GRAN CHACO (gran ch51k6), an ex tensive central tract of South America, extending from the S. tropic to 29° S. lat., and bounded on the E. by the Paraguay and Parana, and on the W. by the Ar gentine provinces of Santiago del Estero and Salta; area, about 180,000 square miles. The portion S. of the Pilcomayo belongs to Argentina, and the remaining third to Paraguay; but the possession of the upper section of the Pilcomayo is dis puted by Bolivia. The country rises grad ually from the Parana toward the N. W. as far as 25° 40' S. lat., when it dips to the valley of the San Francisco—part of a great depression extending through Bolivia nearly to the frontier of Peru, and subject to annual inundations. The Chaco is watered principally by two long, narrow, and tortuous streams, the Ber mejo and the Pilcomayo, flowing S. E. in courses generally parallel, and about 180 miles distant from each other. The
bed of the Bermejo oscillates backward and forward, and in 1870-1872 the river opened up a new channel (known as the Teuco) for nearly 200 miles. The most N. part of the Chaco is an extremely arid zone, but the banks of the upper Pilco rnayo are fertile and its sands auriferous, while S. of the Bermejo the primeval forest extends into Salta. Much of the region is of modern alluvial formation, and exceedingly fertile. Since 1537, when the first explorer, Captain Juan de Ayolas, marched with 250 men into the wilderness from which none ever re turned, numerous expeditions have been sent out from the surrounding countries; but the savage tribes, swamps, lagoons, and floods defeated all early attempts to open up the country. In 1884 garrisons were established along the Bermejo, and since 1885 permanent settlements have been made.