bascom.] QUARTZ-PORPHYRIES. 39 
It is a question whether it is always possible to distinguish between 
a primary and a secondary crystalline groundmass, and no attempt has 
been made to draw a sharp line between the quartz-porphyries and the 
aporhyohtes. In the absence of some of the more marked structures 
of a glass the presence of a secondary crystalline structure has not 
been considered sufficient evidence for the completely secondary char- 
acter of the crystallization. It is very probable that while a large 
portion of the lava flow consolidated as a glass, much of the lava solidi- 
fied at a sufficient depth to have secured a noncrystalline groundmass. 
QUARTZ-PORPHYRIES. 
DISTRIBUTION. 
A deep-red porphyry outcrops along the turnpike leading from the 
west to Monterey Station, and covers a small area north of the high- 
road. 1 One mile north of this, in the neighborhood of u Guni Spring," 
there are several limited areas of blue, purple, and brick-red porphyries. 
About 4 miles northeast, on the "Old Furnace Koad," by the unfinished 
viaduct, is a large area of dark-blue porphyry, passing toward the 
southwest by insensible gradations into typical aporhyohtes. 
These are the more important areas of porphyries. There are other 
porphyries so closely associated with the devitrified glassy lava that 
they will not be separately located. 
MACROSCOPIC AL DESCRIPTION. 
The beauty and variety of color of these porphyries have already 
been noted. The phenocrysts are not large (5 to 11 mm long), but are 
conspicuous against the dark or brilliantly colored matrix. The ortho- 
clase phenocrysts are opaque white, pink, or brick-red. They usually 
possess a well-dehned crystalline outline and show twinning in the hand 
specimen. The quartzes are colorless or a rich wine red. The fracture 
is conchoidal and there is a less-marked tendency to cleavage than in 
the aporhyolites. 
The porphyries show a reddish-brown color on the weathered surface, 
and on decomposing form a red earth. The presence of manganese is 
abundantly exhibited in dendritic markings on the cleavage surfaces. 
This is especially true in the case of the slates developed from the por- 
phyries. (PI. XIV.) 
MICROSCOPICAL DESCRIPTION. 
1'HENOCRYSTS. 
Feldspar. — The porphyritical feldspars are more abundant than the 
quartz phenocrysts, and are remarkably fresh and unaltered. That 
they belong to the group of alkali feldspars, and that both monoclinic 
and triclinic varieties are uudoubtedly present, are indicated by chemi- 
cal analysis (given on page 01), by their specific gravity, and by their 
1 See the geological map of the Mouterey district accompanying this bulletin, PL 111. 
