The
peridotite has been so well described by Branner and Brackett that a
few words of description will suffice here. In the hand specimen the
freshest peridotite is very dark, brownish or greenish black, and
porphyritic with an aphanitic groundmass. The phenocrysts, which make
up about one-quarter of the rock, are mostly of olivine, the color of
which is commonly black, but in some specimens of the rock is yellow or
brown, especially in the less fresh specimens. With these are fewer,
small, glistening plates of a bronzy biotite.
In thin section the olivine phenocrysts are seen to be well-formed from
0.5 to 5.0 mm. long, some of them colorless and fairly fresh in the
interior, but most of them largely altered to serpentine. The biotite
phenocrysts are seen as irregular brownish-yellow patches, highly
pleochroic. The groundmass shows very numerous, small, stout prisms of
colorless augite, and many small grains of magnetite and of
transparent, yellow, isotropic perofskite, embedded in a vitreous
base, which is either colorless or yellow. This is usually isotropic,
but may exhibit faint aggregate polarization through decomposition.
The
fresh peridotite was analyzed chemically by Brackett, and in its
general features it does not differ widely from that found at some
other localities, and his figures accord well with the mineral
composition shown by the microscope. Ferric oxide predominates over
ferrous, which may be ascribed to the somewhat weathered condition of
the rock, and potash is rather high, this being connected with the
presence of biotite.
As
is true of almost all peridotites, the Pike Countv rock weathers
readily, two stages of decomposition being observable here. The first
consists of the mechanical disintegration of the mass into an aggregate
of small, angular fragments, which still preserve nearly their original
hardness, though the olivines are almost wholly altered. This passes
into the stage which is of most interest in connection with the
occurrence of diamonds, the solid rock being reduced to a soft,
friable mass. This is either of a yellowish or, more
characteristically, of a yellowish-green or light bluish-green color,
the two varieties having been called locally the yellow and green
earths. In these the outlines of the original olivines are still well
seen, but the mineral is reduced to a soft, yellow substance, while the
biotites are comparatively little changed. From the preservation of the
form of the olivines it is clear that the decomposition of the rock has
gone on in place, and that the yellow and green earths have not been
transported from a distance.
These
peculiar decomposition products occupy by far the greater portion of
the igneous area, occurring just beneath the thin surface soil covering
all that portion which lies south of the ridge of three hills mentioned
above, while a considerable portion is laid bare and cut into deep
gullies by surface erosion. Apparently the yellow earth overlies the
green and represents the last stage of decomposition, while both
overlie the less decomposed, f ragmental, weathered form, though this
last reaches the surface in places. The exact depth to which the green
earth extends has not yet been ascertained throughout the area, but
drill holes sunk to a depth of 30 feet still show the green earth in
places, while elsewhere it is less deep and the solid peridotite is
struck below it.
The
first diamond was discovered on August 1, 1906, by Mr. John M.
Huddleston, who had purchased the land lying south of the ridge
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