62 REPORT UNITED STATES GEOLOGICAL SURVEY. 



plagioclase crystals are much decayed ; most of the hornblendes have 

 disappeared; a few sanidins can be detected, but have lost most of their 

 polarizing powers. 

 Locality. — Summit of Grizzly Peak. 



No. 26. Hornblende andesite. 

 Locality. — Tower Falls, in breccia. 



Ko. 27. AUGITE ANDESITE. 



Groundmass consists of small plagioclase crystals and many feldspar 

 microlites, disseminated throughout the base, which show a very large 

 amount of aggregate polarization. The base is exceedingly fine grained, 

 and between crossed nicols is so full of luminous points, which yet can- 

 not be distinctly recognized as possessing definite crystalline form, that 

 the diabase is suggested. On the whole, however, there is probably 

 present a considerable amount of amorphous base. 



Locality. — Yellowstone Canon, south side, 5 miles above Lower Falls. 



No. 28. Hornblende andesite. 



Locality. — Northwest spur of Mount Washburn, in breccia. 

 No. 29. Basalt. 



Has very large crystals of beautifully-striated triclinic feldspar, the 

 lines of striation being wonderfully minute in some of the crystals. 

 Small grains of olivine are tolerably numerous, some of which have been 

 much altered. A few curious crystals of augite and large quantities of 

 amorphous augite are disseminated throughout the mass. 



Locality. — Junction Valley, Elk Creek Meadows. 



No. 30. Diabase. 



Locality. — Gardiner Eiver, above junction of East Fork. 



