Personal Narrative of Travels to the Equinoctial Regions of America, During the Years 1799-1804, Vol.3 — 31 - Chapter 3.32, Part 1

CHAPTER 3.

32. GEOGNOSTIC DESCRIPTION OF SOUTH AMERICA, NORTH OF THE RIVER AMAZON, AND EAST OF THE MERIDIAN OF THE SIERRA NEVADA DE MERIDA.

The object of this memoir is to concentrate the geological observations which I collected during my journeys among the mountains of New Andalusia and Venezuela, on the banks of the Orinoco and in the Llanos of Barcelona, Calabozo and the Apure; consequently, from the coast of the Caribbean Sea to the valley of the Amazon, between 2 and 10 1/2 degrees north latitude.

The extent of country which I traversed in different directions was more than 15,400 square leagues.

It has already formed the subject of a geological sketch, traced hastily on the spot, after my return from the Orinoco, and published in 1801. At that period the direction of the Cordillera on the coast of Venezuela and the existence of the Cordillera of Parime were unknown in Europe.

No measure of altitude had been attempted beyond the province of Quito; no rock of South America had been named; there existed no description of the superposition of rocks in any region of the tropics.

Under these circumstances an essay tending to prove the identity of the formations of the two hemispheres could not fail to excite interest.

The study of the collections which I brought back with me, and four years of journeying in the Andes, have enabled me to rectify my first views, and to extend an investigation which, by reason of its novelty, had been favourably received.

That the most remarkable geological relations may be the more easily seized, I shall treat aphoristically, in different sections, the configuration of the soil, the general division of the land, the direction and inclination of the beds and the nature of the primitive, intermediary, secondary and tertiary rocks.

SECTION 1.

Configuration of the Country.

Inequalities of the Soil.

Chains and Groups of Mountains.

Divisionary Ridges.

Plains or Llanos.

South America is one of those great triangular masses which form the three continental parts of the southern hemisphere of the globe.

In its exterior configuration it resembles Africa more than Australia.

The southern extremities of the three continents are so placed that, in sailing from the Cape of Good Hope (latitude 33 degrees 55 minutes) to Cape Horn (latitude 55 degrees 58 minutes), and doubling the southern point of Van Diemen's Land (latitude 43 degrees 38 minutes), we see those lands stretching out towards the south pole in proportion as we advance eastward.

A fourth part of the 571,000 square sea leagues* (* Almost double the extent of Europe.

) which South America comprises is covered with mountains distributed in chains or gathered together in groups.

The other parts are plains forming long uninterrupted bands covered with forests or gramina, flatter than in Europe, and rising progressively, at the distance of 300 leagues from the coast, between 30 and 170 toises above the level of the sea.

The most considerable mountainous chain in South America extends from south to north according to the greatest dimension of the continent;

it is not central like the European chains, nor far removed from the sea-shore, like the Himalaya and the Hindoo-Koosh; but it is thrown towards the western extremity of the continent, almost on the coast of the Pacific Ocean.

Referring to the profile which I have given* of the configuration of South America (* Map of Columbia according to the astronomical observations of Humboldt by A. H.

Brue 1823. ), in the latitude of Chimborazo and Grand Para, across the plains of the Amazon, we find the land low towards the east, in an inclined plane, at an angle of less than 25 seconds on a length of 600 leagues; and if, in the ancient state of our planet, the Atlantic Ocean, by some extraordinary cause, ever rose to 1100 feet above its present level (a height one-third less than the table-lands of Spain and Bavaria), the waves must, in the province of Jaen de Bracamoros, have broken upon the rocks that bound the eastern declivity of the Cordilleras of the Andes.

The rising of this ridge is so inconsiderable compared to the whole continent that its breadth in the parallel of Cape Saint Roche is 1400 times greater than the average height of the Andes.

We distinguish in the mountainous part of South America a chain and three groups of mountains, namely, the Cordillera of the Andes, which the geologist may trace without interruption from Cape Pilares, in the western part of the Straits of Magellan, to the promontory of Paria opposite the island of Trinidad; the insulated group of the Sierra Nevada de Santa Marta; the group of the mountains of the Orinoco, or of La Parime; and that of the mountains of Brazil.

