20 CALIFORNIA ACADEMY OF SCIEXCES. [Proc. 3D Ser. 



irregular, block-like forms, looking, in some cases along the 

 shore near Avalon, like square pillars projecting from the 

 side of the cliff. These break up into smaller block-like 

 masses, and the process is continued until the gravelly con- 

 dition is reached. On the hill slopes the projecting masses 

 frequently present similar forms with smooth surfaces, but 

 it is not uncommon to see also small, boss-like projections, 

 with rough, uneven surfaces. The cause of this difference 

 in form is doubtless a variation in the grain of the rock. 

 The soil formed from these rocks is generally of a dull, yel- 

 lowish color. The porphyrite contains the same minerals 

 that occur in the diorite, except biotite, which was not seen 

 in any of the slides. It presents the same general charac- 

 ters wherever found. 



Mac7'oscofic Characters. — The color of both the unaltered 

 and weathered porphyrite is much the same as that of the 

 diorite, the fresh hand-specimens varying from light to dark 

 gray, most of them with a tinge of green. Little can be made 

 out in the fresher specimens with the unaided eye, except an 

 occasional feldspar, shown by the reflection from a cleavage 

 surface. Hornblende crystals of some length — up to 5 mm. 

 or more — are developed in one or two specimens. As the 

 rock weathers the whitening of the feldspars usually brings 

 out plainly the porphyritic structure. With the lens the por- 

 phyritic feldspars may occasionally be distinguished from 

 the medium-grained ground-mass, though these are usually 

 masked more or less by the fracture of the rock, which 

 leaves minute flakes or splinters clinging to the surface of 

 the specimen. The rock frequently presents a slightly 

 mottled surface, in the dark and light colors. Gleaming 

 bits of pyrite may occasionally be seen in some of the 

 specimens. 



Microscopic Characters. — Microscopically the rock is holo- 

 crystalline and porphyritic, with phenocrysts of a lime-soda- 

 feldspar and of hornblende — occasionally also of augite — 

 and a medium-grained granular ground-mass, composed es- 

 sentially of feldspar and quartz. The phenocrysts vary con- 



