74 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



In only a few of the rocks are all of the minerals unaltered, or nearly 

 fresh. In most cases the hornblende is completely decomposed, while 

 biotite is generally unaltered in most of them. The feldspars are unaltered 

 in nearly all the rocks examined. The least altered rocks were found in 

 the Gray Mountain mass (146, 147), in a heavy sheet in the gulch on the 

 southwest slope of Electric Peak (191), and in the sheet forming the western 

 summit of Electric Peak (197). In these bodies the hornblende is almost 

 entirely fresh. 



The hornblende is greenish brown with the usual pleochroism, between 

 dark greenish brown and light brown. In some cases a zonal structure is 

 exhibited, the zones being different shades of the same color. In other 

 cases, notably in a segregation of hornblende, the color is chestnut brown 

 to purplish brown, passing into greenish brown, and into green at the 

 margin, the zonal arrangement of the colors not being parallel to crystal- 

 lographic forms, but irregular. In some individuals the margin is reddish 

 brown. These tones also occur in phenocrysts in the groundmass that 

 incloses the segregations of hornblende. This particular rock is rich in 

 hornblende and poor in biotite, and appears to be a less siliceous variety. 

 In the more siliceous varieties of these rocks the hornblende has more of 

 the greenish tone. The shape of its crystal is that common to these kinds 

 of rocks — short, stout prisms, generally idiomorphic. Cleavage and twin- 

 ning are also normal. The substance of the unaltered hornblende is quite 

 pure, there being but few inclusions, usually magnetite. It occasionally 

 surrounds biotite and augite (164) as nearly synchronous crystallizations, 

 each being allotriomorphic with respect to the other. Sometimes there is a 

 border of minute biotite plates surrounding the hornblende (146). Decom- 

 position begins as chloritization around the margin of the hornblende and 

 along cracks. When completely altered, there is a pseudomorph consisting 

 of chlorite with grains of iron oxide, and areas or cores of calcite, and 

 sometimes muscovite in confused aggregations. The latter mineral appears 

 to have been derived from the biotite in the rock, which is also decomposed 

 in such cases (201). In an upturned sheet of andesite-porphyry in the 

 southeast spur of Electric Peak the hornblende phenocrysts have been 

 converted into nearly parallel aggregates of actinolite (222). In the rock 

 on the south slope of Gray Mountain, where it appears to be somewhat 

 coarser grained and more easily eroded (148), the hornblende, in part 



