GEMS AND PRECIOUS STONES. 1055
sparingly
through the pegmatite but are more plentiful in the 'upper 8 inches
under the hanging wall where many are in small bunches and horses of
schist. The gems are "frozen" in the pegmatite. Small crystals of
emerald are abundant, and the color of many of them is good
emerald-green. Some are pale colored or yellowish. Occasional gems of
value have been found. Much of the pegmatite matrix containing white
feldspar, gray quartz, black tourmaline, and green emeralds, has been
cut under the name of emerald matrix. Some of this materail is very
pretty.
The
occurrence of emeralds at the Turner mine is similar in many respects
to that at some other localities, but there are here certain associated
rocks which seem to belong naturally with the occurrence of emeralds
that are not present in other deposits.
The
rocks of this portion of the Piedmont Plateau are principally gneisses
and schists of Archean age, cut by masses of later granite, diorite,
and other rocks. In the region around the emerald prospect the types of
rock are varied. There are mica, garnet, cyanite, graphite, and
hornblende gneisses and schists cut by granite, pegmatite, diorite,
gabbro, and other ferromagnesian rocks. Another rock which answers the
description of garnetiferous quartz diorite occurs abundantly at
certain horizons in the gneiss. The strike of the rock formations is
variable between east-west and north-south where the strata are tilted,
but over large areas they are essentially flat with many small rather
gentle folds.
Warping
of the rocks with larger sharp folds occurs and produces abrupt changes
in outcrop, so that mapping the formations is difficult. Another
difficulty is the apparent metamorphism of ferromagnesian rocks to
rocks of less basic composition, and of granite to more basic
composition by reaction between the two at the time of the intrusion of
the granite. In the descriptions of the rocks associated with the
emerald deposit in these reports for 1909 and 1910, tentative names
were given to some rocks, as the material available for examination was
so badly weathered. Less altered specimens obtained from greater depth
have made more accurate determinations possible. Thus, rock called
amphibolite, and amphibolite after
E
yroxenite, proves to
be hornblende hypersthenite and is so called elow. A number of
varieties of basic rocks occur, but all may be placed in one general
class.
The
most common type is hornblende hypersthenite-peridotite, and of this
there are numerous outcrops. In some occurrences olivine is lacking or
scarce, and the rock may be called hornblende hypersthenite. The
minerals composing these rocks, as determined by microscopic
examination, are pale-green hornblende, light-brownish hypersthene,
olivine, augite, biotite, pleonaste, magnetite, and a little sulphide.
Hornblende and hypersthene are present in all occurrences, but one or
all of olivine, augite, biotite, and pleonaste were not observed in
some specimens. The hypersthene and olivine occur in poikilitic
crystals, in some specimens of large size. In some places weathering
has changed hypersthene to bastite, hornblende to chlorite, and added
limonite stains. The result is a rock resembling chloritic soapstone.
On disintegration and decomposition these rocks break down to a dark
greenish-brown lumpy soil.
Gabbro
occurs at several localities and is of two kinds, common and olivine
gabbro. The former exhibits a partial diabasic texture in thin
