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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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.
If we may be allowed to judge of the structure of the whole Sierra
Parime, from the part which I examined in 6 degrees of longitude, and
4 degrees of latitude, we may believe it to be entirely composed of
gneiss-granite; I saw some beds of greenstone and amphibolic slate,
but neither mica-slate, clay-slate, nor banks of green limestone,
although many phenomena render the presence of mica-slate probable on
the east of the Maypures and in the chain of Pacaraina.
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