314 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the central part is pilotaxitic, with some tabular microlites of feldspar. At 

 the margin it is brown glass, with prismatic microlites of feldspar. The 

 minute phenocrysts are hypersthene and augite, and there are' very few 

 of hornblende. 



Another parallel dike, about 25 feet wide, consists of hornblende- 

 andesite (1572) with small hornblendes and still smaller feldspars. The 

 rock is porous and fissile. The groundmass is pilotaxitic, with tabular feld- 

 spars and microcryptocrystalline portions. 



The next dike south is parallel to the last and has a hade of 75° SW 

 It consists of compact dark-gray hornblende-andesite, with many large, 

 phenocrysts of hornblende. There are some small vesicular cavities (1573). 

 The groundmass consists of prisms of labradorite with interstitial material 

 that is nearly isotropic, -is somewhat globulitic, and contains magnetite, 

 minute augite, and considerable serpentine, which colors the whole rock. 

 The phenocrysts are brown hornblende, labradorite, and a little augite. 

 It is probable that the serpentine has been derived from hypersthene. 



The most southerly dike has a general northwest trend, is about 5 feet 

 wide, and consists of compact hornblende-andesite with small scattered 

 phenocrysts of feldspar and hornblende. The rock is dark purplish to red- 

 dish, and has weathered to green, crumbling, rounded masses, leaving a 

 "slide" between indurated breccia walls (1574). The groundmass is pilo- 

 taxitic, with much reddish-brown iron oxide. The rocks of these dikes are 

 all andesitic in habit, and are for the most part glassy in thin section; some 

 are holocrystalline. They are probably not as deeply seated intrusions as 

 those at Sylvan Pass. 



MASSIVE FLOWS AND INTRUSIONS OF LIGHT-COLORED ANDESITE. 



There are several bodies of massive andesite whose mode of occurrence 

 has not been fully discovered. They occur at the present for the most part 

 at the summits of peaks, and give evidence of having been surflcial flows 

 that occupied drainage channels on the ancient surface of the country. But 

 in several instances they appear to have been intrusive bodies, since they 

 are accompanied by branching offshoots or apophyses The porous or 

 vesicular nature of the andesite composing these bodies makes it evident 

 that they must have consolidated under little pressure, and that if they 



