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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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.
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