Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 3 - By Alexander Von Humboldt And Aime Bonpland.
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In That Region Local Circumstances Would Possibly
Lead Us To Regard The Amygdaloids Of Ortiz As Linked To A System
Of
transition rocks (amphibolic serpentine, diorite, and carburetted
slate of Malpasso); but the eruption of the trachytes across rocks
posterior
To the chalk (in the Euganean Mountains and other parts of
Europe) joined to the phenomenon of total absence of fragments of
pyroxenic porphyry, trachyte, basalt and phonolite (The fragments of
these rocks appear only in tufas or conglomerates which belong
essentially to basaltic formations or surround the most recent
volcanoes. Every volcanic formation is enveloped in breccia, which is
the effect of the eruption itself.), in the conglomerates or
fragmentary rocks anterior to the recent tertiary strata, renders it
probable that the appearance of trap rocks at the surface of the earth
is the effect of one of the last revolutions of our planet, even where
the eruption has taken place by crevices (veins) which cross
gneiss-granite, or the transition rocks not covered by secondary and
tertiary formations.
The small volcanic stratum of Ortiz (latitude 9 degrees 28 minutes to
9 degrees 36 minutes) formed the ancient shore of the vast basin of
the Llanos of Venezuela: it is composed on the points where I could
examine it of only two kinds of rocks, namely, amygdaloid and
phonolite. The greyish blue amygdaloid contains fendilated crystals of
pyroxene and mesotype. It forms balls with concentric layers of which
the flattened centre is nearly as hard as basalt. Neither olivine nor
amphibole can be distinguished. Before it shows itself as a separate
stratum, rising in small conic hills, the amygdaloid seems to
alternate by layers with the diorite, which we have mentioned above as
mixed with carburetted slate and amphibolic serpentine. These close
relations of rocks so different in appearance and so likely to
embarrass the observer give great interest to the vicinity of Ortiz.
If the masses of diorite and amygdaloid, which appear to us to be
layers, are very large veins, they may be supposed to have been formed
and upheaved simultaneously. We are now acquainted with two formations
of amygdaloid; one, the most common, is subordinate to the basalt: the
other, much more rare,* (* We find examples of the latter in Norway
(Vardekullen, near Skeen), in the mountains of the Thuringerwald; in
South Tyrol; at Hefeld in the Hartz, at Bolanos in Mexico etc.)
belongs to the pyroxenic porphyry.* (* Black porphyries of M. von
Buch.) The amygdaloid of Ortiz approaches, by its oryctognostic
characters, to the former of those formations, and we are almost
surprised to find it joining, not basalt, but phonolite,* an eminently
felspathic rock, in which we find some crystals of amphibole, but
pyroxene very rarely, and never any olivine. (* There are phonolites
of basaltic strata (the most anciently known) and phonolites of
trachytic strata (Andes of Mexico). The former are generally above the
basalts; and the extraordinary development of felspar in that union,
and the want of pyroxene, have always appeared to me very remarkable
phenomena.) The Cerro de Flores is a hill covered with tabulary blocks
of greenish grey phonolite, enclosing long crystals (not fendillated)
of vitreous felspar, altogether analogous to the phonolite of
Mittelgebirge.
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