The Sierra de Santa Marta being nearly in the meridian of the Cordilleras of Peru and New Grenada, the snowy summits descried by navigators in passing the mouth of the Rio Magdalena are commonly mistaken for the northern extremity of the Andes.

I shall soon prove that the colossal group of the Sierra de Santa Marta is almost entirely separate from the mountains of Ocana and Pamplona which belong to the eastern Cordillera of New Grenada.

The hot plains through which runs the Rio Cesar, and which extend towards the valley of Upar, separate the Sierra Nevada from the Paramo de Cacota, south of Pamplona.

The ridge which divides the waters between the gulf of Maracaibo and the Rio Magdalena is in the plain on the east of the Laguna Zapatoza.

If, on the one hand, the Sierra de Santa Marta has been erroneously considered (on account of its eternal snow, and its longitude) to be a continuation of the Cordillera of the Andes, on the other hand, the connexion of that same Cordillera with the coast mountains of the provinces of Cumana and Caracas has not been recognized.

The littoral chain of Venezuela, of which the different ranges form the Montana de Paria, the isthmus of Araya, the Silla of Caracas and the gneiss-granite mountains north and south of the lake of Valencia, is joined between Porto Cabello, San Felipe and Tocuyo to the Paramos de las Rosas and Niquitao, which form the north-east extremity of the Sierra de Merida, and the eastern Cordillera of the Andes of New Grenada.

It is sufficient here to mention this connexion, so important in a geological point of view;

for the denominations of Andes and Cordilleras being altogether in disuse as applied to the chains of mountains extending from the eastern gulf of Maracaibo to the promontory of Paria, we shall continue to designate those chains (stretching from west to east) by the names of littoral chain, or coast-chain of Venezuela.

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 two extremities of the three Llanos which communicate by land-straits, the Llanos of the Lower Orinoco, the Amazon, and the Rio de la Plata or of Buenos Ayres, are steppes covered with gramina, while the intermediary Llano (that of the Amazon) is a thick forest.

With respect to the two land-straits forming bands directed from north to south (from the Apure to Caqueta across the Provincia de los Llanos, and the sources of the Mamori to Rio Pilcomayo, across the province of Mocos and Chiquitos) they are bare and grassy steppes like the plains of Caracas and Buenos Ayres.

In the immense extent of land east of the Andes, comprehending more than 480,000 square sea leagues, of which 92,000 are a mountainous tract of country, no group rises to the region of perpetual snow; none even attains the height of 1400 toises.

This lowering of the mountains in the eastern region of the New Continent extends as far as 60 degrees north latitude; while in the western part, on the prolongation of the Cordillera of the Andes, the highest Summits rise in Mexico (latitude 18 degrees 59 minutes) to 2770 toises, and in the Rocky Mountains (latitude 37 to 40 degrees) to 1900 toises.

The insulated group of the Alleghenies, corresponding in its eastern position and direction with the Brazil group, does not exceed 1040 toises.

* (* The culminant point of the Alleghenies is Mount Washington in New Hampshire, latitude 44 1/4 degrees.

According to Captain Partridge its height is 6634 English feet.

) The lofty summits, therefore, thrice exceeding the height of Mont Blanc, belong only to the longitudinal chain which bounds the basin of the Pacific Ocean, from 55 degrees south to 68 degrees north latitude, that is to say, the Cordillera of the Andes.

The only insulated group that can be compared with the snowy summits of the equinoctial Andes, and which attains the height of nearly 3000 toises, is the Sierra de Santa Marta; it is not situated on the east of the Cordilleras, but between the prolongation of two of their branches, those of Merida and Veragua.

The Cordilleras, where they bound the Caribbean Sea, in that part which we designate by the name of Coast Chain of Venezuela, do not attain the extraordinary height (2500 toises) which they reach in their prolongation towards Chita and Merida.

Considering separately the groups of the east, those of the shore of Venezuela, of the Parime, and Brazil, we see their height diminish from north to south.

The highest summits of each group are the Silla de Caracas (1350 toises), the peak of Duida (1300 toises), the Itacolumi and the Itambe* (900 toises).

(* According to the measure of MM.

Spix and Martius the Itambe de Villa de Principe is 5590 feet high.

