32 CALIFORNIA ACADEMY OF SCIENCES. [Pnoc. 30 SER. 



eral dull, some of them have an almost greasy luster. A few 

 of the specimens, purplish in color, appear to the eye to be 

 fresh and compact, but with a lens it may generally be seen 

 that they are more or less altered. Though usually compact, 

 the rocks are vesicular in places, the irregular vesicles more 

 or less compressed in the plane of flowage. This was noticed 

 particularly about Isthmus Cove; also in places back of 

 Whitley's Cove. Rarely the rock is amygdaloidal, as in 

 the neighborhood of Whitley's Cove. The rock, though 

 usually fracturing irregularly, at times breaks into plate-like 

 pieces a centimeter or more in thickness. These pieces 

 may sometimes be broken into thinner plates, owing to a 

 laminated condition of the rock. These latter sheets vary 

 in thickness from two millimeters to several centimeters, 

 and their surfaces are generally yellowish from decomposi- 

 tion products. In other cases the rocks break into irregular 

 masses, while showing a phenomenon similar to the fore- 

 going in a series of fine parallel lines on those fractured 

 surfaces at right angles to the bedding. One and only one 

 glassy specimen of the rock was obtained, from near the 

 small bay to the east of Isthmus Cove. 



Microscopic Characters. Microscopically the rocks, with 

 the exception of two of the specimens examined, are pyrox- 

 ene-andesites. They are usually porphyritic with a hyalopil- 

 itic ground-mass. The phenocrysts consist essentially of 

 labradorite, augite, and hypersthene. Secondary silica is 

 usually present in greater or less amounts. This is largely 

 opal with occasional chalcedony. The first mineral to sep- 

 arate from the magma was magnetite, followed by the pyrox- 

 enes, and finally by the feldspars. 



The magnetite occurs either as small octahedrons or in 

 irregular patches, and appears to be in two generations. 

 The largest grains are about .2 mm. in diameter. These 

 are not very numerous. The smaller grains are more abun- 

 dant, and are distributed more or less evenly through the 

 ground-mass. These have an average diameter of about .04 

 mm. This scattered magnetite forms one of the most marked 



