in the highest grade of organic development, namely, the Polypetalæ;
and among these, in the Perigynic class, to the Leguminosæ, Rosaceæ,
Terebinthaceæ, and Juglandeæ; and in the Hypogynic, to the Aurantiaceæ,
Cedrelaceæ, and Sapindaceæ. The beautiful doubly-pinnated leaves which
form one of the principal ornaments of the torrid zone, are most
frequent among the Leguminosæ, in Mimoseæ, also in some Cæsalpinieæ,
Coulterias, and Gleditschias; never, as Kunth remarks, in Papilionaceæ.
“Folia pinnata” and “folia composita” are never found in Gentianeæ,
Rubiaceæ, and Myrtaceæ. In the morphological development presented
by the abundance and variety of form in the appendicular organs of
Dicotyledones, we can at present discern only a small number of general
laws.
ON THE
STRUCTURE AND MODE OF ACTION
OF
VOLCANOS,
IN DIFFERENT PARTS OF THE GLOBE.
[This dissertation was read in a public assembly of the Academy at
Berlin, on the 24th of January, 1823.]
When we reflect on the influence which, for some centuries past, the
progress of geography and the multiplication of distant voyages and
travels have exercised on the study of nature, we are not long in
perceiving how different this influence has been, according as the
researches were directed to organic forms on the one hand, or on the
other to the study of the inanimate substances of which the earth is
composed--to the knowledge of rocks, their relative ages, and their
origin. Different forms of plants and animals enliven the surface of
the earth in every zone, whether the temperature of the atmosphere
varies in accordance with the latitude and with the many inflections
of the isothermal lines on plains but little raised above the level
of the sea, or whether it changes rapidly in ascending in an almost
vertical direction the steep declivities of mountain-chains. Organic
nature gives to each zone of the earth a peculiar physiognomy; but
where the solid crust of the earth appears unclothed by vegetation,
inorganic nature imparts no such distinctive character. The same kinds
of rocks, associated in groups, appear in either hemisphere, from
the equator to the poles. In a remote island, surrounded by exotic
vegetation, beneath a sky where his accustomed stars no longer shine,
the voyager often recognises with joy the argillaceous schists of his
birth-place, and the rocks familiar to his eye in his native land.
This absence of any dependence of geological relations on the present
constitution of climates does not preclude or even diminish the
salutary influence of numerous observations made in distant regions
on the advance and progress of geological science, though it imparts
to this progress something of a peculiar direction. Every expedition
enriches natural history with new species or new genera of plants and
animals: there are thus presented to us sometimes forms which connect
themselves with previously long known types, and thus permit us to
trace and contemplate in its perfection the really regular though
apparently broken or interrupted network of organic forms: at other
times shapes which appear isolated,--either surviving remnants of
extinct genera or orders, or otherwise members of still undiscovered
groups, stimulating afresh the spirit of research and expectation. The
examination of the solid crust of the globe does not, indeed, unfold
to us such diversity and variety; it presents to us, on the contrary,
an agreement in the constituent particles, in the superposition of
the different kinds of masses, and in their regular recurrence, which
excites the admiration of the geologist. In the chain of the Andes,
as in the mountains of middle Europe, one formation appears, as it
were, to summon to itself another. Rocks of the same name exhibit the
same outlines; basalt and dolerite form twin mountains; dolomite,
sandstone, and porphyry, abrupt precipices; and vitreous feldspathic
trachyte, high dome-like elevations. In the most distant zones large
crystals separate themselves in a similar manner from the compact
texture of the primitive mass, as if by an internal development,
form groups in association, and appear associated in layers, often
announcing the vicinity of new independent formations. Thus in any
single system of mountains of considerable extent we see the whole
inorganic substances of which the crust of the earth is composed
represented, as it were, with more or less distinctness; yet, in order
to become completely acquainted with the important phenomena of the
composition, the relative age, and mode of origin of rocks, we must
compare together observations from the most varied and remote regions.
Problems which long perplexed the geologist in his native land in these
northern countries, find their solution near the equator. If, as has
been already remarked, new zones do not necessarily present to us new
kinds of rock (_i. e._ unknown groupings or associations of simple
substances), they, on the other hand, teach us to discern the great and
every where equally prevailing laws, according to which the strata of
the crust of the earth are superposed upon each other, penetrate each
other as veins or dykes, or are upheaved or elevated by elastic forces.
If, then, our geological knowledge is thus promoted by researches
embracing extensive parts of the earth’s surface, it is not surprising
that the particular class of phenomena which form the subject of the
present discussion should long have been regarded from a point of view
the more restricted as the points of comparison were of difficult, I
might almost say arduous and painful, attainment and access. Until
the close of the last century all real or supposed knowledge of
the structure or form of volcanos, and of the mode of operation of
subterranean forces, was taken from two mountains of the South of
Europe, Vesuvius and Etna. The former of these being the easiest of
access, and its eruptions, as is generally the case in volcanos of
small elevation, being most frequent in their occurrence, a hill of
minor elevation became the type which regulated all the ideas formed
respecting phænomena exhibited on a far larger scale in many vast and
distant regions, as in the mighty volcanos arranged in linear series in
Mexico, South America, and the Asiatic Islands. Such a proceeding might
not unnaturally recall Virgil’s shepherd, who thought he beheld in his
humble cottage the type of the eternal City, Imperial Rome.
A more careful examination of the whole of the Mediterranean, and
especially of those islands and coasts where men awoke to the noblest
intellectual culture, might, however, have dispelled views formed from
so limited a consideration of nature. Among the Sporades, trachytic
rocks have been upraised from the deep bottom of the sea, forming
islands resembling that which, in the vicinity of the Azores, appeared
thrice periodically, at nearly equal intervals, in three centuries.
