Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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The Porphyritic Lavas Are Affected By The Action
Of The Sulphuric Acid:
The alumine, magnesia, soda, and metallic
oxides gradually disappear; and often nothing remains but the
silex, which unites in mammillary plates, like opal.
These
siliceous concretions,* (* Opalartiger kieselsinter. The siliceous
gurh of the volcanoes of the Isle of France contains, according to
Klaproth, 0.72 silex, and 0.21 water; and thus comes near to opal,
which Karsten considers as a hydrated silex.) which M. Cordier
first made known, are similar to those found in the isle of Ischia,
in the extinguished volcanoes of Santa Fiora, and in the Solfatara
of Puzzuoli. It is not easy to form an idea of the origin of these
incrustations. The aqueous vapours, discharged through great
spiracles, do not contain alkali in solution, like the waters of
the Geyser, in Iceland. Perhaps the soda contained in the lavas of
the peak acts an important part in the formation of these deposits
of silex. There may exist in the crater small crevices, the vapours
of which are not of the same nature as those on which travellers,
whose attention has been directed simultaneously to a great number
of objects, have made experiments.
Seated on the northern brink of the crater, I dug a hole of some
inches in depth; and the thermometer placed in this hole rose
rapidly to 42 degrees. Hence we may conclude what must be the heat
in this solfatara at the depth of thirty or forty fathoms. The
sulphur reduced into vapour is condensed into fine crystals, which
however are not equal in size to those M. Dolomieu brought from
Sicily.
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