4 i4 



University of California. 



[Vol. 2. 



crystals can be detected whose entire substance has been replaced 

 by the ground-mass of the rock. Decomposition has affected the 

 feldspars of these rocks in a very slight degree. On weathered 

 surfaces they sometimes become opaque and earthy, but in the thin 

 sections studied, the traces of decay were hardly noticeable. In 

 fact, the feldspars seem frequently the last elements of the rock to 

 yield, for specimens were collected in which the ferro-magnesian 

 silicates were completely decomposed, leaving the rock a crumbling 

 mass, and yet the feldspars appear fresh and glassy. In one 

 instance, replacement of feldspar by analcite was observed, and 

 will be described under that mineral. 



The feldspars of the ground-mass are both microlitic and 

 granular in habit. Lath-shaped microlites are most common, and 

 exhibit the uneven boundaries and irregular terminations common 

 to the ground-mass feldspars of andesitic rocks. They are gener- 

 ally twinned, and, like the phenocrysts of similar composition, are 

 clear and glassy, rarely showing signs of decay. Judging from the 

 extinction angles, they have the composition of oligoclase, more 

 rarely of labradorite. 



The presence of orthoclase feldspar in the andesites is con- 

 sidered probable but was not demonstrated. Certain phenocrysts 

 showing no twin or zone structure, and free from the ordinary 

 inclusions of the plagioclase crystals, are believed to consist of this 

 mineral. The repeated occurrence of such crystals in the slides 

 and the presence of a considerable amount of potash in the rocks, 

 as shown by chemical analysis, tend to confirm this determination. 

 But satisfactory optical tests were not obtained. 



Augite is a constant constituent of the andesitic rocks, being 

 almost as abundant as feldspar, and forming, with that mineral, the 

 bulk- of the rock. It occurs in two generations, forming numerous 

 phenocrysts, and as granular individuals making up a large pro- 

 portion of the ground-mass. The augite phenocrysts sometimes 

 attain a large size, and are generally sharply bounded by crystallo- 

 graphic planes in forms which yield characteristic octagonal cross 

 sections; again they are rounded or broken by chemical or 

 mechanical deformation; or they form granular aggregates often 

 mingled with plagioclase grains, the allotriomorphic form of the 



