784 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The first plants brought back from this portion of the Yellowstone 

 National Park by the early Hayden survey parties were submitted to Prof. 

 Leo Lesquereux, and although few in number the specimens and species 

 were nevertheless sufficient to afford some indication of their age. Pro- 

 fessor Lesquereux regarded the plants from Elk Creek and vicinity as 

 indicating an Eocene age, and those from the well-known Fossil Forest on 

 the west side of the Lamar Valley as closely allied to those of the Aurif- 

 erous gravels of California. It is a pleasure to state that this adumbration 

 has been abundantly confirmed by the results of more searching study of 

 a larger amount of material, but at the time this was outlined the facts were 

 so few that the suggestions were not regarded as conclusions, and as it was 

 before any careful detailed geological work had been done, these now clearly 

 defined horizons came to be grouped together under the somewhat non- 

 committal term Volcanic Tertiary. 



Although the geology of the region has been fully discussed by Mr. 

 Arnold Hague in Part I of this monograph, it seems necessary, for the satis- 

 factory understanding of the problems requiring solution, to set forth briefly 

 the p-eolodcal features characterizing this portion of the Park which have 



O O O J. 



a direct bearing upon the remarkable flora found. 



In the first place, all the material constituting the beds in which the 

 Tertiary plants are embedded is of volcanic origin. According to the geol- 

 ogists, this material may be divided into two distinct periods of volcanic 

 eruption, which may be distinguished by their mineral composition. 



The older series of these lavas has been designated as the early acid 

 breccias and flows, and the younger as the early basic breccias and flows. 

 Both these series of rocks carry plant remains. In general the matrix in 

 which the plants are preserved is a fine-grained ash, probably deposited as a 

 mud flow, with all the appearance of stratification and other indications of 

 water-laid deposits. Occasionally the material is much coarser and has the 

 appearance of breccias mixed with fine sediments. The acid rocks are 

 usually light in color — yellow, lavender, or gray — while the basic rocks, 

 which carry more iron, are darker in color — frequently some shade of green 

 or dark brown, passing over into black. In some instances, as might be 

 expected with fine water-laid beds", the deposits in both series of lavas 

 closely resemble one another, while the great mass of lava of the two 

 bodies may be readily distinguished. The acid breccias, the oldest of the 



