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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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. I could discover no fragmentary
stratum (grauwacke) nor kieselschiefer nor chiastolite.
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