Llanos of the Lower Orinoco, the Meta, : 29,000.
and the Guaviare
Plains of the Amazon : 260,400.
Pampas of Rio de la Plata and Patagonia : 135,200.
Plains between the eastern chain of the
Andes of Cundinamarca and the chain of Choco : 12,300.
Plains of the shore on the west of the Andes : 20,000.
TOTAL : 456,900.
The whole surface of South America contains 571,300 square leagues (20 to a degree), and the proportion of the mountainous country to the region of the plains is as 1 to 3.9. The latter region, on the east of the Andes, comprises more than 424,600 square leagues, half of which consists of savannahs; that is to say, it is covered with gramina.
SECTION 2.
GENERAL PARTITION OF GROUND. DIRECTION AND INCLINATION OF THE STRATA. RELATIVE HEIGHT OF THE FORMATIONS ABOVE THE LEVEL OF THE OCEAN.
In the preceding section we have examined the inequalities of the surface of the soil, that is to say, the general structure of the mountains and the form of the basins rising between those variously grouped mountains. These mountains are sometimes longitudinal, running in narrow bands or chains, similar to the veins that preserve their directions at great distances, as the Andes, the littoral chain of Venezuela, the Serra do Mar of Brazil, and the Alleghenies of the United States. Sometimes they are in masses with irregular forms, in which upheavings seem to have taken place as on a labyrinth of crevices or a heap of veins, as for example in the Sierra Parime and the Serra dos Vertentes. These modes of formation are linked with a geognostic hypothesis, which has at least the recommendation of being founded on facts observed in remote times, and which strongly characterize the chains and groups of mountains. Considerations on the aspect of a country are independent of those which indicate the nature of the soil, the heterogeneity of matter, the superposition of rocks and the direction and inclination of strata.
In taking a general view of the geological constitution of a chain of mountains, we may distinguish five elements of direction too often confounded in works of geognosy and physical geography. These elements are:—
1. The longitudinal axis of the whole chain.
2. The line that divides the waters (divortia aquarum).
3. The line of ridges or elevation passing along the maxima of height.
4. The line that separates two contiguous formations into horizontal
sections.
5. The line that follows the fissures of stratification.
This distinction is the more necessary, there existing probably no chain on the globe that furnishes a perfect parallelism of all these directing lines. In the Pyrenees, for instance, 1, 2, 3, do not coincide, but 4 and 5 (that is, the different formations which come to light successively, and the direction of the strata) are obviously parallel to 1, or to the direction of the whole chain. We find so often in the most distant parts of the globe, a perfect parallelism between 1 and 5, that it may be supposed that the causes which determine the direction of the axis (the angle under which that axis cuts the meridian) are generally linked with causes that determine the direction and inclination of the strata. This direction of the strata is independent of the line of the formations, or their visible limits at the surface of the soil; the lines 4 and 5 sometimes cross each other, even when one of them coincides with 1, or with the direction of the longitudinal axis of the whole chain. The RELIEF of a country cannot be precisely explained on a map, nor can the most erroneous opinions on the locality and superposition of the strata be avoided, if we do not apprehend with clearness the relation of the directing lines just mentioned.
In that part of South America to which this memoir principally relates, and which is bounded by the Amazon on the south, and on the west by the meridian of the Snowy Mountains (Sierra Nevada) of Merida, the different bands or zones of formations (4) are sensibly parallel with the longitudinal axis (1) of the chains of mountains, basins or interposed plains. It may be said in general that the granitic zone (including under that denomination the rocks of granite, gneiss and mica-slate) follows the direction of the Cordillera of the shore of Venezuela, and belongs exclusively to that Cordillera and the group of the Parime mountains; since it nowhere pierces the secondary and tertiary strata in the Llanos or basin of the Lower Orinoco. Thence it results that the same formations do not constitute the region of plains and that of mountains.