University of California. 



[Vol. 2. 



prismatic habit, showing characteristic prismatic cleavage and trans- 

 verse basal parting. It is nearly colorless and is weakly but dis- 

 tinctly pleochroic :c = pale greenish; a = 6 = pale reddish yellow. Its 

 intergrowths with augite have already been noted, hypersthene in 

 all cases being the older mineral. Many of the crystals exhibit a 

 network of cracks, which give a granular appearance to the sur- 

 ace. This condition is accompanied by and seems to precede a 

 decomposition into serpentine. The weak pleochroism of the 

 hypersthene suggests that it is a variety of the mineral with low 

 content of iron, an idea confirmed by the absence of dark borders 

 of separated iron oxide about decomposed crystals, which appears 

 to be a common characteristic of normal hypersthene in andesitic 

 rocks. 



Olivine, like hypersthene, is a sparing constituent of certain 

 facies of the andesites, and, so far as observed, the two minerals do 

 not occur in the same rock. While the evidence on this point is, 

 of necessity, only negative, and therefore not conclusive, it suggests 

 that olivine is formed at the expense of the hypersthene, or, vice 

 versa, as local conditions may determine. This example confirms 

 the opinion expressed by Zirkel * on this point. Olivine occurs as 

 idiomorphic phenocrysts of characteristic form and properties, 

 commonly much altered to serpentine. As the mineral is fully 

 described under another group of rocks in which it is an impor- 

 tant constituent, it will receive here but -passing mention. 



Magnetite is abundantly present in all facies of the andesite, 

 particularly in those containing the least proportion of glassy base. 

 It is found in porphyritic individuals of varying size, with irregu- 

 lar crystalline form, in crystalline grains as a constituent of the 

 ground-mass, and as inclusions in all the other porphyritic constitu- 

 ents. It is also sparingly formed as a secondary product through 

 Jthe alteration of olivine and augite. 



Apatite is a constant constituent of the andesites. It is seen 

 included in feldspar and augite in slender, colorless prisms, with 

 hexagonal cross sections and characteristic optical properties. In 

 certain facies of the rock apatite assumes a different appearance. 

 It forms rather stout prisms with irregular terminations and hexa- 

 *F. Zirkel Lehrbuch der Petrographie, Leipzig, 1894, II, p. 813. 



