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.