) But, as I have elsewhere observed, it would be erroneous to judge the height of a chain of mountains solely from that of the most lofty summits.

The peak of the Himalayas, accurately measured, is 676 toises higher than Chimborazo (* The Peak Iewahir, latitude 30 degrees 22 minutes 19 seconds; longitude 77 degrees 35 minutes 7 seconds east of Paris, height 4026 toises, according to MM.

Hodgson and Herbert.

); Chimborazo is 900 toises higher than Mont Blanc; and Mont Blanc 653 toises higher than the peak of Nethou.

* (* This peak, called also peak of Anethou or Malahita, or eastern peak of Maladetta, is the highest summit of the Pyrenees.

It rises 1787 toises and consequently exceeds Mont Perdu by 40 toises.

) These differences do not furnish the relative average heights of the Himalayas, the Andes, the Alps and the Pyrenees, that is, the height of the back of the mountains, on which arise the peaks, needles, pyramids, or rounded domes.

It is that part of the back where passes are made, which furnishes a precise measure of the minimum of the height of the great chains.

In comparing the whole of my measures with those of Moorcroft, Webb, Hodgson, Saussure and Ramond, I estimate the average height of the top of the Himalayas, between the meridians of 75 and 77 degrees, at 2450 toises; the Andes* (at Peru, Quito and New Grenada), at 1850 toises (* In the passage of Quindiu, between the valley of the Magdalena and that of the Rio Cauca, I found the culminant point (la Garita del Parama) to be 1798 toises; it is however, regarded as one of the least elevated.

The passages of the Andes of Guanacas, Guamani and Micuipampa, are respectively 2300, 1713, and 1817 toises above sea-level.

Even in 33 degrees south latitude the road across the Andes between Mendoza and Valparaiso is 1987 toises high.

I do not mention the Col de l'Assuay, where I passed, near la Ladera de Cadlud, on a ridge 2428 toises high, because it is a passage on a transverse ridge joining two parallel chains.

);

the summit of the Alps and Pyrenees at 1150 toises.

The difference of the mean height of the Cordilleras (between 5 degrees north and 2 degrees south latitude) and the Swiss Alps, is consequently 200 toises less than the difference of their loftiest summits; and in comparing the passes of the Alps, we see that their average height is nearly the same, although peak Nethou is 600 toises lower than Mont Blanc and Mont Rosa.

Between the Himalaya* (* The passes of the Himalaya that lead from Chinese Tartary into Hindostan (Nitee-Ghaut, Bamsaru, etc. ) are from 2400 to 2700 toises high.

) and the Andes, on the contrary, (considering those chains in the limits which I have just indicated), the difference between the mean height of the ridges and that of the loftiest summits presents nearly the same proportions.

Taking an analogous view of the groups of mountains at the east of the Andes, we find the average height of the coast-chain of Venezuela to be 750 toises; of the Sierra Parime, 500 toises; of the Brazilian group, 400 toises; whence it follows that the mountains of the eastern region of South America between the tropics are, when compared to the medium elevation of the Andes, in the relation of one to three.

The following is the result of some numerical statements, the comparison of which affords more precise ideas on the structure of mountains in general.

* (* The Cols or passes indicate the minimum of the height to which the ridge of the mountains lowers in a particular country.

Now, looking at the principal passes of the Alps of Switzerland (Col Terret, 1191 toises, Mont Cenis, 1060 toises; Great Saint Bernard, 1246 toises; Simplon, 1029 toises; and on the neck of the Pyrenees, Benasque, 1231 toises; Pinede, 1291 toises; Gavarnic, 1197 toises; Cavarere, 1151 toises; it would be difficult to affirm that the Pyrenees are lower than the average height of the Swiss Alps.

) TABLE OF HEIGHTS OF VARIOUS RANGES.

COLUMN 1 : NAMES OF THE CHAINS OF MOUNTAINS.

COLUMN 2 : THE HIGHEST SUMMITS IN TOISES.

COLUMN 3 : MEAN HEIGHT OF THE RIDGE IN TOISES.

COLUMN 4 : PROPORTION OF THE MEAN HEIGHT OF THE RIDGES TO THAT A THE HIGHEST SUMMITS.

