The great breadth of this zone of savannahs (from 100 to 120 leagues) renders the denomination of land-strait somewhat improper, at least if it be not geognostically applied to every communication of basins bounded by high Cordilleras. Perhaps this denomination more properly belongs to that part in which is situated the group of almost unknown mountains that surround the sources of the Rio Negro. In the basin comprehended between the eastern declivity of the Andes of New Grenada and the western part of the Sierra Parime, the savannahs, as we have observed above, stretch far beyond the equator; but their extent does not determine the southern limits of the basin here under consideration. These limits are marked by a ridge which divides the waters between the Orinoco and the Rio Negro, a tributary stream of the Amazon. The rising of a counter-slope almost imperceptible to the eye, forms a ridge that seems to join the eastern Cordillera of the Andes to the group of the Parime. This ridge runs from Ceja (latitude 1 degree 45 minutes), or the eastern slope of the Andes of Timana, between the sources of the Guayavero and the Rio Caguan, towards the isthmus that separates the Tuamini from Pimichin. In the Llanos, consequently, it follows the parallels of 20 degrees 30 minutes and 2 degrees 45 minutes. It is remarkable that we find the divortia aquarum further westward on the back of the Andes, in the knot of mountains containing the sources of the Magdalena, at a height of 900 toises above the level of the Llanos, between the Caribbean Sea and the Pacific ocean, and almost in the same latitude (1 degree 45 minutes to 2 degrees 20 minutes). From the isthmus of Javita towards the east, the line of the partition of waters is formed by the mountains of the Parime group; it first rises a little on the north-east towards the sources of the Orinoco (latitude 3 degrees 45 minutes ?) and the chain of Pacaraina (latitude 4 degrees 4 minutes to 4 degrees 12 minutes); then, during a course of 80 leagues, between the portage of the Anocapra and the banks of the Rupunuri, it runs very regularly from west to east; and finally, beyond the meridian 61 degrees 50 minutes, it again deviates towards lower latitudes, passing between the northern sources of the Rio Suriname, the Maroni, the Oyapoc and the southern sources of Rio Trombetas, Curupatuba, and Paru (latitude 2 degrees to 1 degree 50 minutes). These facts suffice to prove that this first line of partition of the waters of South America (that of the northern hemisphere) traverses the whole continent between the parallels of 2 and 4 degrees. The Cassiquiare alone has cut its way across the ridge just described. The hydraulic system of the Orinoco displays the singular phenomenon of a bifurcation where the limit of two basins (those of the Orinoco and the Rio Negro) crosses the bed of the principal recipient. In that part of the basin of the Orinoco which runs in the direction of from south to north, as well as in that running from west to east, the maxima of depression are found at the foot of the Sierra Parime, we may even say, on its outline.
4. THE BASIN OF THE RIO NEGRO AND THE AMAZON.
This is the central and largest basin of South America. It is exposed to frequent equatorial rains, and the hot and humid climate develops a force of vegetation to which nothing in the two continents can be compared. The central basin, bounded on the north by the Parime group, and on the south by the mountains of Brazil, is entirely covered by thick forests, while the two basins at the extremities of the continent (the Llanos of Venezuela and the Lower Orinoco, and the Pampas of Buenos Ayres or the Rio de la Plata) are savannahs or prairies, plains without trees and covered with gramina. This symmetric distribution of savannahs bounded by impenetrable forests, must be connected with physical revolutions which have operated simultaneously over great surfaces.
(4a.) PART OF THE BASIN OF THE AMAZON, RUNNING FROM EAST TO WEST, BETWEEN 2 DEGREES NORTH AND 12 DEGREES SOUTH; 880 LEAGUES IN LENGTH.
The western shore of this basin is formed by the chain of the Andes, from the knot of the mountains of Huanuco to the sources of the Magdalena. It is enlarged by the spurs of the Rio Beni,* (* The real name of this great river, respecting the course of which geographers have been so long divided, is Uchaparu, probably water (para) of Ucha; Peni also signifies river or water; for the language of the Maypures has very many analogies with that of the Moxos; and veni (oueni) signifies water in Maypure, as una in Moxo. Perhaps the river retained the name of Maypure, after the Indians who spoke that language had emigrated northward in the direction of the banks of the Orinoco.) rich in gem-salt, and composed of several ranges of hills (latitude 8 degrees 11 minutes south) which advance into the plains on the eastern bank of the Paro. These hills are transformed on our maps into Upper Cordilleras and Andes of Cuchao. Towards the north the basin of the Amazon, of which the area (244,000 square leagues) is only one-sixth less than the area of all Europe, rises in a gentle slope towards the Sierra Parime. At 68 degrees of west longitude the elevated part of this Sierra terminates at 3 1/2 degrees north latitude. The group of little mountains surrounding the source of the Rio Negro, the Inirida and the Xie (latitude 2 degrees) the scattered rocks between the Atabapo and the Cassiquiare, appear like groups of islands and rocks in the middle of the plain. Some of those rocks are covered with signs or symbolical sculpture. Nations, very different from those who now inhabit the banks of the Cassiquiare, penetrated into the savannahs; and the zone of painted rocks, extending more than 150 leagues in breadth, bears traces of ancient civilization. On the east of the sporadic groups of rocks (between the meridian of the bifurcation of the Orinoco and that of the confluence of the Essequibo with the Rupunuri) the lofty mountains of the Parime commence only in 3 degrees north latitude; where the plains of the Amazon terminate.
