Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 3 - By Alexander Von Humboldt And Aime Bonpland.
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It Is Surrounded By Pyroxenic Amygdaloid; It Would No
Doubt Be Seen Below, Issuing Immediately From Gneiss-Granite, Like The
Phonolite Of Biliner Stein, In Bohemia, Which Contains Fragments Of
Gneiss Embedded In Its Mass.
Does there exist in South America another group of rocks, which may be
preferably designated by the name of volcanic rocks, and which are as
distinct from the chain of the Andes, and advance as far towards the
east as the group that bounds the steppes of Calabozo?
Of this I
doubt, at least in that part of the continent situated north of the
Amazon. I have often directed attention to the absence of pyroxenic
porphyry, trachyte, basalt and lavas (I range these formations
according to their relative age) in the whole of America eastward of
the Cordilleras. The existence even of trachyte has not yet been
verified in the Sierra Nevada de Merida which links the Andes and the
littoral chain of Venezuela. It would seem as if volcanic fire, after
the formation of primitive rocks, could not pierce into eastern
America. Possibly the scarcity of argentiferous veins observed in
those countries may be owing to the absence of more recent volcanic
phenomena. M. Eschwege saw at Brazil some layers (veins?) of diorite,
but neither trachyte, basalt, dolerite, nor amygdaloid; and he was
therefore much surprised to see, in the vicinity of Rio Janeiro, an
insulated mass of phonolite, exactly similar to that of Bohemia,
piercing through gneiss. I am inclined to believe that America, on the
east of the Andes, would have burning volcanoes if, near the shore of
Venezuela, Guiana and Brazil, the series of primitive rocks were
broken by trachytes, for these, by their fendillation and open
crevices, seem to establish that permanent communication between the
surface of the soil and the interior of the globe, which is the
indispensable condition of the existence of a volcano. If we direct
our course from the coast of Paria by the gneiss-granite of the Silla
of Caracas, the red sandstone of Barquisimeto and Tocuyo, the slaty
mountains of the Sierra Nevada de Merida, and the eastern Cordillera
of Cundinamarca to Popayan and Pasto, taking the direction of
west-south-west, we find in the vicinity of those towns the first
volcanic vents of the Andes still burning, those which are the most
northerly of all South America; and it may be remarked that those
craters are found where the Cordilleras begin to present trachytes, at
a distance of eighteen or twenty-five leagues from the present coast
of the Pacific Ocean.* (* I believe the first hypotheses respecting
the relation between the burning of volcanoes and the proximity of the
sea are contained in Aetna Dialogus, a very eloquent though
little-known work by Cardinal Bembo.) Permanent communications, or at
least communications frequently renewed, between the atmosphere and
the interior of the globe, have been preserved only along that immense
crevice on which the Cordilleras have been upheaved; but subterranean
volcanic forces are not less active in eastern America, shaking the
soil of the littoral Cordillera of Venezuela and of the Parime group.
In describing the phenomena which accompanied the great earthquake of
Caracas,* on the 26th March, 1812, I mentioned the detonations heard
at different periods in the mountains (altogether granitic) of the
Orinoco.
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