Another
specimen is a cream-yellow piece of chert which show3 black dendritic
markings similar to those occurring in moss agate, and stdl another is
a slice of translucent chalcedony with inclusions of tufts or globules
of white chalcedony, which, in turn, inclose small spots of green prase.
WYOMING.
The
following notes on the occurrence of moss agate in Fremont County,
Wyo., along the Sweetwater Valley, have been supplied by C. J. Hares,
of the United States Geological Survey. The agate deposits are
scattered over several townships, including Tps. 30 and 31 N., Rs. 89,
90, and 91 W., mostly north of Sweetwater River, and among the isolated
granitic masses of the Granite Mountains. The locality is about 35
miles west of the Pathfinder Dam and 60 miles west of Casper.
Occasionally a few agates are found farther down Sweetwater River, but
the best agate beds are limited to from 1 mile to 3 miles north of the
Mcintosh ranch, located on Sage Hen Creek, in sec. 15, T. 30 N., R. 90
W., and north of the granite in that vicinity.
The
agates occur as pebbles scattered over the surface of the ground on
terracelike slopes or in valleys. As they are gathered up from the
surface a fresh supply is uncovered among the grass, sagebrush, cacti,
and soil by hard showers or by the almost daily heavy winds. They
apparently represent residual wind-swept gravel from the
disintegration of the flat-lying White River formation, of Oligocene
ago.
The
White River formation in this area is composed of a great variety of
rocks—very coarse granitic conglomerate, agglomerate containing
fragments of Tertiary eruptive rocks, grits, arkosic sandstone, shale,
fine volcanic tuffs containing rhyolitic bombs, opalized auartz, opal
containing mossy markings in limestone, much unin-urated volcanic ash,
and clay. In the vicinity of the agate beds the formation lies
unconformably on and fills in around the old irregular bald masses of
pre-Cambrian granite. The formation is flat lying and, as far as
ascertained, has not been materially deformed since its deposition.1
The
present altitude of the agate beds is about 6,400 feet above sea level,
which is approximately 1,000 feet below the highest White River beds in
near-by localities—along the northern edge of the Sweetwater escarpment
and about 200 feet above Sweetwater River. The crest of the Sweetwater
escarpment, composed of the White River formation, about 25 miles to
the west of the agate beds, is composed of limestones which, in
places, contain opal with mosslike markings. These opalized limestone
beds possibly represent the last stages in the deposition of the White
River formation. It is probable that the agates are derived from the
disintegration of the White River beds and now remain on the surface,
from which the finer and softer material has been blown and washed away.
The
agate pebbles range in size up to 2 inches or more in diameter, and are
usually well rounded and some are partly polished. Many of the agates
have the dreikanter shape and the wind-blown facets characteristic of
wind action. Some are elongated and others are flattened or lens
shaped. The good specimens are uncommon, being
1 For a fuller description of the geology of this region, see Hares, C J., The anticlines in central Wyoming. [In preparation.]
