270 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



of the Park, and mention the lavas forming- the mountains east of the 

 Yellowstone River, from the northern boundary southward. Those in the 

 vicinity of Soda Butte Creek and east of Lamar River have been described 

 in connection with the dissected volcano of Crandall Basin, but their men- 

 tion again here will serve to make clear the connection of that volcano 

 with others that combined to form the Absaroka Range. In proceeding 

 from the north southward, the rocks encountered will follow one another 

 more nearly according to the order of their eruption, the youngest being 

 found farthest south. 



EAREY ACID BRECCIA. 



The volcanic ejectamenta of the Absaroka Range rest upon crystalline 

 schists and sedimentary rocks in the vicinity of the northern border of the 

 Yellowstone Park. The contact is exposed along the valley of Clark Fork, 

 Soda Butte Creek, Slough Creek, and lower Lamar and Yellowstone rivers. 

 In all of these localities there are exposures of light-colored andesitic 

 breccia, often variegated in color. These represent masses of various 

 dimensions, sometimes very large. They rest immediately upon the schists 

 and sedimentary rocks, and are overlain by dark-colored breccia. In some 

 places the two grade into each other gradually; in others there is a well- 

 defined plane of contact, and evidences of a period of ei'osion, between the 

 deposition of the two breccias. The gradation between the two indicates 

 continuous deposition, or that both belong to a prolonged series of erup- 

 tions, during which the composition of the lavas changed. A precisely 

 similar relation between lower acid and upper basic breccias obtains at 

 Sepulchre Mountain, where the volcanic activity was synchronous with 

 that of the volcanoes of the Absaroka Range. 



Exposures of the early acid breccia are few, and their areas are com- 

 paratively small, in the region about to be described In the vicinity of Junc- 

 tion Butte, immediately over the gneiss there is tuff-breccia of light-colored 

 acid audesite and trachytic rhyolite, quite the same as those west of Yellow- 

 stone River in the neighborhood of Crescent Hill, and undoubtedly part of 

 the same formation. These breccia deposits have been more or less worked 

 over by water and rearranged, and include many fragments of gneiss and 

 schist. Similar lavas and breccia with buff-colored tuff (1025, 1026, 1032) 

 and some massive hornblende-andesite (1024) form the top of the north- 

 western end of Specimen Ridge. The trachyte is brecciated with lumps of 



