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 Crystals Which Mr. Friesleben And Myself Have
Made Known Under The Denomination Of Foliated Olivine (Blattriger
Olivin) Belong, According
To Mr. Karsten, to the pyroxene augite.
Journal des Mines de Freiberg 1791 page 215.) Amphibole is in
general very
Rare at Teneriffe, not only in the modern lithoid
lavas, but also in the ancient basalts, as has been observed by M.
Cordier, who resided longer at the Canaries than any other
mineralogist. Nepheline, leucite, idocrase, and meionite have not
yet been seen at the peak of Teneriffe; for a reddish-grey lava,
which we found on the slope of Monte Verde, and which contains
small microscopic crystals, appears to me to be a close mixture of
basalt and analcime.* (* This substance, which M. Dolomieu
discovered in the amygdaloids of Catania in Sicily, and which
accompanies the stilbites of Fassa in Tyrol, forms, with the
chabasie of Hauy, the genus Cubicit of Werner. M. Cordier found at
Teneriffe xeolite in an amygdaloid which covers the basalts of La
Punta di Naga.) In like manner the lava of Scala, with which the
city of Naples is paved, contains a close mixture of basalt,
nepheline, and leucite. With respect to this last substance, which
has hitherto been observed only at Vesuvius and in the environs of
Rome, it exists perhaps at the peak of Teneriffe, in the old
currents of lava now covered by more recent ejections. Vesuvius,
during a long series of years, has also thrown out lavas without
leucites: and if it be true, as M. von Buch has rendered very
probable, that these crystals are formed only in the currents which
flow either from the crater itself, or very near its brink, we must
not be surprised at not finding them in the lavas of the peak. The
latter almost all proceed from lateral eruptions, and consequently
have been exposed to an enormous pressure in the interior of the
volcano.
In the plain of Retama, the basaltic lavas disappear under heaps of
ashes, and pumice-stone reduced to powder. Thence to the summit,
from 1500 to 1900 toises in height, the volcano exhibits only
vitreous lava with bases of pitch-stone* (* Petrosilex resinite.
Hauy.) and obsidian. These lavas, destitute of amphibole and mica,
are of a blackish brown, often varying to the deepest olive green.
They contain large crystals of feldspar, which are not fissured,
and seldom vitreous. The analogy of those decidedly volcanic masses
with the resinite porphyries* (* Pechstein-porphyr. Werner.) of the
valley of Tribisch in Saxony is very remarkable; but the latter,
which belong to an extended and metalliferous formation of
porphyry, often contain quartz, which is wanting in the modern
lavas. When the basis of the lavas of the Malpays changes from
pitchstone to obsidian, its colour is paler, and is mixed with
grey; in this case, the feldspar passes by imperceptible gradations
from the common to the vitreous. Sometimes both varieties meet in
the same fragment, as we observed also in the trappean porphyries
of the valley of Mexico.
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