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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These
Geological Questions Can Be Solved Only So Far As They Are Directed
By The Actual State Of Things, That Is, Of Facts Susceptible Of
Being Verified By Observation.
Considering rocks according to the succession of eras, we find that
primitive formations exhibit very few caverns.
The great cavities
which are observed in the oldest granite, and which are called
fours (ovens) in Switzerland and in the south of France, when they
are lined with rock crystals, arise most frequently from the union
of several contemporaneous veins of quartz,* (* Gleichzeitige
Trummer. To these stone veins which appear to be of the same age as
the rock, belong the veins of talc and asbestos in serpentine, and
those of quartz traversing schist (Thonschiefer). Jameson on
Contemporaneous Veins, in the Mem. of the Wernerian Soc.) of
feldspar, or of fine-grained granite. The gneiss presents, though
more seldom, the same phenomenon; and near Wunsiedel,* (* In
Franconia, south-east of Luchsburg.) at the Fichtelgebirge, I had
an opportunity of examining crystal fours of two or three feet
diameter, in a part of the rock not traversed by veins. We are
ignorant of the extent of the cavities which subterranean fires and
volcanic agitations may have produced in the bowels of the earth in
those primitive rocks, which, containing considerable quantities of
amphibole, mica, garnet, magnetic iron-stone, and red schorl
(titanite), appear to be anterior to granite. We find some
fragments of these rocks among the matters ejected by volcanoes.
The cavities can be considered only as partial and local phenomena;
and their existence is scarcely any contradiction to the notions we
have acquired from the experiments of Maskelyne and Cavendish on
the mean density of the earth.
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