The Orinoco is one of those remarkable rivers which, after numerous windings, first towards the west and then to the north, finally return towards the east in such a manner as to bring both its estuary and its source into nearly the same meridian. From the Chiguire and the Gehette as far as the Guaviare, the course of the Orinoco inclines westward, as if it would pour its waters into the Pacific. Here branches off to the south, the Cassiquiare, a remarkable river, but little known to Europeans, which unites with the Rio Negro, or as the natives call it, the Guainia: furnishing the only example of a bifurcation which forms in the very interior of a continent a natural connection between two great river valleys. The nature of the soil, and the junction of the Guaviare and Atabapo with the Orinoco, cause the latter to deflect suddenly northwards. From a want of correct geographical data, the Guaviare, flowing in from the west, was long regarded as the true source of the Orinoco. The doubts advanced since 1797 by an eminent geographer, M. Buache, regarding the possibility of a connection with the Amazon, have, I trust, been completely set at rest by my expedition. In an uninterrupted voyage of 920 miles, I penetrated through a remarkable net-work of rivers, from the Rio Negro, along the Cassiquiare, into the Orinoco; across the interior of the continent, from the Brazilian boundary to the coast of Caracas. In the upper portion of this fluvial district, between 3° and 4° north lat., nature has exhibited, at many different points, the puzzling phenomenon of the so-called _black waters_. The Atabapo, whose banks are adorned with _Carolinias_ and arborescent _Melastomas_, the Temi, Tuamini, and Guainia, are all rivers of a brown or coffee colour, which, under the deep shade of the palms, assumes a blackish, inky tint. When placed in a transparent vessel, the water appears of a golden yellow colour. These black streams reflect the images of the southern stars with the most remarkable clearness. Where the waters flow gently they afford the astronomer, who is making observations with reflecting instruments, a most excellent artificial horizon. An absence of crocodiles as well as of fish—greater coolness—less torment from stinging mosquitoes—and salubrity of atmosphere, characterize the region of the black rivers. They probably owe their singular colour to a solution of carburetted hydrogen, to the rich luxuriance of tropical vegetation, and to the abundance of plants on the soil over which they flow. Indeed, I have observed that on the western declivity of the Chimborazo, towards the shores of the Pacific, the overflowing waters of the Rio de Guayaquil gradually assume a golden yellow, approaching to a coffee colour, after they have covered the meadows for several weeks.