Of the three insulated groups of mountains, that is to say, those which are not branches of the Cordillera of the Andes and its continuation towards the shore of Venezuela, one is on the north, and the other two on the west of the Andes: that on the north is the Sierra Nevada de Santa Marta; the two others are the Sierra de la Parime, between 4 and 8 degrees of north latitude, and the mountains of Brazil, between 15 and 28 degrees south latitude. This singular distribution of great inequalities of soil produces three plains or basins, comprising a surface of 420,600 square leagues, or four-fifths of all South America, east of the Andes. Between the coast-chain of Venezuela and the group of the Parime, the plains of the Apure and the Lower Orinoco extend; between the group of Parime and the Brazil mountains are the plains of the Amazon, of the Rio Negro and the Madeira, and between the groups of Brazil and the southern extremity of the continent are the plains of Rio de la Plata and of Patagonia. As the group of the Parime in Spanish Guiana, and of the Brazil mountains (or of Minas Geraes and Goyaz), do not join the Cordillera of the Andes of New Grenada and Upper Peru towards the west, the three plains of the Lower Orinoco, the Amazon, and the Rio de la Plata, are connected by land-straits of considerable breadth. These straits are also plains stretching from north to south, and traversed by ridges imperceptible to the eye but forming divortia aquarum. These ridges (and this remarkable phenomenon has hitherto escaped the attention of geologists) are situated between 2 and 3 degrees north latitude, and 16 and 18 degrees south latitude. The first ridge forms the partition of the waters which fall into the Lower Orinoco on the north-east, and into the Rio Negro and the Amazon on the south and south-east; the second ridge divides the tributary streams of the right bank of the Amazon and the Rio de la Plata. These ridges, of which the existence is only manifested, as in Volhynia, by the course of the waters, are parallel with the coast-chain of Venezuela; they present, as it were, two systems of counter-slopes partially developed, in the direction from west to east, between the Guaviare and the Caqueta, and between the Mamori and the Pilcomayo. It is also worthy of remark that in the southern hemisphere the Cordillera of the Andes sends an immense counterpoise eastward in the promontory of the Sierra Nevada de Cochabamba, whence begins the ridge stretching between the tributary streams of the Madeira and the Paraguay to the lofty group of the mountains of Brazil or Minas Geraes. Three transversal chains (the coast-mountains of Venezuela, of the Orinoco or Parime, and the Brazil mountains) tend to join the longitudinal chain (the Andes) either by an intermediary group (between the lake of Valencia and Tocuyo), or by ridges formed by the intersection of counter-slopes in the plains. The