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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(* 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. It is surrounded by pyroxenic amygdaloid; it would no
doubt be seen below, issuing immediately from gneiss-granite, like the
phonolite of Biliner Stein, in Bohemia, which contains fragments of
gneiss embedded in its mass.
Does there exist in South America another group of rocks, which may be
preferably designated by the name of volcanic rocks, and which are as
distinct from the chain of the Andes, and advance as far towards the
east as the group that bounds the steppes of Calabozo? Of this I
doubt, at least in that part of the continent situated north of the
Amazon. I have often directed attention to the absence of pyroxenic
porphyry, trachyte, basalt and lavas (I range these formations
according to their relative age) in the whole of America eastward of
the Cordilleras. The existence even of trachyte has not yet been
verified in the Sierra Nevada de Merida which links the Andes and the
littoral chain of Venezuela. It would seem as if volcanic fire, after
the formation of primitive rocks, could not pierce into eastern
America.
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