450 A. Hague — Age of the Igneous Rocks 



as Sepulchre Mountain. They are, however, best exposed in 

 the northwest corner of the Park on both sides of the Yellow- 

 stone River, where the overlying lavas have undergone the 

 greatest amount of denudation. Here they rest directly upon 

 the Archean rocks of the Snowy Range, and in sharp contrast 

 to the later lavas carry a large amount of crystalline gneisses 

 and schists imbedded in the breccias. On Crescent Hill, just 

 south of the Yellowstone River, these early acid breccias are 

 well shown over 1,000 feet in height, in a steep slope facing 

 northward. The finer tuffs, ashes, and mud flows have 

 afforded exceptionally favorable conditions for the growth and 

 preservation of a rich and varied flora. At Crescent Hill, 

 Elk Creek, and on both banks of the Yellowstone River, be- 

 low the mouth of Elk Creek, large collections of plant re- 

 mains were obtained, which have since been studied by 

 Professor Knowlton. From this abundant flora he has identi- 

 fied 79 species, of which 42 species are either new to science 

 or now specifically determined. Of the remaining 37 species 

 that have been found elsewhere, 17, or nearly one-half of them, 

 are already known in beds belonging to the Fort Union for- 

 mation. Of the remaining species, 5 come up from the 

 Laramie, 5 are found in the Livingston and Denver beds, and 

 11 species are also common to the Auriferous Gravels of Cali- 

 fornia. This flora from the acid breccias is otherwise so 

 closely allied to early Tertiary forms that Professor Knowlton 

 has no hesitation in referring the entire group to the Eocene 

 period, and correlating it with the Fort Union horizon. 



The following characteristic species have been determined 

 from the acid breccias : Sapmdus ojjinis, Cornus acuminata, 

 Populus speciosa, Sequoia couttsice, Asplenium magnum, 

 Taxites ulriki, Populus daphnogenoides, Populus xantholi- 

 iheusis, Betula iddingsii, Fagus undulata, Aralia notata. 

 This reference to the Eocene is in accord with the geological 

 evidence which makes these andesitic lavas carrying the flora 

 younger than the Livingston formation, and among the earliest 

 of the extrusive rocks of the Park. 



Overlying this group of early acid breccias comes a great 

 accumulation, in places several thousand feet in thickness, of 

 volcanic ejectamenta, and also like the earlier rocks, made up 

 of coarse and fine material presenting great differences in 

 texture and physical habit, whose character is determined in 

 great part by their mode of occurrence and the distance from 

 their sources of eruption. Like the early acid breccias, they 

 have been grouped together into one series representing a dis- 

 tinct phase of the volcanic phenomena of the region. Unlike 

 the earlier breccias, however, they are distinctly basic in com- 



