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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What Dr. Gillan Calls Somewhat
Vaguely, Mountains Of Hard Ferruginous Clay, Are Nothing But An
Alluvium Which We Find At The Foot Of Every Volcano.
Strata of clay
accompany basalts, as tufas accompany modern lavas.
Neither M.
Cordier nor myself observed in any part of Teneriffe a primitive
rock, either in its natural place, or thrown out by the mouth of
the Peak; and the absence of these rocks characterizes almost every
island of small extent that has an unextinguishied volcano. We know
nothing positive of the mountains of the Azores; but it is certain,
that the island of Bourbon as well as Teneriffe, exhibits only a
heap of lavas and basalts. No volcanic rock rears its head, either
on the Gros Morne, or on the volcano of Bourbon, or on the colossal
pyramid of Cimandef, which is perhaps more elevated than the Peak
of the Canary Islands.
Bory St. Vincent nevertheless asserted, that lavas including
fragments of granite have been found on the elevated plain of
Retama; and M. Broussonnet informed me, that on a hill above
Guimar, fragments of mica-slate, containing beautiful plates of
specular iron, had been found. I can affirm nothing respecting the
accuracy of this latter statement, which it would be so much the
more important to verify, as M. Poli, of Naples, is in possession
of a fragment of rock thrown out by Vesuvius,* which I found to be
a real mica-slate. (* In the valuable collection of Dr. Thomson,
who resided at Naples till 1805, is a fragment of lava enclosing a
real granite, which is composed of reddish feldspar with a pearly
lustre like adularia, quartz, mica, hornblende, and, what is very
remarkable, lazulite.
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