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University of California. 



[Vol. 2. 



sometimes attain a size of one half an inch in greatest dimension, 

 are characterized by abundant inclusions of augite, magnetite, glass, 

 and serpentine. These have the same mode of arrangement and 

 the same net-like aspect as was described in the case of the 

 andesites, but here even more abundant than as there described. 



The feldspars frequently exhibit irregular cracks, which, as well 

 as the cleavage cracks, are filled with pale yellow serpentine. 

 Alteration seems scarcely to have begun in this mineral, which 

 seems to be the most stable constituent, appearing fresh and glassy 

 in the most rotten specimens of the rock. The feldspars of the 

 ground-mass are both microlitic and granular in habit, more com- 

 monly the former. They are, apparently, quite basic in composi- 

 tion, their extinction angles indicating labradorite. Twins are the 

 rule in even the minutest microlites. The microlites sink to almost 

 sub-microscopic dimensions, and on the other hand by increase of 

 size, merge with the porphyritic crystals. 



Augite, like feldspar, is in two generations. Phenocrysts of 

 this mineral are almost universally present and generally exhibit 

 sharp idiomorphic boundaries with characteristic form and cleav- 

 age. The mineral is colorless to pale fawn color, non-pleochroic, 

 with extinction angle c/\c as large as forty degrees. Twins of 

 normal structure are common, and zoned crystals were observed, 

 which gave undulatory extinction. In certain very local forms 

 augite was observed forming ophitic intergrowths with feldspar, 

 the irregular angular areas of augite between the feldspar laths 

 having the same optical orientation over considerable spaces. As 

 a constituent of the ground-mass augite is invariably in small 

 rounded granules, filling interstices between the feldspar individ- 

 uals, and, apparently, the last product of crystallization of the 

 magma. Alteration of augite appears to be usually to a yellowish 

 serpentine, rarely to chlorite, and has taken place extensively 

 especially in the augite grains of the inclusions and the ground- 

 mass. The calcite observed in the slides is probably due to lime 

 set free from the augite by this decomposition. 



Olivine is abundant in most specimens of basalt, and is prob- 

 ably present in all, though not always observed. It is always in 

 porphyritic crystals sometimes of large size and of characteristic 



