VOLCANIC ROCKS OF SEPULCHRE MOUNTAIN. 127 



both in color and in the extent to which it has been resorbed. In some 

 cases there has been no resorption. The crystals when idiomorphic are 

 bounded by the prism, clinopinacoid, and the usual terminal planes. In 

 many cases the form of the crystals is not sharply defined. The color 

 varies from intense red in some rocks to reddish brown, chestnut brown, 

 greenish brown, and brownish green, with the corresponding pleochroisrn. 

 The color bears no fixed relation to the presence or absence of opaque 

 border, nor to the amount of resorption. It does not appear to be due to 

 secondary alteration of the hornblendes, since the rocks are all fresh and 

 glassy. 



The character of the border is not always constant for all the horn- 

 blendes in one rock section. Around it in some cases is a narrow margin 

 of magnetite grains; in others the margin is heavy and opaque. Other 

 hornblendes in the same section are surrounded by crystals of pyroxene, 

 plagioclase, and magnetite. In many sections all the hornblendes are alike, 

 with or without borders. There seems to be no relation between the char- 

 acter or degree of resorption and the degree or kind of crystallization of 

 the groundmass ; and different phases of resorption occur within very 

 short distances of one another in the same rock. Crystals which do not 

 exhibit other signs of resorption sometimes have large "bays" or pockets 

 of groundmass as inclusions, which may have been originally inclosed 

 at the time of the crystallization of the hornblende. The position of 

 the hornblende with respect to adjacent crystals of pyroxene and felds- 

 par indicates that they were contemporaneous crystallizations in part. 

 The latest feldspars and pyroxenes are always younger than the horn- 

 blende. 



The feldspar phenocrysts are all plagioclase, in most cases labradorite, 

 less commonly andesite or oligoclase. Their microscopical characters are 

 very nearly the same as those of the feldspars in the pyroxene-andesites. 

 The groundmass of these andesites in some cases is brown globulitic glass 

 with microlites of pyroxene, feldspar, and magnetite. In most sections it 

 is colorless glass crowded with the same kinds of microlites. It carries 

 microscopic crystals of these minerals which are porphyritical with respect 

 to the groundmass when seen with a microscope, but which in turn form 

 part of the groundmass that carries the megascopic phenocrysts. 



