Salitre limestones and marbles, about 300 meters thick and of Jurassic age.
Diamonds
and carbonados are found together in this region, and their mining
constitutes an important industry of Bahia. The source of the diamonds
and carbonados has been pretty definitely traced to the Lavras series,
though Dr. Branner has never seen one in place in these rocks. The
streams and river gravels have been found richest in diamonds where
they flow through or over large areas of the Lavras beds. No eruptive
rocks occur in the Lavras beds which could possibly give rise to all
the diamonds and carbonados of the region. Areas of serpentine have
been found in the crystalline rocks that underlie the sedimentary
rocks. It is possible the diamonds originated in peridotite in these
crystalline rocks, and were later washed out and deposited with the
Lavras beds.
The
most productive area has been that between Sincoro on the south and
Morro do Chapeo on the north. Whether this is due to a particular
richness of the beds in this area or to the favorable supply of water,
or to both, is not known. Mining methods used have been crude,
practically all work being done by hand. The possibilities of
mechanical apparatus, especially dredges, should be tested. In the
stream beds and places where the natives have been able to work there
is but little virgin gravel left. In swamp and marsh lands along the
river not accessible to the natives it is probable rich deposits still
exist and could be worked with dredges. Dredges must operate under
difficulties in this region, however, as transportation facilities are
poor and repair shops and factories a long way off.
British Guiana.—The exports of diamonds from British Guiana during the calendar year 1908 amounted to 4,968 carats,0
valued at $40,872, as against 2,220 carats, valued at $17,550, in 1907.
The shipments between January 1 and May 12, 1909, amounted to 1,095
carats,6 valued at $7,350. A new deposit of diamonds was
discovered near the Dukwarri Cataract, about 115 miles from the mouth
of Cuyuna River.c Two parcels of stones weighed 138 carats,
showing that the diamonds of this deposit are small in size, as are
those from the other deposits of British Guiana.
INDIA.
The production of diamonds in India during 1908 amounted to 140.75 carats, valued at £940,d against 628 carats, valued at £2,784e in
1907. These figures represent the production in the States of Panna,
Charkhari, and Ajaigarh, in central India. A few diamonds are reported
found in the Anantapus district of the Anadras Presidency, though no
statistics have been obtained.
AUSTRALIA.
New South Wales—The
production of diamonds in New South Wales during 1907 amounted to 2,539
carats, valued at £2,056, a decrease of 288 carats in quantity and of
£64 in value from 1906.
aMin. Jour., London, February 6,1909. & Jlin. Jour., London, June 5, 1909. . cMin. Jour., London, May 5,1909.
d Advance
statement of the production of minerals in India in 1908 by the
director of the Geological Survey of India, June 10, 3909. eRec. Geol.
Survey India, vol. 37, pt. 1,1908. /Ann. Kept. Dept. Mines, New South
Wales, 1907, p. 59.
