AGATE, MOSS AGATE, ETC.
WYOMING.
The
production of moss agate in the United States comes principally from
the Wilde and Deercorn mine, 2 miles northwest of Guernsey, in Laramie
County, Wyo. This mine contains three claims and is located near the
top of a hill or small mountain about 400 feet above the adjoining
valley. The base and lower slopes of this hill are composed of red
quartzites, phyllites, hornblende, and greenstone schists. These rocks
are mapped under the name Whalen group by W. S. T. Smith a and
are referred to the Algonkian age. The top of the hill is composed of
limestone and quartzites resting unconform-ably on the Whalen group and
dipping to the west at a low angle. These rocks belong principally to
the Guernsey formation of Carboniferous age as mapped by Smith.
The
moss agate occurs in an irregularly shaped vein, varying from less than
1 inch to nearly 2 feet in thickness and cutting nearly vertically
across the bedded limestones. This vein strikes northeast and has been
opened at two places about 200 yards apart. At the southwest opening an
open cut and drift about 75 feet long have been made on the vein. The
openings do not reach a greater depth than 15 feet. The upper few feet
of the limestone exposed in the opening has a light flesh color, and
the lower layer is red and is banded. The vein appears to pinch out in
places in the light-colored layer of limestone and does not reach the
surface a few feet above the tunnel. In the floor of the tunnel the
moss agate had a thickness of nearly 2 feet in one place and pinched
down to a few inches in a short distance. Small stringers of moss agate
occur in some of the vertical seams crossing or branching out from the
main vein. The moss agate does not appear to be firmly attached to the
wall rock, but is separated from it by a deposit of white chalky
chalcedony or silica, and in places by layers of
columnarcalcitecrystals. The vein filling is chalcedony or agate with a
few small botryoidal chalcedony and drusy quartz lined cavities through
it. The greater part of the chalcedony has abundant black moss-like
arborescent and dendritic markings throughout. The agate varies in
quality from opaque cloudy white to subtranslucent to translucent or
subtransparent. The latter material furnishes the finest stone for gem
purposes. The white and subtranslucent agate is plentiful, and contams
smaller portions of clearer fine gem material distributed through it.
The translucent agate is also found in smaller rounded masses with a
chalky coating over their surfaces. The black stains (of manganese
oxide) occur through both the cloudy and the translucent agate, with
all the variations of form characteristic of the mocha stone or moss
agate. The better grades furnish very fine gem material, and the cloudy
varieties are suitable for mosaic and small ornaments, for which a
portion is used. Blocks of several hundred pounds weight of cloudy
white agate with translucent portions were seen around the mine, and it
is reported that a 1,000-pound block was once obtained which was almost
entirely composed of moss agate of good quality. About 3-1/2 tons of
rough moss agate were mined during 1908, though none was sold.
aHartvillefolio (No. 91), Geol. Atlas U. S., U. S. Geol. Survey, 1903..
