|
1250
|
MINERAL RESOURCES.
|
||
|
|
|||
|
of
the ground, in addition to the petrographic and other evidence,
rendered the presumption in favor of their derivation from the
peri-dotite almost a certainty.
Subsequent
to this visit extensive prospecting has been undertaken according to
plans suggested by the authors, large amounts of the green earth (which
disintegrates in water to a fine, impalpable mud) being washed and
screened in Little Missouri River. This work was done under the
supervision of Mr. Theodore Hartman, a civil engineer of Little Rock.
In the course of these operations two small diamonds were found in the
concentrates. This would have settled definitely the question of their
source had not some doubt existed through the possible accidental
admixture of small amounts of the surface soil with the underlying
green earth, as Mr. Hartman's careful precautionary measures to guard
against this were not followed by some of the men.
But
final and absolutely definite proof that the diamonds occur in the
peridotite and that those found have been derived from it was furnished
by the discovery, about the middle of March, of a diamond embedded in
the green earth, about 3 feet below the surface, while this was being
excavated for washing, a careful watch being also kept by the men for
just such a discovery. This specimen was brought to New York by one of
the parties interested, and was carefully examined by both of the
authors. The stone is white, apparently a flattened octahedron, firmly
embedded in the decomposed peridotite, so that only a portion of it is
visible, this being about 12 mm. long by 2 to i mm. wide. The
most careful scrutiny failed to reveal any evidence that it had been
artificially inserted, and no other conclusion was possible than that
it was actually in situ. Taking all the facts into consideration,
therefore, the occurrence of diamonds in the peridotite of Murfreesboro
may be regarded as unquestionable.
The
number of diamonds found up to the date of writing is 130, the weights
varying from one thirty-second of a carat up to 6-1/2 carats. The
majority are distorted octahedrons, a few being flattened and
triangular, and a small number are almost perfect octahedrons. No cubes
have been found. Most of the stones are white, a large proportion
being of good water and the white of exceptional purity, finer than
most African stones. A smaller number are brown; some are yellow, and
several small individuals are of bort.
The
mass is now being examined with the diamond drill, and fresh and solid
peridotite is found beneath varying depths of green earth, the greatest
depth yet reached being 186 feet.
As
this is the only place outside of South Africa where diamonds have been
found in peridotite, a brief comparison of the two localities will be
of interest, a more detailed statement being reserved for the future.
While, petrographically and chemically, the rocks around Kimberley and
near Murfreesboro are much alike, there are some decided differences.
The Murfreesboro rock is a true porphyritic lava, although the portion
now visible had not reached the surface, and it was evidently ejected
through a volcanic vent as a relatively quiet liquid flow, while the
peridotite of the South African pipes seems to be uniformly an igneous
breccia and to have been ejected by explosive eruptions in a more or
less fragmentary condition and probably mingled with a considerable
proportion of water. At Murfreesboro inclosed fragments of the rocks
traversed by the lava are wholly lack-
|
|||
|
|
|||