60 REPORT UNITED STATES GEOLOGICAL SURVEY. 



quartz porphyry when it is viewed with a very low magnifying power. 

 The amount of aggregate polarization is considerably less than is seen 

 in most of the quartz porphyries. 



In the vicinity of some of the quartz crystal the base has a grayish 

 color, forming a grayish band around the crystal. In some instances 

 this grayish band gradually shades into the body of the crystal itself in 

 such a manner that there is no definii e boundary, suggesting the pos- 

 sibility that the secretion of the crystal from the base had jjroeeeded 

 from the center or axis outwards, gradually absorbing the base, and con- 

 verting it into crystalline substance. 



Between crossed nicols the groundmass shows many spots which are 

 much brighter than the adjoining portions, while between parallel 

 nicols the same spots are darker than the adjoining j)ortions. These 

 spots obviously contain crystalMne matter in some form, and may be in- 

 cipient crystalloids. 



Locality. — Grand Canon of the Yellowstone, at foot of Great Falls. 



1^0. 12. Ehyolite. 



Contains comparatively few crystals of sanidin, and many small crys- 

 tals of quartz. Base consists of granular felsite, with decided aggre- 

 gate polarization. The quartz crystals contain glass cavities. There 

 is no accessory hornblende or mica. 



Locality. — Madison Plateau, opposite the junction of Gibbon Eiver 

 with the Madison. 



!N"o. 13. Ehyolite. 

 Locality. — Mount Sheridan. 



"So. 14. Ehyolite. 

 Locality. — Mount Sheridan. 



IsTos. 15, 16, and 17. Trachyte. 



Much altered, probably by solfataric action. Feldspars much decom- 

 posed, and do not polarize excejjt occasionally in very small portions 

 of the original crystals, and then only imperfectly. Crystals porphy- 

 ritie, the base having a iine-grained felsitic character, very nearly 

 amorphous. Fragments of hornblende very much decomposed and 

 largely replaced with hydrated peroxyd of iron. 



In the green specimen the hornblende shows the ordinary alteration 

 into viridite and epidote. 



Locality. — Junction Valley, near the base of Garnet Hill. 



No. 18. POEPHYRITIC TRACHYTE. 



This rock has the usual characters common in the laccolite tra- 

 chytes ; very large and beautiful crystals of sanidin, showing less 

 than the usual amount of decomposition ; polarizing sharply, many of 

 the larger crystals showing beautifal twins which have the zonal struc- 

 ture. The base is micro- crystalline, and shows a large amount of polar- 

 ization, approaching to that of some granite porphyries. There is con- 

 siderable biotite, also grains of magnetite, disseminated throughout the 

 ground mass. Small crystals of ijlagioclase are present. 



Locality. — The great laccolite, Indian Creek, East Gallatin Mount- 

 ains. 



No. 19. HORNBLENDIC TRACHYTE. 



Locality. — Summit of Mount Chittenden. 



