at
an average altitude of 4,000 feet, divide the rivers of the north coast
line countries of South America from the great basin of the Amazon,
south of which are the diamond fields of Bahia, Brazil. The diamonds
are found in a somewhat remote part of Demarara. The route from
Georgetown is up the coast about 20 miles to the mouth of the Essequibo
river; up the river about 50 miles to Bartica, and thence by small
boats, which must be carried around numerous falls, up the Mazaruni
river to San-San-Kopai landing. This latter part of the journey
consumes usually 14 days, the distance being between 90 and 100 miles.
The diamondiferous area lies on the left bank of the Mazaruni river,
between two of its tributaries, the Putareng creek, and a river which
Alfred de Andrade, one of the pioneers of the fields, calls the Puruni
river. Diamonds have been found also on the left bank of the Curibrong
river near its junction with the Potari river.
The
diamond-bearing gravel occurs- usually under an overburden of gray
sandy soil and rests on a clayey subsoil. The crystals are found
chiefly in the lower part of the gravel and sticking in the upper part
of the clay, associated with jasper and other siliceous pebbles. The
diamonds are usually of good quality but very small. Most of them run
about 15 to the carat. Stones of three-quarters of a carat and over are
extremely rare, and none of importance have yet been registered from
these fields. An early report of a day's work on a promising deposit,
by one of the companies with a working force of 18 men, gave 22 cubic
yards of ground handled, yielding 90 diamonds which weighed 5.7
carats. In the year ending June 30, 1902, 132,077 stones were