The limits of the plains of the Amazon are still less known towards the south than towards the north. The mountains that exceed 400 toises of absolute height do not appear to extend in Brazil northward of the parallels 14 or 15 degrees of south latitude, and west of the meridian of 52 degrees; but it is not known how far the mountainous country extends, if we may call by that name a territory bristled with hills of one hundred or two hundred toises high. Between the Rio dos Vertentes and the Rio de Tres Barras (tributary streams of the Araguay and the Topayos) several ridges of the Monts Parecis run northward. On the right bank of the Topayos a series of little hills advance as far as the parallel of 5 degrees south latitude, to the fall (cachoeira) of Maracana; while further west, in the Rio Madeira, the course of which is nearly parallel with that of the Topayos, the rapids and cataracts indicate no rocky ridges beyond the parallel of 8 degrees. The principal depression of the basin of which we have just examined the outline, is not near one of its banks, as in the basin of the Lower Orinoco, but at the centre, where the great recipient of the Amazon forms a longitudinal furrow inclining from west to east, under an angle of at least 25 degrees. The barometric measurements which I made at Javita on the banks of the Tuamini, at Vasivia on the banks of the Cassiquiare and at the cataract of Rentema, in the Upper Maranon, seem to prove that the rising of the Llanos of the Amazon northward (at the foot of the Sierra Parime) is 150 toises, and westward (at the foot of the Cordillera of the Andes of Loxa), 190 toises above the sea-level.
(4b.) PART OF THE BASIN OF THE AMAZON STRETCHING FROM SOUTH TO NORTH.
This is the zone or land-strait by which, between 12 and 20 degrees of south latitude, the plains of the Amazon communicate with the Pampas of Buenos Ayres. The western bank of this zone is formed by the Andes, between the knot of Porco and Potosi, and that of Huanuco and Pasco. Part of the spurs of the Rio Beni, which is but a widening of the Cordilleras of Apolobamba and Cuzco and the whole promontory of Cochabamba, advance eastward into the plains of the Amazon. The prolongation of this promontory has given rise to the idea that the Andes are linked with a series of hills which the Serras dos Parecis, the Serra Melgueira, and the supposed Cordillera of San Fernando, throw out towards the west. This almost unknown part of the frontiers of Brazil and Upper Peru merits the attention of travellers. It is understood that the ancient mission of San Jose de Chiquitos (nearly latitude 17 degrees, longitude 67 degrees 10 minutes, supposing Santa Cruz de la Sierra, in latitude 17 degrees 25 minutes, longitude 66 degrees 47 minutes) is situated in the plains, and that the mountains of the spur of Cochabamba terminate between the Guapaix (Rio de Mizque) and the Parapiti, which lower down takes the names of Rio San Miguel and Rio Sara. The savannahs of the province of Chiquitos communicate on the north with those of Moxos, and on the south with those of Chaco; but a ridge or line of partition of the waters is formed by the intersection of two gently sloping plains. This ridge takes its origin on the north of La Plata (Chuquisaca) between the sources of the Guapaix and the Cachimayo, and it ascends from the parallel of 20 degrees to that of 15 1/2 degrees south latitude, consequently on the north-east, towards the isthmus of Villabella. From this point, one of the most important of the whole hydrography of America, we may follow the line of the partition of the water to the Cordillera of the shore (Serra do Mar). It is seen winding (latitude 17 to 20 degrees) between the northern sources of the Araguay, the Maranhao or Tocantines, the Rio San Francisco and the southern sources of the Parana. This second line of partition which enters the group of the Brazil mountains on the frontier of Capitania of Goyaz separates the flowings of the basin of the Amazon from those of the Rio de la Plata, and corresponds, south of the equator, with the line we have indicated in the northern hemisphere (latitude 2 to 4 degrees), on the limits of the basins of the Amazon and the Lower Orinoco.
If the plains of the Amazon (taking that denomination in the geognostic sense we have given it) are in general distinguished from the Llanos of Venezuela and the Pampas of Buenos Ayres, by the extent and thickness of their forests, we are the more struck by the continuity of the savannahs in that part running from south to north. It would seem as though this sea of verdure stretched forth an arm from the basin of Buenos Ayres, by the Llanos of Tucuman, Manso, Chuco, the Chiquitos, and the Moxos, to the Pampas del Sacramento and the savannahs of Napo, Guaviare, Meta and Apure. This arm crosses, between 7 and 3 degrees south latitude, the basin of the forests of the Amazon; and the absence of trees on so great an extent of territory, together with the preponderance which the small monocotyledonous plants have acquired, is a phenomenon of the geography of plants which belongs perhaps to the action of ancient pelagic currents or other partial revolutions of our planet.