The upper Orinoco basin includes some of the best known, as well as some of the least explored districts in the whole territory. The old mission station of Esmeralda (longitude 65° 40´ W., latitude 3° 11´ N.) marks the limit of any attempt at civilisation on the upper Orinoco, and beyond this point our knowledge of the country is very scant indeed. Below this point the river and forests and savannahs near its bank are comparatively well known, from the number of travellers and small rubber prospectors, as far as San Fernando, and below this again little exploration has been carried out away from the river, which, nevertheless, is the main line of communication with Pericos, below the Atures Rapids, whence steamers run down the lower Orinoco to Ciudad Bolivar, and so afford communication with the outside world. The Atures Rapids, the biggest on the Orinoco, form at present an effectual barrier to through communication by steamer, between the upper and lower river, a difficulty formerly obviated by the construction of a now disused cart-road from Pericos to Salvajitos, above the rapids, a distance of 14 kilometres. But though the rapids have thus barred the advance of civilisation, their great beauty and the possibility that one day they may afford the power for an electric railway along the line of the old cart-road beyond compensate for any such disadvantage. Humboldt thus describes the Maipures and Atures Rapids in his “Ansichten der Natur.” They are, he says, “to be regarded as a countless number of small cascades succeeding each other like steps. The _Raudal_ (as the Spanish term this kind of cataract) is formed by an archipelago of islands and rocks, which so contract the bed of the river that its natural width of more than 8,500 feet is often reduced to a channel scarcely navigable to the extent of 20 feet. At the present day the eastern side is far less accessible and far more dangerous than the western. “... It was with surprise I found, by barometrical measurements, that the entire fall of the Raudal (of Maipures) scarcely amounted to more than 30 or 32 feet.... I say with surprise, for I hence discovered that the tremendous roar and wild dashing of the stream arose from the contraction of its bed by numerous rocks and islands, and the counter-currents produced by the form and position of the masses of rock.