312 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



hornblende is greenish brown. The feldspars are labradorite. There is a 

 little augite in phenocrysts, and some altered hypersthene and magnetite. 



On the peak south of Mount Langford a small dike trending east and 

 west consists of very dark-colored and dense hornblende-andesite, with 

 many small hornblende phenocrysts and none of feldspar (1561). The 

 groundmass is globulitic glass crowded with microlites of feldspar and 

 pyroxene. There are many small phenocrysts of augite, besides those of 

 greenish-brown hornblende. 



The dikes cutting the ridge east of the south fork of Middle Creek 

 have a general trend to the northwest, but vary somewhat and intersect one 

 another. They may be seen traversing the ridges to the northwest and also 

 to the southeast. Other dikes were observed traversing the ridge northeast 

 of the farthest point visited, and it is probable that the country south of 

 Middle Creek is filled with dikes, but none have been mapped in that 

 localit} T . 



The most northerly dike reached on the ridge in question is about 20 

 feet wide, and trends a little south of west, with steep hade to the north- 

 west. It is hornblende-andesite (1562), compact and dark purplish gray, 

 with prominent phenocrysts of hornblende and white feldspar. It splits 

 in plates parallel to the walls of the dike. The groundmass is a crowded 

 mixture of feldspar prisms, brown globulitic particles, and magnetite grains, 

 and is possibly glassy. The hornblende is green, and there is a little brown 

 biotite. The feldspars are probably labradorite. Immediately south of 

 this dike a steeply dipping sheet or dike of hornblende-andesite (1563, 

 1564) caps the ridge for- a distance of three-eighths of a mile. It is at 

 present from 20 to 50 feet thick, but the upper wall has probably been 

 eroded. The dip is to the southeast. At the northern end of the exposure 

 it is relatively dense and prismatic and light gray; at the southern end it 

 is more porous, or almost vesicular and fissile, and is dark gray (1563). At 

 the bottom plane of contact it is dense, bluish black, and glassy (1564), 

 is rich in phenocrysts of hornblende, with smaller feldspars, and carries 

 scattered segregations of coarsely crystalline hornblende and feldspar. The 

 lighter-colored part of the rock mass is noncrystalline, but microlitic, and 

 slightly granular in places. In the darker parts it is pilotaxitic, and at the 

 margin is glassv, consisting- of beautiful brown globulitic and microlitic 

 glass. The phenocrysts are alike throughout — brown hornblende, labra- 



