From the west its most important affluents are the Pilcomayo and the Vermejo, which fall into it below Assumption:--both flow through a prodigious extent of country, having their sources in the rich districts of Upper Peru. The first passes not far from Potosi, and, after a thousand windings through the chaco, or desert, falls into the Paraguay by two branches, the one called the Araquay, in lat. 25° 21´ 29", according to an observation taken by Azara; the other, about nine leagues below it. M. de Angelis has I think clearly shown that the river to the north of Assumption, which Azara has laid down as the most northern branch of the Pilcomayo, is the Fogones of Quiroga. In 1741 Father Castañares attempted an exploration of the Pilcomayo, in the expectation that it would facilitate a communication with the Jesuit missions in the province of Chiquitos; but after many hardships and difficulties, at the end of eighty-three days, he was obliged to give it up, from the river becoming too shallow for his canoes to pass on. In 1785 Azara attempted to ascend it by the Araquay, in a small vessel; but after proceeding about twenty leagues, was obliged to return, for the same reason,--want of water; although it was at the season of the floods, and the river was more than ordinarily full. The Vermejo, on the contrary, which falls into the Paraguay still further down, has been more than once proved to afford a navigable communication with the province of Salta: First by Cornejo, in 1790; who, starting from the confluence of the rivers Centa and Tarija, reached the Paraguay in fifty-five days; the distance by the river being, according to his computation, no less than 407-1/2 leagues. And more recently, in 1826, by Don Pablo Soria, the agent of some spirited individuals in Buenos Ayres, who about that time formed an association for the purpose of endeavouring to open a water-communication between the capital and the rich districts of the Upper Provinces. The vessel they built for the purpose was fifty-two feet long, and drew about two feet water; which, with but little more assistance than was necessary to keep in the mid-stream, was floated down from the neighbourhood of Oran by the current, and in fifty-seven days entered the Paraguay, without any other impediment than a feeble attempt on the part of some Indians, armed with bows and arrows, to annoy them as they passed through their lands.