162 ECONOMIC GEOLOGY
10 million carats, and from S.W. Africa, 7 million carats.
Owing to its hardness and resistance to weathering the dia-
mond is widely distributed in alluvial deposits, which were
for long the only source. Brazil vielded most of the supply
from 1721 till 1870.
Soutn Arrica—The first South African diamonds! were
discovered in alluvial deposits in 1867. They were found
at Kimberley in 1870, in an oval patch of a yellow ground
covering the unoxidized * blue-ground (kimberlite) which
proved to be an ultra-basic intrusion. After the workings had
been amalgamated into one company in 1888 deep mining
became possible, and has been carried to 3500 feet. The
country consists of the Upper Carboniferous Ecca shale,
which rests on pre-Palzozoic quartzites and basic lava;
they lie on crystalline schists, which have been intruded by
quartz-porphyry and pegmatite. The diamonds. are found
in the blue-ground with olivine, pyroxenes, biotite, and
garnet, and the largest number of mineral species found
in any one rock. The kimberlite includes pseudo-spherulites
similar to those formed around geysers; it was probably
saturated with superheated steam, and was viscous rather
than molten. The diamonds are scattered through the rock,
and as some of them are broken, they were formed before
its final consolidation. At Newlands, Kimberley, some dia-
monds were found in eclogite boulders; but 20 tons of
these boulders from the Kimberley Mine did not yield a
single diamond. Diamonds have been found in other
igneous plugs in South Africa, but they are absent, or practi-
cally absent, from nine-tenths of the kimberlite occurrences.
The Premier Mine yielded the largest known diamond, the
Cullinan, which weighed 22 0z., or 3025 carats, and has
been cut into 105 gems.
BraziL—Elsewhere diamonds have been derived mostly
from pneumatolytic contact rocks and pegmatites, as in
Brazil, India, Southern Rhodesia, and West Africa. The
Brazilian diamond fields are of two different types. Dia-
mantina in Central Minas Geraes, is a belt 250 miles long
by 20 miles wide, and consists of pre-Pal®ozoic quartzites,
schists, and pegmatites, and the overlying Diamantina Con-
glomerate. The diamonds occur in the conglomerate, but
LP. A, Wagner, Diamond Fields, Southern Africa, 1014.
