Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
- Page 50 of 208 - First - Home
This
Idea Had For A Long Time Occupied The Mind Of A Person Highly
Distinguished For His Talents And Reputation At Quito, Who,
Unacquainted With The Labours Of The Mineralogists Of Europe, Had
Devoted Himself To Researches On The Volcanoes Of His Country.
Don
Juan de Larea, one of those men lately sacrificed to the fury of
faction, had been struck with the phenomena exhibited by obsidians
exposed to a white heat.
He had thought, that, wherever volcanoes
act in the centre of a country covered with porphyry with base of
obsidian, the elastic fluids must cause a swelling of the liquified
mass, and perform an important part in the earthquakes preceding
eruptions. Without adopting an opinion, which seems somewhat bold,
I made, in concert with M. Larea, a series of experiments on the
tumefaction of the volcanic vitreous substances at Teneriffe, and
on those which are found at Quinche, in the kingdom of Quito. To
judge of the augmentation of their bulk, we measured pieces exposed
to a forge-fire of moderate heat, by the water they displaced from
a cylindric glass, enveloping the spongy mass with a thin coating
of wax. 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? This interesting problem has been considered by the
naturalists of Lord Macartney's expedition, and by those who
accompanied captain Baudin in his voyage to the Austral regions.
Their opinions are in direct opposition to each other; and the
contradiction is the more striking, as the question does not refer
to one of those geological reveries which we are accustomed to call
systems, but to a positive fact.
Doctor Gillan imagined that he observed, between Laguna and the
port of Orotava, in very deep ravines, beds of primitive rocks.
This, however, is a mistake. What Dr. Gillan calls somewhat
vaguely, mountains of hard ferruginous clay, are nothing but an
alluvium which we find at the foot of every volcano. Strata of clay
accompany basalts, as tufas accompany modern lavas. Neither M.
Cordier nor myself observed in any part of Teneriffe a primitive
rock, either in its natural place, or thrown out by the mouth of
the Peak; and the absence of these rocks characterizes almost every
island of small extent that has an unextinguishied volcano. We know
nothing positive of the mountains of the Azores; but it is certain,
that the island of Bourbon as well as Teneriffe, exhibits only a
heap of lavas and basalts. No volcanic rock rears its head, either
on the Gros Morne, or on the volcano of Bourbon, or on the colossal
pyramid of Cimandef, which is perhaps more elevated than the Peak
of the Canary Islands.
Bory St. Vincent nevertheless asserted, that lavas including
fragments of granite have been found on the elevated plain of
Retama; and M. Broussonnet informed me, that on a hill above
Guimar, fragments of mica-slate, containing beautiful plates of
specular iron, had been found.
Enter page number
PreviousNext
Page 50 of 208
Words from 49893 to 50893
of 211363