different times after the coming in of the Pleisto- 

 cene, but there is no direct evidence that such was the case. 

 If they did break out they were limited in extent and were 

 only evidences of continued internal heat, such as is shown by 

 the hydrothermal phenomena for which this region is still so 

 celebrated. 



The facts brought together here clearly demonstrate that 

 the pouring out of igneous rocks began with the post-Laramie 

 uplift or closely followed it, and from the time of the first 

 appearance of these rocks, volcanic eruptions continued with 

 greater or less energy throughout Tertiary time. It is evident 

 that from the time of the post-Laramie uplift there was, as 

 shown in the geological history of the region, a succession of 

 events of great importance in the development of the Pocky 

 Mountains, and that each period of this history was character- 

 ized by distinct phases of volcanic phenomena. The great 

 value of paleobotany as an aid in determining the age of the 

 geological formations is singularly well illustrated in the re- 

 gion under discussion. At least five distinct and important 

 geological periods are defined by their fossil flora, and four of 

 them are exposed in the Park within a few miles of each 

 other. Along the abrupt west wall of Mt. Everts, the Laramie 

 sandstones contain several species of plants, sufficiently 

 characteristic to determine the Laramie age of the beds. It 

 has been shown that the Livingston formation resting uncon- 

 formably upon tjie Laramie carries a distinct flora, well defined 

 from the Laramie below and the Fort Union above. 



About ten miles east of Mt. Everts the early acid andesitic 

 breccias contain a varied Eocene flora, referred to the Fort 

 Union formation. At a number of localities within a few 

 miles of Crescent Hill, the breccias afford a special grouping 

 of fossil plants designated as the Intermediate flora, and re- 

 garded as more closely related to the Lamar than to the Fort 

 Union flora, They have been referred to the Lower Miocene. 



Above these latter beds in a great development of basic 

 breccias occurs a flora referred to the Upper Miocene period. 

 It has been named the Lamar flora, and correlated with the 

 Auriferous Gravels of California, with which it is closely 

 allied. 



The following table may serve to bring out more clearly the 

 relationships between the different geological formations and 

 the floras which characterize them : 



