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PRECIOUS STONES.
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probably
to be found at several localities; it appeared to be fairly plentiful
at the only locality visited, about 10 miles north of Fort Defiance, a
mile or two west of the Arizona- New Mexico line. This locality is on
and around the ranch of Navajo Charlie. An Indian guide reported the
occurrence of gem peridot and garnet on two prominent knobs several
miles east of this locality in Xew Mexico. Small peridots, occasionally
large enough for cutting, are found at Black liock, an outcrop of basic
rock near Fort Defiance. The peridot locality near Navajo Charlie's
could be reached from Fort Defiance,, though the trip was made from
Ganado, 35 miles west southwest, across by the Zilh-Tusayan Butte.
The
rocks between Ganado and the peridot locality consist chiefly of red
and grayish sandstones and conglomerate, with an outcrop of volcanic
rock forming Zilh-Tusayan Butte. Petrified wood is scattered over much
of the region, especially between Ganado and the Butte. The red
sandstone probably belongs chiefly to the undifferentiated Triassic as
mapped by N. H. Darton.a East of Zilh-Tusayan Butte the red
sandstone forms a large area of mesa and gently sloping country
extending to near and around the peridot area. The peridot is
associated with volcanic rocks which occupy a basin or depression 200
to 300 feet deep, partly surrounded by red sandstone mesa or plateau
country. The volcanic rocks outcrop over an area a mile long east and
west and three-fourths or more of a mile north and south. The basin is
drained by a wash which enters from the northwest, turns cast across
it, then south along the sandstone contact on the east side, and
finally cuts across the sandstone to the east near Navajo Charlie's
house. Several hollows and washes enter from different sides. A
prominent wash from the southwest, running in part along the sandstone
contact on that side, enters the other wash at the outlet of the basin,
where a gorge has been cut through the red sandstone. The A'olcanic
rock outcrops from small isolated hills in the valley or from ridges
extending from the sides into the valley. The valley floor, where many
of the peridots are found, is low and flat in places.
The
character of the volcanic rock varies in different exposures, some of
the differences being due to texture and grain and others due to
variation of composition. It was not possible to make a careful
E
etrographic study of
the different types, though it is hoped this may e done later, hence
type names will be used in a provisional way. The volcanic rocks are of
three types—coarse monzonite porphyry, orthoclase basalt, and
peridotite agglomerate. These rocks are associated with one another in
places apparently in an intricate-way, and their relations were not
determined.
The
monzonite porphyry is a spotted gray rock with white orthoclase and
oligoclase phenocrysts measuring up to 2 centimeters across. Biotite
phenocrysts are abundant also. The groundmass is very fine grained and
consists of feldspar laths, with some biotite and aegirite. A few
rounded, corroded quartz crystals are present. All of the monzonite
porphyry seen was badly altered, and in some the biotite had gone over
to chlorite completely, giving the rock a dull greenish cast which
resembled serpentine except for the remnants of white feldspar crystals
scattered throughout.
a Reconnaissance of part of western New Mexico and northern Arizona: Bull. U. S. Geol. Survey. (In preparation.)
13250—ii E 1U0S, it 2------53
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