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PRECIOUS STONES.
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825
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The
garnets are found on a series of mesa-like benches rising from Gypsum
Valley on the southeast, and between 6,000 and 7,000 feet (barometric
measurement) above sea level. The elevation increases slightly to the
west in the mountains around Monument Pass. The benches and mesa on
which the garnet deposits occur are nearly level in places and dip to
the southeast in others. They are formed by different beds of the red
sandstone formation, the edges of some of the beds standing as small
cliffs over the next lower. In places the sandstone floors are bare;
in others they are covered by wind-blown sand in layers varying from a
few inches in depth to dunes many feet high. This sand is brought up
principally from the red-sandstone country to the southwest, from which
direction the prevailing winds of the region blow. The garnets are
found in the sand and on the sandstone floors, associated with pebbles
of feldspar sometimes with a moonstone luster, occasionally
emerald-green diopside, red sugary quartz, and such rocks as granite,
diorite, trap, etc. Some of the garnets and hard-rock fragments are
rounded and polished on one or more sides by the action of the
wind-blown sand as they lie exposed on the surface. This accounts for
the smooth rounded surfaces so prevalent on many of the garnets from
the Navajo country. The garnets may be uncovered by a wind from one
direction and then covered up by that from another, or vice versa. By
the shifting of the dunes the position of the garnets is changed so
that different sides are exposed for polishing by the wind-blown sand.
The
source of the garnet over the mesa country is in a stratum of coarse,
unconsolidated drift or gravel that rests on the more elevated part of
the red sandstone on the northwest of the area examined. This drift is
over 100 feet thick and is composed of bowlders, which vary from stones
weighing many tons to cobble size, mixed through a matrix of pebbles
and sand. The gravel and bowlders consist of biotite granite gneiss,
porphyritic biotite granite gneiss, hornblende or diorite gneiss,
partly epidotized trap and basaltic rocks, epidote hornstone,
soapstone, tremolite asbestos, sugary quartz, and large blocks of light
gray colored fossiliferous limestone of Carboniferous age. Just where
the origin of this conglomeration is to be sought is not known. The
general appearance of the drift is that of a glacial deposit.
Glaciation has taken place in the San Francisco Mountains ° of Coconino
County, Ariz., and moraine deposits have been formed. The latter are
thought to be of rather recent age, probably Quaternary. Whether there
has been glaciation in the slightly higher country west and northwest
of the garnet deposits is not known. It is probable that the
garnet-bearing drift deposits are of greater age than the glacial
deposits of the San Francisco Mountains, for the former are covered
with a stratum of hard white sandstone and are at almost as great an
elevation as any of the surrounding region. The presence of such
quantities of crystalline and ancient rocks in the drift can not be
explained by very recent action, as these rocks do not outcrop near the
locality.
The
garnets are scattered through the drift, though not plentifully, and
are carried down with it to the mesa country below during erosion. It
seems the garnets undergo a partial concentration on the mesas
a Ward, L. F., Glaciation of the San Francisco Mountains, Arizona: Jour. Geology, vol. 13, 1905, pp. 276-279.
Robinson, H. H., Geology of the San Franciscan volcanic field, Arizona: Prof. Paper U. S. Geol. Survey. (In preparation.)
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