Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
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At Piedras Azules These
Slates, Mingled With Hornblende, Cover In Concordant Stratification A
Blackish-Blue Slate, Very Fissile, And Traversed By Small Veins Of
Quartz.
The green slates include some strata of grunstein, and even
contain balls of that substance.
I nowhere saw the green slates
alternate with the black slates of the ravine of Piedras Azules: at
the line of junction these two slates appear rather to pass one into
the other, the green slates becoming of a pearl-grey in proportion as
they lose their hornblende.
Farther south, towards Parapara and Ortiz, the slates disappear. They
are concealed under a trap-formation more varied in its aspect. The
soil becomes more fertile; the rocky masses alternate with strata of
clay, which appear to be produced by the decomposition of the
grunsteins, the amygdaloids, and the phonolites.
The grunstein, which farther north was less granulous, and passed into
serpentine, here assumes a very different character. It contains balls
of mandelstein, or amygdaloid, eight or ten inches in diameter. These
balls, sometimes a little flattened, are divided into concentric
layers: this is the effect of decomposition. Their nucleus is almost
as hard as basalt, and they are intermingled with little cavities,
owing to bubbles of gas, filled with green earth, and crystals of
pyroxene and mesotype. Their basis is greyish blue, rather soft, and
showing small white spots which, by the regular form they present, I
should conceive to be decomposed feldspar. M. von Buch examined with a
powerful lens the species we brought.
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