124 



University of California . 



[Vol. 3. 



The feldspar phenocrysts invariably exhibit polysynthetie 

 twinning. Carlsbad twins also occur in a majority of eases, so 

 that Michel Levy's method of determination may conveniently 

 be applied. In the main central portion of a doubly twinned 

 crystal, the concurrent angles were 19° and 34.5°, corresponding 

 with a composition of about AI13 A114. Distinct zonal banding is 

 always present, and the successive shells, while showing a general 

 decrease in basicity, give evidence of the oscillation that seems to 

 be usual in the growth of phenocrysts. The most acid shells 

 have the composition of andesines. The inclusions comprise 

 crystals of augite, magnetite, and apatite, but the most abundant 

 are of glass, usually in the form of negative crystals, each con- 

 taining one or more bubbles. The feldspar substance is generally 

 fresh, but a certain zone, or the centre of a crystal, is sometimes 

 replaced by a green fibrous or scaty mineral, and red oxide of 

 iron is often deposited along cracks or cleavages. 



The augite is of a green variety, very feebly pleochroic, and 

 has an extinction angle of about 45? The stout prisms, of 

 octagonal cross-section, are in general of sharply idiomorphic 

 outline. Twinning on the orthopinacoid is of frequent occur- 

 rence. In contrast with the hypersthene, the augite is entirely 

 unaltered. 



Hypersthene is somewhat more abundant than augite. It 

 occurs in well formed octagonal prisms, with greater relative 

 elongation in the direction of the vertical axis than is shown by 

 the monoclinic pyroxene. It is also distinguished by weak double 

 refraction, straight extinction and a marked pleoehroism. The 

 axes colors are: 8, pale reddish brown; b, yellow; C, pale green, 

 very similar to the color of the augite. 



The alteration of the hypersthene is deserving of especial 

 notice. The process of decomposition begins at the surface and, 

 in the transverse cracks, extends most rapidly in the direction of 

 the vertical axis. Almost every crystal has been attacked, and 

 the replacement of the original substance in many cases has been 

 complete. The final product is a pseudomorph composed of fibres 

 that lie nearly parallel to the vertical axis of the hypersthene, 

 so as to extinguish almost uniformly in polarized light. The 

 structure is thus quite analogous to that of bastite, but the fibres 



