HOLMES.] PETROGEAPHIC NOTES. 61 



Is"o. 20. Pkopylite (?) 



This specimen lias many of the characters of a propylite. It consists 

 of a tolerably homogeneous base, holding crystals of plagioclase and 

 orthoclase and an abundance of what was once probably hornblende, 

 but which has been converted into peroxide of iron. Presuming these 

 to be hornblendes, the mode of their occurrence, their dissemination in 

 fragments of fine dust throughout the base, is quite characteristic of 

 propylite. The larger feldspar crystals are in most cases sanidins, hav- 

 ing the zonal structure and often exhibiting Carlsbad twins. The 

 plagioclases are far more numerous, but smaller in size than the sani- 

 dins. Very many of them show a curious fibrous structure, as if the 

 crystals were built up of minute microlitic stuffs, and myriads of these 

 swarm throughout the base. Traces of brown hornblende not wholly 

 decomposed are sometimes seen, but ordinarily the characters of the 

 hornblende are past recognition. 



Locality. — In breccia, Terrace Point, head of Yellowstone Lake. 



Ko. 21. HORNBLENDIC ANDESITE. 



A much- altered rock, externally reddish, differing from the hornblende 

 andesites, which are usually bluish, greenish, and graj'ish; the color 

 is undoubtedly due to weathering or solfataric action. The character- 

 istic color of epidote is seen in macroscopically greenish-yellow spots, 

 which occur abundantly in the groundmass, but do not sensibly afi'ect 

 the reddish tone of the rock itself. Crystals quite small and rare, 

 though the microscope shows great multitudes of small ledge-formed 

 crystals of triclinic feldspar and a great abundance of two varieties of 

 hornblende, dark brown and pale green, the latter being very strongly 

 dichroitic, the former moderately so. The brown hornblendes have the 

 characteristic black shaded border, which is seldom wanting in the 

 hornblende andesites. Green stains, arising doubtless from the decom- 

 position of hornblende, are abundant throughout the base. There are 

 man>' feldspar microlites. The base is for the most part amorphous. 



Locality. — Crossing of Beaver-dam Creek, near head of Yellowstone 

 Lake, in breccia. 



No. 22. HOENBLENDE ANDESITE. 



DiCering notably from the one just described, containing, however, 

 a large number of similar plagioclase crystals, man.v of which seem to 

 be broken and shattered transversely. There is much microfelsitic mat- 

 ter desseminated throughout the base, which polarizes considerably. 

 The hornblende seems to be confined to crystals of notable size. There 

 is an absence of minute hornblende dust disseminated throughout the 

 base. The rock is, therefore, grayish rather than green. The large 

 hornblendes have characteristic black shaded borders. They polarize 

 splendidly and are strongly dichroitic. 



Locality. — Summit of Mount Chittenden. 



No. 23. HOENBLENDE ANDESITE. 



Closely approximating No. 21. 



Locality. — Summit of Mount Chittenden. 



No. 24. HOENBLENDE ANDESITE. 



Locality. — Summit of Mount Chittenden. 



No. 25. HOENBLENDE ANDESITE. 



This is almost wholly a feldspathic rock, much altered by weathering; 

 base highly charged with microlites, which must be plagioclase; larger 



