The Difference Of The Level Is Very Remarkable In The Bottom Of The Two Parallel Basins Of Cauca And Magdalena.

The former, between Cali and Cantago, is from 500 to 404 toises; the latter, from Neiva to Ambalema, is from 265 to 150 toises high.

According to different geological hypotheses, it may be said either that the secondary formations have not accumulated to the same thickness between the eastern and central, as between the central and western chains; or, that the deposits have been made on the base of primitive rocks, unequally upheaved on the east and west of the Andes of Quindiu. The average difference of the thickness of these formations is 300 toises. The rocky ridge of the Angostura of Carare branches from the south-east, from the spur of Muzo, through which winds the Rio Negro. By this spur, and by those that come from the west, the eastern and central chains approach between Nares, Honda, and Mendales. In fact, the bed of the Rio Magdalena is narrowed in 5 and 5 degrees 18 minutes, on the east by the mountains of Sergento, and on the west by the spurs that are linked with the granitic mountains of Maraquito and Santa Ana. This narrowing of the bed of the river is in the same parallel with that of the Cauca, near the Salto de San Antonio; but, in the knot of the mountains of Antioquia the central and western chains join each other, while between Honda and Mendales, the tops of the central and eastern chains are so far removed that it is only the spurs of each system that draw near and are confounded together. It is also worthy of remark that the central Cordillera of New Grenada displays the loftiest summit of the Andes in the northern hemisphere. The peak of Tolima (latitude 4 degrees 46 minutes) which is almost unknown even by name in Europe, and which I measured in 1801, is at least 2865 toises high. It consequently surpasses Imbabura and Cotocache in the province of Quito, the Chiles of the table-lands of Los Pastos, the two volcanoes of Popayan and even the Nevados of Mexico and Mount Saint Elias of Russian America. The peak of Tolima, which in form resembles Cotapaxi, is perhaps inferior in height only to the ridge of the Sierra Nevada de Santa Marta, which may be considered as an insulated system of mountains.

The eastern chain, also called the chain of Choco and the east coast (of the Pacific), separates the provinces of Popayan and Antioquia from those of Barbacoas, Raposo and Choco. It is in general but little elevated, compared to the height of the central and eastern chains; it however presents great obstacles to the communications between the valley of Cauca and the shore. On its western slope lies the famous auriferous and platiniferous land,* which has during ages yielded more than 13,000 marks of gold annually. (* Choco, Barbacoas and Brazil are the only countries in which the existence of grains of platinum and palladium has hitherto been fully ascertained. The small town of Barbacoas is situated on the left bank of the Rio Telembi (a tributary of Patias or the Rio del Castigo) a little above the confluence of Telembi and the Guagi or Guaxi, nearly in latitude 1 degree 48 minutes. The ancient Provincia, or rather the Partido del Raposo, comprehends the insalubrious land extending from the Rio Dagua, or San Buenaventura, to the Rio Iscuande, the southern limit of Choco.) This alluvial zone is from ten to twelve leagues broad; its maximum of productiveness lies between the parallels of 2 and 6 degrees latitude; it sensibly impoverishes towards the north and south, and almost entirely disappears between 1 1/4 degree north latitude and the equator. The auriferous soil fills the basin of Cauca, as well as the ravines and plains west of the Cordillera of Choco; it rises sometimes nearly 600 toises above the level of the sea, and descends at least 40 toises.* (* M. Caldas assigns to the upper limit of the zone of gold-washings, only the height of 350 toises. Semanario tome 1 page 18; but I found the Seraderos[?] of Quilichao, on the north of Popayan, to be 565 toises high.) Platinum (and this fact is worthy of attention) has hitherto been found only on the west of the Cordillera of Choco, and not on the east, notwithstanding the analogy of the fragments of rocks of greenstone, phonolite, trachyte, and ferruginous quartz, of which the soil of the two slopes is composed. From the ridge of Los Robles, which separates the table-land of Almaguer from the basin of Cauca, the western chain forms, first, in the Cerros de Carpinteria, east of the Rio San Juan de Micay, the continuation of the Cordillera of Sindagua, broken by the Rio Patias; then, lowering northward, between Cali and Las Juntas de Dagua, and at the elevation of 800 to 900 toises, it sends out considerable spurs (latitude 4 1/4 to 5 degrees) towards the source of the Calima, the Tamana and the Andagueda. The two former of these auriferous rivers are tributary streams of the Rio San Juan del Choco; the second empties its waters into the Atrato. This widening of the western chain forms the mountainous part of Choco: here, between the Tado and Zitara, called also Francisco de Quibdo, lies the isthmus of Raspadura, across which a monk traced a navigable line of communication between the two oceans. The culminant point of this system of mountains appears to be the Peak of Torra, situated south-east of Novita.

The northern extremity of this enlargement of the Cordillera of Choco, which I have just described, corresponds with the junction formed on the east, between the same Cordillera and the central chain, that of Quindiu. The mountains of Antioquia, on which we have the excellent observations of Mr. Restrepo, may be called a knot of mountains, and on the northern limit of the plains of Buga, or the basin of Cauca, they join the central and western chains.

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