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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According To Our Experiments, The Obsidians Swelled Very
Unequally:
Those of the Peak and the black varieties of Cotopaxi
and of Quinche increased nearly five times their bulk.
The colour of the pumice-stones of the Peak leads to another
important observation. The sea of white ashes which encircles the
Piton, and covers the vast plain of Retama, is a certain proof of
the former activity of the crater: for in all volcanoes, even when
there are lateral eruptions, the ashes and the rapilli issue
conjointly with the vapours only from the opening at the summit of
the mountain. Now, at Teneriffe, the black rapilli extend from the
foot of the Peak to the sea-shore; while the white ashes, which are
only pumice ground to powder, and among which I have discovered,
with a lens, fragments of vitreous feldspar and pyroxene,
exclusively occupy the region next to the Peak. This peculiar
distribution seems to confirm the observations made long ago at
Vesuvius, that the white ashes are thrown out last, and indicate
the end of the eruption. In proportion as the elasticity of the
vapours diminishes, the matter is thrown to a less distance; and
the black rapilli, which issue first, when the lava has ceased
running, must necessarily reach farther than the white rapilli. The
latter appear to have been exposed to the action of a more intense
fire.
I have now examined the exterior structure of the Peak, and the
composition of its volcanic productions, from the region of the
coast to the top of the Piton: - I have endeavoured to render these
researches interesting, by comparing the phenomena of the volcano
of Teneriffe with those that are observed in other regions, the
soil of which is equally undermined by subterranean fires. This
mode of viewing Nature in the universality of her relations is no
doubt adverse to the rapidity desirable in an itinerary; but it
appears to me that, in a narrative, the principal end of which is
the progress of physical knowledge, every other consideration ought
to be subservient to those of instruction and utility. By isolating
facts, travellers, whose labours are in every other respect
valuable, have given currency to many false ideas of the pretended
contrasts which Nature offers in Africa, in New Holland, and on the
ridge of the Cordilleras. The great geological phenomena are
subject to regular laws, as well as the forms of plants and
animals. The ties which unite these phenomena, the relations which
exist between the varied forms of organized beings, are discovered
only when we have acquired the habit of viewing the globe as a
great whole; and when we consider in the same point of view the
composition of rocks, the causes which alter them, and the
productions of the soil, in the most distant regions.
Having treated of the volcanic substances of the isle of Teneriffe,
there now remains to be solved a question intimately connected with
the preceding investigation. Does the archipelago of the Canary
Islands contain any rocks of primitive or secondary formation; or
is there any production observed, that has not been modified by
fire?
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