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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The Morros Of San Juan Are Formed Of Limestone Of A Crystalline
Texture; Sometimes Very Compact, Sometimes Spongy, Of A Greenish-Grey,
Shining, Composed Of Small Grains, And Mixed With Scattered Spangles
Of Mica.
This limestone yields a strong effervescence with acids.
I
could not find in it any vestige of organized bodies. It contains in
subordinate strata, masses of hardened clay of a blackish blue, and
carburetted. These masses are fissile, very heavy, and loaded with
iron; their streak is whitish, and they produce no effervescence with
acids. They assume at their surface, by their decomposition in the
air, a yellow colour. We seem to recognize in these argillaceous
strata a tendency either to the transition-slates, or to the
kieselschiefer (schistose jasper), which everywhere characterise the
black transition-limestones. When in fragments, they might be taken at
first sight for basalt or hornblende.* (* I had an opportunity of
examining again, with the greatest care, the rocks of San Juan, of
Chacao, of Parapara, and of Calabozo, during my stay at Mexico, where,
conjointly with M. del Rio, one of the most distinguished pupils of
the school of Freyberg, I formed a geognostical collection for the
Colegio de Mineria of New Spain.) Another white limestone, compact,
and containing some fragments of shells, backs the Morros de San Juan.
I could not see the line of junction of these two limestones, or that
of the calcareous formation and the diabasis.
The transverse valley which descends from Piedras Negras and the
village of San Juan, towards Parapara and the Llanos, is filled with
trap-rocks, displaying close affinity with the formation of green
slates, which they cover.
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