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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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. The feldsparry lavas of the Peak, of a
much less black tinge than those of Arso in the island of Ischia,
whiten at the edge of the crater from the effect of the acid
vapours; but internally they are not found to be colourless like
that of the feldsparry lavas of the Solfatara at Naples, which
perfectly resemble the trappean porphyries at the foot of
Chimborazo. In the middle of the Malpays, at the height of the
cavern of ice, we found among the vitreous lavas with pitch-stone
and obsidian bases, blocks of real greenish-grey, or mountain-green
phonolite, with a smooth fracture, and divided into thin laminae,
sonorous and keen edged. These masses were the same as the
porphyrschiefer of the mountain of Bilin in Bohemia; we recognised
in them small long crystals of vitreous feldspar.
This regular disposition of lithoid basaltic lava and feldsparry
vitreous lava is analogous to the phenomena of all trappean
mountains; it reminds us of those phonolites lying in very ancient
basalts, those close mixtures of augite and feldspar which cover
the hills of wacke or porous amygdaloids: but why are the
porphyritic or feldsparry lavas of the Peak found only on the
summit of the volcano? Should we conclude from this position that
they are of more recent formation than the lithoid basaltic lava,
which contains olivine and augite? I cannot admit this last
hypothesis; for lateral eruptions may have covered the feldsparry
nucleus, at a period when the crater had ceased its activity. At
Vesuvius also, we perceive small crystals of vitreous feldspar only
in the very ancient lavas of the Somma. These lavas, setting aside
the leucite, very nearly resemble the phonolitic ejections of the
Peak of Teneriffe. In general, the farther we go back from the
period of modern eruptions, the more the currents increase both in
size and extent, acquiring the character of rocks, by the
regularity of their position, by their division into parallel
strata, or by their independence of the present form of the ground.
The Peak of Teneriffe is, next to Lipari, the volcano that has
produced most obsidian. This abundance is the more striking, as in
other regions of the earth, in Iceland, in Hungary, in Mexico, and
in the kingdom of Quito, we meet with obsidians only at great
distances from burning volcanoes. Sometimes they are scattered over
the fields in angular pieces; for instance, near Popayan, in South
America; at other times they form isolated rocks, as at Quinche,
near Quito. In other places (and this circumstance is very
remarkable), they are disseminated in pearl-stone, as at
Cinapecuaro, in the province of Mechoacan,* (* To the west of the
city of Mexico.) and at the Cabo de Gates, in Spain. At the peak of
Teneriffe the obsidian is not found towards the base of the
volcano, which is covered with modern lava: it is frequent only
towards the summit, especially from the plain of Retama, where very
fine specimens may be collected. This peculiar position, and the
circumstance that the obsidian of the Peak has been ejected by a
crater which for ages past has thrown out no flames, favour the
opinion, that volcanic vitrifications, wherever they are found, are
to be considered as of very ancient formation.
Obsidian, jade, and Lydian-stone,* (* Lydischerstein.) are three
minerals, which nations ignorant of the use of copper or iron, have
in all ages employed for making keen-edged weapons. We see that
wandering hordes have dragged with them, in their distant journeys,
stones, the natural position of which the mineralogist has not yet
been able to determine. Hatchets of jade, covered with Aztec
hieroglyphics, which I brought from Mexico, resemble both in their
form and nature those made use of by the Gauls, and those we find
among the South Sea islanders. The Mexicans dug obsidian from
mines, which were of vast extent; and they employed it for making
knives, sword-blades, and razors. In like manner the Guanches, (in
whose language obsidian was called tabona,) fixed splinters of that
mineral to the ends of their lances. They carried on a considerable
trade in it with the neighbouring islands; and from the consumption
thus occasioned, and the quantity of obsidian which must have been
broken in the course of manufacture, we may presume that this
mineral has become scarce from the lapse of ages. We are surprised
to see an Atlantic nation substituting, like the natives of
America, vitrified lava for iron. In both countries this variety of
lava was employed as an object of ornament: and the inhabitants of
Quito made beautiful looking-glasses with an obsidian divided into
parallel laminae.
There are three varieties of obsidian at the Peak. Some form
enormous blocks, several toises long, and often of a spheroidal
shape. We might suppose that they had been thrown out in a softened
state, and had afterwards been subject to a rotary motion. They
contain a quantity of vitreous feldspar, of a snow-white colour,
and the most brilliant pearly lustre.
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