section.
The constituent minerals determined microscopically in one specimen are
labradorite, hornblende, part original and part formed from pyroxene,
augite, hypersthene, biotite, magnetite, apatite, and
p
yrrhotite. Olivine
gabbro is associated with the hornblende hypersthenite-peridotite, and
the two rocks are probably segregations from the same magma. Some
specimens contain such minerals as to make it difficult to determine
whether the rock should be called olivine gabbro or hornblende
hypersthenite-peridotite. The olivine gabbro weathers to a soil very
similar to that formed by the perido-tite. Spheroidal bowlders are
generally left over in the outcrops.
Diorite
and hornblende gneiss are common rocks of the region. Microscopic
examination of sections reveal hornblende, andesine, biotite, iron ore,
apatite, zircon, and, in some specimens, quartz. On weathering the
dioritic rocks form dark-brown clay soils.
The
granite of this region is medium-grained biotite granite. Peg-matitic
phases are associated with it, especially near the contact with other
rocks, and in some places the small bodies may equally be called
pegmatite or granite. On disintegration the granite breaks down to a
light sandy soil.
The
relations between the granite and the basic rocks are interesting.
Besides the types of basic rocks mentioned, there are gradations from
them into granite by the presence of granite minerals. This gradation
is present where gabbro, olivine gabbro, and hypersthenite-peridotite
are inclosed in granite. Thus, a specimen from a spheroidal bowlder of
weathering near a gabbro and granite contact contained andesine or
labradorite, hornblende after pyroxene, biotite, and considerable
quartz along with iron ore, apatite, and pyrrhotite. The true gabbro
from this body has more calcic feldspar and ferromagnesian minerals
with no quartz. A spheroidal bowlder from a similar contact at the
emerald mine contained hornblende, biotite, labradorite, quartz,
magnetite, and apatite. Diorite contacts with granite show a similar
gradation by the presence of quartz.
The
ages of all the different rocks have not been determined. The mica and
garnet gneisses and schists are of Archean age and belong to the
Carolina gneiss, as designated by Keith in the geologic folios on the
southern Appalachian region. The diorite and associated hornblende
gneiss and schist are also Archean and belong to the Roan gneiss. The
gabbro, olivine gabbro, and hornblende hypersthenite-peridotite rocks
are probably of later age. They were intruded into the other gneisses
and schist before the granite, and Keith thinks much of the granite of
this region is probably of post-Carboniferous age. The intrusion of the
granite before the complete consolidation of the basic rock magmas
would make easier an explanation of the apparent mutual absorption
described above that has taken place between the two. If the basic
rocks are considered much older than the granite, the reaction between
the two and absorption by the granite appear to offer a better
explanation than would be offered by magmatic differentiation.
Pegmatite
probably representing later stages of activity of the granite magmas
have filled openings and fissures in the rocks. Some of these are in
the basic rocks, and it is in such fillings that the emeralds have been
found on the Turner place. Several pegmatite veins have been uncovered
during the prospecting, but so far emeralds
