VOLCANIC EOCKS OF SEPULCHRE MOUNTAIN. 125 



rocks. In this case the rock is glassy and vesicular, and shows no signs of 

 any kind of metamorphosing action. The inclusions in the hypersthene 

 appear to be primary, inclosed at the time of the crystallization of the 

 mineral. The lighter-colored hypersthenes are free from them. Occasion- 

 ally the color varies in concentric zones. In some rocks both kinds of 

 pyroxene have narrow reddish-brown borders that are analogous to the 

 black borders around some hornblendes, and appear to be of similar origin. 

 The pyroxene microlites of the groundmass are affected in the same manner 

 as the phenocrysts; hence the change of condition must have followed the 

 crystallization of the microlites. 



The color of the augite is light green in thin section, and without ple- 

 ochroism. It is like the augite in the diorites of Electric Peak, both optic- 

 ally and as regards cleavage. Hypersthene and augite are occasionally 

 intergrown in such a manner as to indicate their nearly synchronous crys- 

 tallization ; but when one incloses the other, it is hypersthene that is within, 

 and hence the older. Hypersthene is the more easily decomposed of the 

 two, and changes into a green fibrous mineral, probably bastite. 



The feldspar phenocrysts are all lime-soda feldspar, and are mostly 

 labradorite, judging from optical characters. Their outline in sections is 

 rectangular, sometimes with more than four sides. Zonal structure is pro- 

 nounced, but the difference in the optical character of the various zones is 

 not marked. Glass inclusions are frequent. In the larger crystals the 

 central portion is often crowded with inclusions of brown glass containing 

 the same microlites as the surrounding groundmass. The shape of these 

 inclusions is usually rectangular. Many smaller feldspars are almost free 

 from them. Inclusions of magnetite and pyroxene occur. When feldspars 

 and pyroxenes have crystallized in juxtaposition, it is seen that the feldspar 

 is the younger, but that its crystallization began before that of the pyroxene 

 ceased. They were in part contemporaneous. 



Magnetite occurs as microscopic phenocrysts. There are five sections 

 of pyroxene-andesites carrying small amounts of hornblende and consti- 

 tuting transitional varieties between these rocks and hornblende-pyroxene- 

 andesites. 



The hornblende is in small irregular crystals, some being rounded and 

 others in angular shapes. It is reddish brown and brownish green, with 

 strong pleochroism. Many of the individuals, especially the rounded ones, 



