Geol.— Vol. I.] SMITH— SANTA CATALLVA ISLAND. 29 



or with a tinge of pink. It appears to be considerably 

 altered. It contains scattered pheriocrysts of quartz (up to 

 3 mm. in diameter) with smaller and more numerous crystals 

 of biotite. The vesicles indicate flow by their pronounced 

 compression in one plane. Some of the cavities have a 

 smooth, lustrous surface, and appear to be regular in shape, 

 as if due to the leaching out of phenocrysts once contained 

 in them. The form of two or three of these cavities strongly 

 suggested a simply twinned feldspar. 



Microscofic Charactei's. — In thin section the open-textured 

 facies of the rock is seen to be composed largely of a dirty 

 brown, very vesicular ground-mass, in which occur scattered 

 phenocrysts of quartz and biotite, besides more or less mag- 

 netite in small grains or crystals. No phenocrysts of feld- 

 spar were seen, but one cavity was found which clearly 

 had the form of a Carlsbad twin of feldspar. This cavity 

 had a very narrow border of some secondary product strongly 

 stained with limonite. 



The quartz is in general quite clear, and occurs in idio- 

 morphic forms which are usually more or less corroded. 

 Frequent cracks traverse the sections. Besides brownish 

 patches of included glass, the quartz contains occasional 

 small spherulites, and sections of biotite partly or wholly 

 included. No liquid inclusions were seen. 



Biotite occurs in scattered, idiomorphic sections, generally 

 with clear boundaries. A few of the rectangular sections 

 are somewhat frayed at the ends. The crystals range in 

 length from .15 mm. to .7 mm. The mineral exhibits the 

 usual strong pleochroism. 



The ground-mass consists in large part of feebly polarizing 

 feldspar microHtes in a dark isotropic matrix. A few of the 

 vesicular cavities of the slides are nearly round, the rest 

 being elliptical in form, and occasionally drawn out at the 

 ends. Some contain a small amount of a clear, secondary 

 mineral, and others, nearly spherical, are completely filled 

 with almost opaque secondary products, dirty brown to black 

 in color. 



