GEOL. VOL. L] SMITH SANTA CAT A LIN A ISLAND. 35 



extinction angle, most of them extinguishing parallel to their 

 length, thus placing them in the oligoclase-andesine series. 

 Many of them appear to be simply twinned. In length they 

 range up to about .04 mm. In nearly all cases they show a 

 general parallelism due to flow. The, amount of glass in 

 the ground-mass is very variable, some of the slides being 

 almost holo crystalline. In other cases the glass and second- 

 ary silica compose nearly one-half the ground-mass. This 

 glass is readily distinguished from the opal by its color. 

 The latter is yellowish brown and is quite clear, while the 

 glass is dark and usually filled with small, irresolvable dots 

 which may be magnetite. 



Glassy Fades. The glassy facies of the andesite, found 

 as a very small occurrence near the isthmus, differs both 

 macroscopically and microscopically from that just described. 

 The rock is dark, almost black, and glassy, with yellowish 

 patches scattered over the surface, the largest seen being 

 13 mm. in length. Occasional kaolinized feldspars occur, 

 either within these yellowish patches or alone, up to a length 

 of about 2 mm. With a lens very small hexagonal crystals 

 of biotite are seen here and there. The dark ground-mass 

 constitutes the bulk of the rock. 



Under the microscope scattered and aggregated crystals 

 of magnetite, hypersthene, augite, biotite, and feldspar are 

 seen here and there in a glassy matrix. Magnetite, in grains 

 and small octahedra, occurs as inclusions in the other min- 

 erals, and scattered through the ground-mass. There is 

 only a moderate amount of biotite in the slides. It occurs 

 as isolated idiomorphic sections, and was seen nowhere in 

 contact with the other minerals. No basal sections were 

 seen. The mineral everywhere exhibits the usual strong 

 pleochroism. 



Augite occurs either alone or with hypersthene, and while 

 generally more or less rounded, it occasionally shows good 

 crystal boundaries. It is pale green in color and non-pleo- 

 chroic. One section of the augite showing rough crystal 

 boundaries is wholly surrounded by a somewhat rectangular 



