opinion
that the diamonds were originally connected with eclogite, a
metamorphic rock carrying somewhat more silica than characterizes the
present blue ground, and containing considerable garnet.
About
ninety-five per cent of the world's supply of diamonds comes at the
present time from the South African mines, their annual production
being about 2,500,000 carats. Other countries which produce small
quantities of diamonds, besides those already mentioned, are Borneo,
Australia, British Guiana, and the United States.
The
diamonds of Borneo come from two portions of the island, one field
being in the western and the other in the southern part. These fields
have been known and worked from time immemorial; but have afforded only
a small supply, the product varying from two thousand to six thousand
carats annually. In the western part of the island the diamonds occur
in alluvial gravels, and their parent rock is not known. In the
southern part they occur in a conglomerate overlying strata of Eocene
age. The majority of the diamonds obtained are small and of rather poor
quality. Their mining is performed in a desultory way by native Malays
and Chinese, and the supply seems gradually to be decreasing.
The
first discovery of diamonds in Australia was made in 1851 in placer
gold-mining in New South Wales. The locality was not far from Bathurst.
Since then in this locality, and the neighboring head waters of the
Macquarie River, a number of small diamonds have been found. The
largest number were found along the Cudgegong River, northwest of
Mudgee, in an old river drift covered with basalt. About 2,500 stones
were obtained there in 1869. Accompanying the diamonds are gold,
garnet, zircon, tinstone, or cassiterite, tourmaline, and magnetite.
The gold and diamonds are obtained as in California by tunneling under
the basalt so as to excavate the gravels. Another locality in New South
Wales which has yielded diamonds is in the vicinity of Bingera. Here
the diamonds occur in gold and ruby-bearing sands, the accompanying
minerals being quite similar to those mentioned above.
In
Southern and Western Australia and in Tasmania a few diamonds have
also been found. The Australian diamonds are all small, none of over 6
carats weight being known. The yield from New South Wales in 1899 was
reported to be 25,874 carats.
Small
diamonds have been found at several points in the Ural Mountains. The
first were obtained about 1829 in the vicinity of Bissersk, Government
of Perm, occurring in alluvial sands with gold, platinum, quartz,
magnetite, and anatase. It is said that Alexander Humboldt
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