DIAMOND MINING                 207
	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