Himalayas (between north latitude : 4026 : 2450 : 1 : 1. 6. 30 degrees 18 minutes and 31 degrees 53 minutes, and longitude 75 degrees 23 minutes and 77 degrees 38 minutes) Cordillera of the Andes (between : 3350 : 1850 : 1 : 1. 8. latitude 5 and 2 degrees south) Alps of Switzerland : 2450 : 1150 : 1 : 2. 1. Pyrenees : 1787 : 1150 : 1 : 1. 5. Littoral Chain of Venezuela : 1350 : 750 : 1 : 1. 8. Group of the Mountains of the Parime : 1300 : 500 : 1 : 2. 6. Group of the Mountains of Brazil : 900 : 500 : 1 : 2. 3. If we distinguish among the mountains those which rise sporadically, and form small insulated systems,* (* As the groups of the Canaries, the Azores, the Sandwich Islands, the Monts-Dores, and the Euganean mountains.

) and those that make part of a continued chain,* (* The Himalayas, the Alps, and the Andes.

) we find that, notwithstanding the immense height* of the summits of some insulated systems (* Among the insulated systems, or sporadic mountains, Mowna-Roa is generally regarded as the most elevated summit of the Sandwich Islands.

Its height is computed at 2500 toises, and yet at some seasons it is entirely free from snow.

An exact measure of this summit, situated in very frequented latitudes, has for 25 years been desired in vain by naturalists and geologists.

), the culminant points of the whole globe belong to continuous chains--to the Cordilleras of Central Asia and South America.

In that part of the Andes with which I am best acquainted, between 8 degrees south latitude and 21 degrees north latitude, all the colossal summits are of trachyte.

It may almost be admitted as a general rule that whenever the mass of mountains rises in that region of the tropics much above the limit of perpetual snow (2300 to 2470 toises), the rocks commonly called primitive (for instance, gneiss-granite or mica-slate) disappear, and the summits are of trachyte or trappean-porphyry.

I know only a few rare exceptions to this law, and they occur in the Cordilleras of Quito where the Nevados of Conderasto and Cuvillan, situated opposite to the trachytic Chimborazo, are composed of mica-slate and contain veins of sulphuret of silver.

Thus in the groups of detached mountains which rise abruptly from the plains the loftiest summits, such as Mowna-Roa, the Peak of Teneriffe, Etna and the Peak of the Azores, present only recent volcanic rocks.

It would, however, be an error to extend that law to every other continent, and to admit, as a general rule, that, in every zone, the greatest elevations have produced trachytic domes: gneiss-granite and mica-slate constitute the summits of the ridge, in the almost insulated group of the Sierra Nevada of Grenada and the Peak of Malhacen,* (* This peak, according to the survey of M. Clemente Roxas, is 1826 toises above the level of the sea, consequently 39 toises higher than the loftiest summit of the Pyrenees (the granitic peak of Nethou) and 83 toises lower than the trachytic peak of Teneriffe.

The Sierra Nevada of Grenada forms a system of mountains of mica-slate, passing to gneiss and clay-slate, and containing shelves of euphotide and greenstone.

), as they also do in the continuous chain of the Alps, the Pyrenees and probably the Himalayas.

* (* If we may judge from the specimens of rocks collected in the gorges and passes of the Himalayas or rolled down by the torrents.

) These phenomena, discordant in appearance, are possibly all effects of the same cause: granite, gneiss, and all the so-styled primitive Neptunian mountains, may possibly owe their origin to volcanic forces, as well as the trachytes; but to forces of which the action resembles less the still-burning volcanoes of our days, ejecting lava, which at the moment of its eruption comes immediately into contact with the atmospheric air; but it is not here my purpose to discuss this great theoretic question.

After having examined the general structure of South America according to considerations of comparative geology, I shall proceed to notice separately the different systems of mountains and plains, the mutual connection of which has so powerful an influence on the state of industry and commerce in the nations of the New Continent.

I shall give only a general view of the systems situated beyond the limits of the region which forms the special object of this memoir.

Geology being essentially founded on the study of the relations of juxtaposition and place, I could not treat of the littoral chain and the chain of the Parime separately, without touching on the other systems south and west of Venezuela.