Precious Gem stones production in 1908 Page 41



	818	MINERAL RESOURCES.

	All of the diamonds are to be cut in Germany, and it is estimated that the wages paid for the work will amount to $500,000 annually at the present rate of production of the field. The diamonds are to be taxed at such a rate that the miners will secure about one-half the profit. Mining consists in simple sieving, washing, and picking out of diamonds. Almost no capital for machinery has been required so far and the work is done by natives with white overseers. The extent of the diamond field is being found to be greater than at first thought. Diamonds have been found along the coast to the south about 150 miles from Luderitz Bay and 75 miles from Orange River.^a That the deposits have formed from alluvial deposits is thought probable, and Orange River is pointed out as a possible original source. SOUTH AMERICA. Brazil.—The geology of the diamond-bearing highlands of Bahia— Chapada Diamantina, as a portion of it is called—has been described by J. C. Branner.⁶ This region lies northwest of Bahia and south and southeast of Rio Sao Francisco. About it comparatively little is known. Two railroads approach but do not penetrate this area, and travel must be accomplished with mules. The region is semiarid, and the climate hot, though healthful. Much of the country is covered with catinga forests, a tough, scrubby growth of timber. The country is fairly well watered along the streams, though subject to droughts away from them. The following is an outline of the geology with probable ages of the formations: Along the coast to the north of the city of Bahia is a series of Cretaceous and Tertiary strata resting on pre-Cambrian schists, gneisses, and granites. The latter extend from the sediment-aries along the coast westward to the highlands and form a nearly level plain with a few scattered hills and peaks over it. The Serra de Jacobina is the first mountain range of the highlands and is com- p osed of the conglomerates, shales, schists, and quartzites of the Minas series, 1,000 meters thick, and of Cambrian age. The bedding of this series strikes nearly north with the range and dips steeply to the east or is vertical. The crystalline rocks also appear on the west of the Jacobina Range. To the west of this is the great Tombador Range, composed of 400 meters of nearly horizontal Cambrian sandstones and quartzites resting directly on the crystallines. The edges of these beds of the Tombador formation, which dip gently westward, form great walls or scarps on the east. The Tombador beds can be traced northward toward Rio Sao Francisco, where they cap the flat-topped mountains or form the monoclinal ridges of that region. Above the Tombador beds are the Jacuipe flints, about 100 meters thick. Above these are 500 meters of the Caboclo formation, of Devonian age, composed of gray, red, yellow, black, and cream-colored shales. The upper part of this formation is in contrast with the overlying false-bedded pinkish sandstones, conglomerates, and quartzites of the Lavras series, of Carboniferous age. This series carries the diamonds and is about 700 meters thick. The Lavras series is overlain by 350 meters of red Triassic sandstones, very like the Triassic sandstones of New Jersey and Connecticut. These have been called the Estancia red beds. The last series exposed is the oMin. Jour., London, March 20,1909. ⁶ Eng. and Min. Jour., May, 15,1909.