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1218 MINERAL RESOURCES.
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outcrop
is being vigorously prospected, and it is expected that a small
experimental washing plant will be installed. Mr. Cooney kindly
furnished specimens of certain rocks he considered to be typical of
the formation near Oroville and to be identical with the kimberlite and
associated rocks of South Africa. The specimens marked "blue"' were
soft, highly serpentinized rocks whose original nature could not be
determined. In a thin section under the microscope the rock was found
to contain rounded crystals of some mineral, probably olivine, entirely
altered to serpentine. The section contained a great deal of serpentine
throughout, with some in little streaks and veinlets. There were small
fragments, apparently a variety of feldspar, of some larger crystals,
mostly lost in grinding. In hand specimens portions of the rock have a
brecciated appearance, while other pieces appear to have a more even
texture. The color is dull, ranging from greenish to bluish green to
bluish black. Slickenside partings are not uncommon in various
directions through the small pieces examined. Specimens of two other
types of rock were "bull's-eyes." or spherical balls with concentric
layer structure, and concretions or nodules of calcium carbonate. The
"bull's-eyes" range in size from that of an egg up, and have been
formed by the weathering of a fine-grained basic rock, probably of the
basalt or diabase family. It has a fine porphyritic texture with a
slight development of amygdules.
According to Mr. Cooney, the " bull's-eyes " and lime nodules were found on the surface and to a depth of 20 feet,
mixed with earthy material and somewhat cemented together. This
gradually gave place to soft j'ellow ground at 25 feet. The yellow
ground held out to a depth of 40 feet, where a semisiliceous stratum,
"somewhat like the ' floating reefs' encountered in the diamond chutes
or pipes of South Africa," was met. Below this came in the "blue
ground" described above. The following minerals are reported by Mr.
Cooney in the Oroville serpentine or " blue" and "j'ellow " earths as
similar to those minerals commonly associated with the diamonds in
South Africa: Menaccanite, magnetite, olivine, garnets, spinel rubies,
topaz, beryl, chrysoprase, agate and other forms of chalcedony,
zircons, etc.
The
specimens sent to the Survey by Mr. Cooney as typical '"blue earth " of
the Oroville locality do not bear much resemblance to the genuine
kimberlite of South Africa. Points of likeness are the extensive
serpentinization in each, a general bluish-green color, and probable
brecciation of the California rock compared with the evident extreme
brecciation of the kimberlite. On the other hand, the general
appearance of the two rocks on close inspection is very unlike. The
California serpentine apparently does not contain inclusions of other
types of rocks forming the walls, while the kimberlite contains these
in quantity, as black shale, conglomerate, quartzite, melaphyre, etc.
The numerous plates of biotite common in the true kimberlite were not
observed in the California rock. The presence of feldspathic material
in the California serpentine indicates a quite different type of rock
from the kimberlite. The latter is regarded as a serpentinized volcanic
peridotite breccia, with the serpentine probably derived from a less
basic rock, possibly of the gabbro or diorite class. As far as can be
learned, the presence of " bull's-eyes" is not a prominent feature of
the South African diamond mines, while the occurrence of lime
concretions is not limited to the outcrop of diamond pipes alone, but
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