The former consists of a rocky reef running like a wall across its bed, which at low water is at times crossed by the gauchos of the country on horseback, though during the floods it is passable in boats, by which, and canoes, the river is navigable without further danger as high up as the Missions. Beautiful specimens of silicified wood and variegated pebbles are found in the upper parts of the bed of this river, of which I brought many to this country. The Negro, which runs into it from the Banda Oriental, derives its name (the black river) from the sarsaparilla plant, which, at a particular season, rots upon its banks, and falls into the stream in such immense quantities as to discolour its waters, which are found to be highly medicinal, and much in request in consequence. The little village of Mercedes, near its mouth, has of late years been much resorted to by invalids from Buenos Ayres to drink these waters. The river Paraguay, as high as the Jaurú, was carefully laid down after the treaty of 1750; and the Spanish officers appointed to determine the boundaries, in virtue of that subsequently signed in 1777, surveyed the Paranã as high as the Tieté, as well as the whole of the Uruguay, and determined the courses of all their most important affluents in the course of the eighteen years during which they were employed in laying down the southern division only of this survey. The results of their labours, which were only stopped by the renewal of war, may justly be ranked amongst the most beautiful and perfect geographical works ever produced. Copies of the whole existed at Buenos Ayres during my time in the hands of Colonel Cabrer, one of the officers originally attached to the commissioners; and the Government of Buenos Ayres were in treaty for the purchase of them for the use of the topographical department of the state, where, it is to be hoped, they will not be buried in unprofitable obscurity. When the war with Brazil for the Banda Oriental broke out, in 1826, Colonel Cabrer drew a MS. map from these materials for the use of General Alvear, the Buenos Ayrean Commander-in-chief, which he was afterwards kind enough to present to me. By a curious coincidence, about the same time, I obtained possession of one upon a large scale of the southern provinces of Brazil, drawn, by the Emperor's order, from the best data to be collected at Rio de Janeiro, for the Marquis of Barbacena, who commanded the Brazilian army, and lost it at the battle of Ituzaingo. They have, I believe, afforded Mr. Arrowsmith data for materially improving his last maps of that part of South America. FOOTNOTES: [50] His positions will be found in the tables of fixed points given in the Appendix.