EARLY BASIC BRECCIA AiSTD FLOWS. 275 



EARLY BASIC BRECCIA AND ASSOCIATED BASALTIC FLOWS. 



This breccia includes all of the darker-colored breccia, with some light- 

 colored breccia, which directly overlies the early acid breccia and which 

 consists mainly of pyroxene-andesites, with some hornblende-pyroxene- 

 andesites and basaltic andesites and basalts. 



By correlation it corresponds to the basic breccia of Sepulchre Moun- 

 tain and that west and south of the Gallatin Mountains. It constitutes the 

 great accumulation of basic breccia that formed the bulk of the volcano of 

 Crandall Basin, including the mountain masses from Index Peak, through 

 those on both sides of Soda Butte Creek to Sloug'h Creek, south through 

 Fossil Forest and Mirror Plateau, to the mountains surrounding the drain- 

 age of Lamar River and the drainage of Crandall Creek. Basic breccia 

 connected with this extends along the mountain range east of the head of 

 the Stinkingwater River and west up to the tributary canyons, and under- 

 lies the summits of the northern half of the Absaroka Range within the 

 boundary of the Yellowstone Park. 



Basic breccia forms the mountains north of Lamar River, including 

 Bison and Druid peaks and the high ridges on both sides of Pebble Creek 

 and Soda Butte Creek, the bedding in all of these masses being' nearly 

 horizontal, with a slight dip toward the south. They appear to be continu- 

 ous with the breccia south of Lamar River, and, as already pointed out in 

 Chapter VII, they may be considered as the outlying base of the Crandall 

 volcano. In these breccias pyroxene-andesite is the prevalent rock, horn- 

 blendic varieties being less common, and basaltic varieties subordinate. 



Associated with these breccias — that is, intercalated in them at the base — 

 are several sheets of basaltic rock, which are exposed in disconnected 

 bodies along the bottom of the valley of Lamar River and Soda Butte 

 Creek. They distinctly underlie the great mass of the basic breccia where 

 they are exposed on Soda Butte Creek and near its junction with Lamar 

 River. They also overlie some of the same kind of breccia. The charac- 

 teristics of these basaltic rocks are sufficiently pronounced to distinguish 

 them from a much more recent, basalt, whose eruption was posterior to the 

 excavation of the present valley of Lamar River, and which will be 

 described later on. The older basaltic rock is found over the acid breccia 

 and trachytic breccia in the vicinity of Junction Butte and about the mouth 



