200 WILLIAM H. EMMONS 



by the Haystack stock. At its contacts with the Haystack stock 

 the breccia is greatly indurated and weathers at some places like a 

 massive rock. On account of its greater hardness this contact at 

 most places forms a ridge. Since the border facies of the Haystack 

 stock is fine-grained, it is impossible at many places to locate pre- 

 cisely the contact, though the doubtful zone is usually not more 

 than a few yards wide. 



Basic andesitic breccia. — The basic andesitic breccia is darker 

 than the acid breccia. Chocolate, dark grays, and somber shades 

 of red and yellow predominate. It is composed of pyroxene-andesite 

 and hornblende-pyroxene-andesite, with a subordinate amount of 

 dacite and latite. Fragments of gneiss or of quartzite are much less 

 abundant than in the lower breccia. The fragments are generally 

 smaller than those in the basal portion of the acid breccia and beds 

 of fine-grained tuff are less conspicuous than in the acid breccia. 

 Basaltic lava flows which are very common, especially near the top 

 of the formation, are best developed in the southwest portion of the 

 area. The thickness of the upper breccia is very great. The canyon 

 of Hell Roaring Creek, several miles southwest of the area mapped, is 

 nearly 3,000 feet deep and is cut entirely in this formation. It is 

 probable that this breccia also was formerly more extensive and it 

 may have completely covered the acid breccia. The basic breccia 

 is of Neocene age.^ 



ANDESITE-DACITE INTRUSIVES 



Andesite-dacite sills. — Andesite-dacite sills occur as sheets injected 

 between the Cambrian strata. In this area they are present wherever 

 a section of Cambrian rocks is exposed. At most places there is 

 a sheet between the quartzite and the limestone at the horizon of 

 the shales which occur at the base of the limestone and several sheets 

 are intercalated within the limestone. The cartographic representa- 

 tion is necessarily generalized, showing from one to three of these 

 sheets, but where the Cambrian beds are thickest there are sometimes 

 more. The maximum thickness of the sills is about two hundred feet. 



The andesite-dacite is dark gray, reddish gray, or brown, and 

 contains phenocrysts of feldspar, hornblende, quartz, and biotite. 



I J. p. Iddings, Folio 30, "Yellowstone National Park," Geological Atlas of U. S., 

 U. S. Geological Survey. 



