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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On The African Soil Excessive Heat And Lengthened
Drought Retard The Growth Of Cryptogamous Plants.
The basalts of Graciosa are not in columns, but are divided into
strata ten or fifteen inches thick.
These strata are inclined at an
angle of 80 degrees to the north-west. The compact basalt
alternates with the strata of porous basalt and marl. The rock does
not contain hornblende, but great crystals of foliated olivine,
which have a triple cleavage.* (* Blaettriger olivin.) This
substance is decomposed with great difficulty. M. Hauy considers it
a variety of the pyroxene. The porous basalt, which passes into
mandelstein, has oblong cavities from two to eight lines in
diameter, lined with chalcedony, enclosing fragments of compact
basalt. I did not remark that these cavities had the same
direction, or that the porous rock lay on compact strata, as
happens in the currents of lava of Etna and Vesuvius. The marl,* (*
Mergel.) which alternates more than a hundred times with the
basalts, is yellowish, friable by decomposition, very coherent in
the inside, and often divided into irregular prisms, analogous to
the basaltic prisms. The sun discolours their surface, as it
whitens several schists, by reviving a hydro-carburetted principle,
which appears to be combined with the earth. The marl of Graciosa
contains a great quantity of chalk, and strongly effervesces with
nitric acid, even on points where it is found in contact with the
basalt. This fact is the more remarkable, as this substance does
not fill the fissures of the rock, but its strata are parallel to
those of the basalt; whence we may conclude that both fossils are
of the same formation, and have a common origin.
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