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GEMS AND PRECIOUS STONES. 1055
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Even
with the decreased activity in gem mining in southern California in
1912, some rich finds were made in San Diego County. Dr. L. P. Gratacap
mentions three magnificent specimens of rubellite of fine, deep
gerardia color, found in a pocket at Pala, that have been added to the
Morgan collection in the Natural History Museum, New York. One of these
is attached to a large, well-developed quartz crystal. The specimens
are composed of compound fascicled groups with fine gem nuclei. One,
irregular in shape, is 8 inches high and 3-1/2 inches in diameter at
the top, tapers to a base, and has small, divergent crystals. The
second is 4 inches high and 3 J inches thick, and the third is 8 inches
high and 2 to 2\ inches thick and is associated with albite and lepidolite.
TURQUOISE.
Two
interesting articles on turquoise appeared in 1912. These were "The
origin of turquoise in the Burro Mountains, New Mexico," by Sidney
Paige,1 and "The aboriginal use of turquois in North America," by Joseph E. Pogue.2
Data for Paige's article were obtained during the course of geologic
mapping of the Silver City quadrangle and the conclusions reached are
the result of study of the local occurrence of turquoise and its
relations to the general geology of the region. Pogue's paper is an
extract from a manuscript report dealing with the ethnology, mythology,
mineralogy, geology, and technology of turquoise, prepared during work
for the United States National Museum.
Several
deposits of turquoise have been operated in the Burro Mountains of
Grant County, N. Mex. The most extensive mining was conducted by the
Azure Co., and a large quantity of very fine grade gem material was
obtained. Other deposits were developed by the American Gem &
Turquoise Co. and M. M. Porterfield with varying success.
The
occurrence is somewhat similar at the different deposits, the turquoise
being found in seams, veinlets, and nodules in fracture zones in rocks
of granitic and quartz monzonitic composition which have undergone more
or less decomposition and alteration. Paige's article deals chiefly
with the occurrence at the Azure mine and the following notes have been
abstracted from it.
The
turquoise occurs in granitic and quartz monzonitic intrusive recks. The
granite belongs to a pre-Cambrian complex and was intruded in late
Cretaceous or post-Cretaceous time by a mass of quartz monzonite
porphyry followed by dikes of similar rock. The region has been
strongly fractured, and the turquoise occurs in marked fracture zones.
Fracturing was followed or accompanied by solutions probably of
magmatic origin, which deposited cupriferous pyrite and quartz, formed
sericite, and completely destroyed or altered hornblende and biotite.
The
quartz monzonite intrusions were exposed by a prolonged period of
erosion following the uplift of Cretaceous rocks. An important feature
was the extensive subaerial erosion of Pleistocene time in which
planated surfaces were formed. The turquoise occurs from the surface to
shallow depths and is related to the surface of plana-tion. During this
period copper-bearing sulphides were altered to
i Econ. Geology, Juno, 1912, pp. 382-392.
8 Am. Anthropologist, new ser., vol. 14, No. 3,1912, pp. 437-466.
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