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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Igneous Phenomena (If
Their Existence Be Really Well Certified) Are Attributed By The People
To The Granitic Peaks Of Duida And Guaraco, And Also To The Calcareous
Mountain Of Cuchivano.
From these observations it results that gneiss-granite predominates in
the immense group of the mountains of the Parime,
As mica-slate-gneiss
prevails in the Cordillera of the coast; that in the two systems the
granitic soil, unmixed with gneiss and mica-slate, occupies but a very
small extent of country; and that in the coast-chain the formations of
clayey slate (thonschiefer), mica-slate, gneiss and granite succeed
each other in such a manner on the same line from east to west
(presenting a very uniform and regular inclination of their strata
towards the north-west), that, according to the hypothesis of a
subterraneous prolongation of the strata, the granite of Las
Trincheras and the Rincon del Diablo may be superposed on the gneiss
of the Villa de Cura, of Buenavista and Caracas; and the gneiss
superposed in its turn on the mica-slate and clay-slate of Maniquarez
and Chuparuparu in the peninsula of Araya. This hypothesis of a
prolongation of every rock, in some sort indefinite, founded on the
angle of inclination presented by the strata appearing at the surface,
is not admissible; and according to similar equally vague reasoning we
should be forced to consider the primitive rocks of the Alps of
Switzerland as superposed on the formation of the compact limestone of
Achsenberg, and that [transition, or identical with zechstein?] in
turn, as being superposed on the molassus of the tertiary strata.
2. FORMATION OF THE CLAY-SLATE (THONSCHIEFER) OF MALPASSO.
If, in the sketch of the formations of Venezuela, I had followed the
received division into primitive, intermediary, secondary and tertiary
strata, I might be doubtful what place the last stratum of mica-slate
in the peninsula of Araya should occupy. This stratum, in the ravine
(aroyo) of Robalo, passes insensibly in a carburetted and shining
slate, into a real ampelite. The direction and inclination of the
stratum remain the same, and the thonschiefer, which takes the look of
a transition-rock, is but a modification of the primitive mica-slate
of Maniquarez, containing garnets, cyanite, and rutile titanite. These
insensible passages from primitive to transition strata by clay-slate,
which becomes carburetted at the same time that it presents a
concordant position with mica-slate and gneiss, have also been
observed several times in Europe by celebrated geologists. The
existence of an independent formation of primitive slate
(urthonschiefer) may even be doubted, that is, of a formation which is
not joined below by strata containing some vestiges of monocotyledonous
plants.
The small thonschiefer bed of Malpasso (in the southern chain of the
littoral Cordillera) is separated from mica-slate-gneiss by a
co-ordinate formation of serpentine and diorite. It is divided into
two shelves, of which the upper presents green steatitous slate mixed
with amphibole, and the lower, dark-blue slate, extremely fissile, and
traversed by numerous veins of quartz.
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