of the Yellowstone Park. 449 



This volcanic energy continued throughout a long period of 

 time with intervals of comparative rest, as shown by the evi- 

 dences of erosion of the ejected material, and followed by re- 

 newed activity in the outpouring of fresh lavas. In many 

 localities in the world the age of volcanic flows has been more 

 or less definitely determined by the fortunate preservation of 

 a fauna or flora in some lake beds formed in a depression or 

 basin in the volcanic rocks, and these again preserved by beds 

 of later compact lava or mud flows. These in a number of in- 

 stances determine the age of the underlying and overlying 

 flows, but are of slight value in determining the relative age 

 of other eruptions in the adjoining regions. It is doubtful if 

 any region in the world furnishes a more complete geological 

 record of volcanic eruption from beginning to end throughout 

 so long a period of time as the Yellowstone Park and the ad- 

 joining mountains. 



As regards the problem connected with the relative age of 

 the different masses of igneous rocks, the prosecution of the 

 work has been greatly advanced by the field study of the 

 so-called 'fossil forests and the many localities where a varied 

 flora has been well preserved in the tuffs and waterlain 

 deposits. The mode of occurrence of the beds carrying a fossil 

 flora has been carefully examined and large collections made 

 from time to time. Prof. Lester F. Ward and Prof. F. H. 

 Knowlton passed the season of 1887 in the Park. In the fol- 

 lowing year, Professor Knowlton accompanied me to the field 

 for the purpose of collecting from all the known localities, 

 adding largely to the store of material previously obtained. 

 All these collections were submitted to him for study, and*his 

 results when published will form an important chapter in the 

 monograph upon the Yellowstone Park, soon to be published 

 by the U. S. Geological Survey. 



All references in this paper to the genera and species found 

 here are based upon the identifications of Professor Knowlton. 



The oldest extrusive flows recognized in the Park are a 

 series of light- colored coarse breccias and flne tuffs composed 

 of hornblende-andesite, and hornblende-mica andesite, varying 

 greatly in texture and physical habit, but grouped together 

 under the general description of early acid breccias and flows. 

 It is evident that they were thrown out from different centers 

 of eruption, and in every instance, so far as can be told, they 

 were the earliest eruptions from those recognized centers. In 

 general, these early acid breccias occur as relatively small 

 isolated bodies exposed by erosion of the later rocks. In this 

 way they occupy the bottom of the deep valley of Cache 

 Creek in the Absaroka Range, they are found in the deeper 

 gorges of Tower Creek basin, and are found as far westward 



