of the Yellowstone ParTt. 451 



position and consist mainly of hornblende-pyroxene-andesites, 

 pyroxene-andesites, and basalts. 



By far the greater part of this erupted material is formed of 

 coarse agglomerates, somber in color, with interbedded sheets 

 of compact basalt varying in extent, and of greater or less 

 thickness. These basic breccias cover large areas of country, 

 extending from the Gallatin Range westward across the Snowy 

 Range, along the nothern border of the Park. They consti- 

 tute nearly all the northern portion of the Absaroka Range. 

 Indeed, they may be said to cover the greater part of the 

 northwest corner of the Park. Over this extensive area the 

 basic breccias are only seen in a few localities resting directly 

 upon the earlier acid rocks, but where exposed, they are for 

 the most part easily recognized by their sharp contrast in 

 color, texture, and mineral composition. In places along their 

 contact the acid breccias show evidence of considerable erosion 

 before the pouring out of basic flows, but in other localities 

 they exhibit a transition from one series of rocks to the other 

 and occasionally a mingling of both groups. 



On the ridge about one mile south of Yanceys Station and 

 within a short distance of Elk Creek, and again near Lost Creek, 

 these basic rocks lie directly upon the acid rocks in a series of 

 fine agglomerates and mud flows. The acid rocks present a 

 very hilly and uneven surface, with the basic rocks frequently 

 lying at a lower level and occupying depressions in the older 

 lava. 



Possibly in some instances erosion washed down the softer 

 material from both series of rocks, causing a mingling of both 

 acid and basic lavas, and rendering it impossible to discrimi- 

 nate between beds. Apparently similar conditions prevailed 

 on the opposite side of the Yellowstone River along the north- 

 ern escarpment of Specimen Ridge, extending eastward as far 

 as Crystal Creek, where both series of rocks are again seen 

 together. It is evident that the belt of country including the 

 localities mentioned was* at one time favorable to a vigorous 

 and varied vegetable growth. 



At a number of localities the same flora occurs, carrying 

 well preserved leaf impressions, together with silicified trunks 

 of trees still standing firmly planted in the muds and breccias. 



The flora from this limited area contains 30 species, of which 

 18 are new to science. Only 2 of the new species are found 

 in the acid breccias, and only 3 have been collected from the 

 basic breccias. Nine of the species identified are common to 

 both acid and basic breccias. Of the species previously 

 described as occurring elsewhere than in the Park, most of 

 them have a wide geological and geographical distribution, 

 ranging from the Laramie well up through the Miocene. The 



