GOLD MOUNTAIN RIDGE AND NEAR-BY HILLS, IGNEOUS ROCKS. 185 
The predominant type in the vicinity of Gold Mountain is an 
'uneven-grained, coarse, pink biotite granite. The constituents are 
pink feldspar, locally in good crystals one-half inch long which show 
Carlsbad twinning, slightly smoky quartz, and black- biotite. Micro- 
scopic examination shows this granite to be formed of quartz, micro- 
cline, microperthitic orthoclase (in some specimens), oligoclase, bio- 
tite, and magnetite. Quartz shows strong undulose extinction. Seri- 
cite and kaolin are alteration products of the feldspars. The granite 
at the west end of the ridge is more variable in character, the most 
abundant type being a fine- to medium-grained gray biotite granite. 
This is composed of quartz, orthoclase, plagioclase, and biotite, with 
titanite, magnetite, muscovite, zircon, and apatite as accessory min- 
erals. Plagioclase (oligoclase) occurs in rude laths. The coarse pink 
variety of Gold Mountain is also present, and it grades into a gray 
granite porphyry, with fine-grained groundmass of quartz, biotite, 
and feldspar. The phenocrysts, which reach a maximum length of 
one-half inch, are tabular gray feldspars rimmed by white bands, 
some of them Carlsbad twinned; rounded, strongly smoky quartz, and 
hexagonal biotite. The microscope shows that this granite porphyry 
has a hypidiomorphic groundmass, composed of orthoclase, quartz, 
biotite, and plagioclase, with magnetite, apatite, and titanite as acces- 
sory minerals. Quartz in this granite is notably variable, being abun- 
dant in some outcrops and inconspicuous in others. With decrease in 
quartz the normal granite porphyry grades into a quartz-poor horn- 
blende-bearing porphyry in which some pink feldspar phenocrysts 
are 1 inch long. Under the microscope this proves to be a quartz- 
monzonite porphyry, with rare tabular microcline and orthoclase and 
lath -shaped plagioclase phenocrysts. The groundmass, which is 
allotriomorphic granular, is composed of quartz, biotite, hornblende, 
orthoclase, and plagioclase. The accessory minerals are titanite, 
apatite, zircon, and magnetite. A little epidote is secondary to horn- 
blende and biotite. 
Both areas of granite are cut by dikes of a fine-grained pink apatite, 
in some places as rich in biotite as the granite and in others poor in 
this mineral. The granite is also cut by coarse pegmatite dikes and 
grades into similar pegmatitic masses. Graphic granite is not un- 
common and at many points is the transition facies between granite 
and coarsely crystalline pegmatite. The aplite is, as a rule, older 
than the pegmatite, although in one case an aplitic dike passes along 
its strike into a dike with narrow aplitic border and pegmatitic center. 
The yellowish outcrops of granite at the base of the ridge are dis- 
continuous, being, as a rule, mere heaps of bowlders partially buried 
in granite soil. The outcrops near the crest are much more continu- 
ous. The granite intrudes Cambrian sediments, and rhyolite flows 