The Peloponnesus has, between Epidaurus and Trœzene, near Methone, a
Monte Nuovo described by Strabo and seen again by Dodwell, which is
higher than the Monte Nuovo of the Phlegræan Fields near Baiæ, and
perhaps even higher than the new volcano of Jorullo in the plains of
Mexico, which I found surrounded by several thousand small basaltic
cones which had been protruded from the earth and were still smoking.
In the Mediterranean and its shores, it is not only from the permanent
craters of isolated mountains having a constant communication with
the interior, as Stromboli, Vesuvius, and Etna, that volcanic fires
break forth: at Ischia, on the Monte Epomeo, and also, as it would
appear by the accounts of the ancients, in the Lelantine plain near
Chalcis, lavas have flowed from fissures which have suddenly opened at
the surface of the earth. Besides these phænomena, which fall within
the historic period, or within the restricted domain of well-assured
tradition, and which Carl Ritter will collect and elucidate in his
masterly work on Geography,--the shores of the Mediterranean exhibit
numerous remains of more ancient volcanic action. In the south part
of France, in Auvergne, we see a separate complete system of volcanos
arranged in lines, trachytic domes alternating with cones of eruption,
from which streams of lava have flowed in narrow bands. The plain of
Lombardy, as level as the surface of the sea, and forming an inner Gulf
of the Adriatic, surrounds the trachyte of the Euganean Hills, where
rise domes of granular trachyte, obsidian, and pearl-stone, masses
connected by a common origin, which break through the lower cretaceous
rock and nummulitic lime-stone, but have never flowed in narrow
streams. Similar evidences of ancient revolutions of nature are found
in several parts of the mainland of Greece and in Asia Minor, countries
which will one day offer a rich field for geological investigation,
when intellectual light shall revisit the seats from which it has
radiated to the western world, and when oppressed humanity shall no
longer be subject to the barbarism of Turkish rule.
I recall the geographical proximity of these various phænomena, in
order to shew that the basin of the Mediterranean, with its series
of islands, might have offered to an attentive observer much that
has been recently discovered, under various forms, in South America,
Teneriffe, and the Aleutian Islands near the polar circle. The objects
to be observed were assembled within a moderate distance; yet distant
voyages, and the comparison of extensive regions in and out of Europe,
have been required for the clear perception and recognition of the
resemblance between volcanic phænomena and their dependence on each
other.
Our ordinary language, which often gives permanency and apparent
authority to the first-formed erroneous views of natural phænomena,
but which also often points instinctively to the truth,--our ordinary
language, I repeat, applies the term “volcanic” to all eruptions of
subterranean fires or molten substances; to columns of smoke and
vapour rising from rocks, as at Colares after the great earthquake
of Lisbon; to “Salses” or mud volcanos, argillaceous cones emitting
mud, asphalte, and hydrogen, as at Girgenti in Sicily, and at Turbaco
in South America; to the Geysers, hot springs in which, as in those
of Iceland, the waters, pressed by elastic vapours, rise in jets to a
considerable altitude; and, in general, to all operations of natural
forces having their seat in the interior of our planet. In Central
America (Guatimala), and in the Philippine Islands, the natives even
distinguish formally between water- and fire-volcanos, Volcanes de
agua y de fuego, giving the former name to those mountains from which
subterranean waters issue from time to time with violent earthquake
shocks and a hollow noise.
Not denying the connexion of the different phenomena which have been
referred to, it yet appears desirable to give greater precision to the
terms employed in the physical as well as in the mineralogical part
of geology, and not to apply the word “volcano” at one moment to a
mountain terminating in a permanent igneous opening or fiery crater,
and at another to every subterranean cause of volcanic phenomena. In
the present state of our planet the most ordinary form of volcanos
is indeed in all parts of the globe that of an isolated conical
mountain, such as Vesuvius, Etna, the Peak of Teneriffe, Tunguragua,
and Cotapaxi. I have myself seen such volcanos varying in size from
the smallest hill to an elevation of 18000 (19184 English) feet above
the sea. But besides these isolated cones there are also permanent
openings or craters, having established channels of communication
with the interior of the earth, which are situated on long chains of
mountains with serrated crests, and not even always on the middle of
the ridge, but sometimes at its extremity: such is Pichincha, situated
between the Pacific and the city of Quito, and which acquired celebrity
in connection with Bouguer’s earliest barometric formulæ, and such
are the volcanos which rise in the elevated Steppe de los Pastos,
itself ten thousand (10657 English) feet high. All these summits,
which are of various shapes, consist of trachyte, formerly called
Trap-porphyry: a granular vesicular rock composed of different kinds
of feldspar (Labradorite, Oligoklase, and Albite), augite, hornblende,
and sometimes interspersed mica, and even quartz. In cases where the
evidence of the first outburst or eruption, or I might say where the
ancient structure or scaffolding remain entire, the isolated conical
mount is surrounded by an amphitheatre or lofty circular rampart of
rocky strata superimposed upon each other. Such walls or ring-formed
ramparts are called “craters of elevation,” a great and important
phenomenon, concerning which a memorable treatise was presented to our
Academy five years ago (_i. e._ in 1818), by the first geologist of our
time, Leopold von Buch, from whose writings I have borrowed several of
the views contained in the present discussion.
Volcanos which communicate with the atmosphere through permanent
openings, conical basaltic hills, and craterless trachytic domes,
sometimes as low as Sarcouy, sometimes as lofty as the Chimborazo, form
various groups. Comparative geography shows us sometimes small clusters
or distinct systems of mountains, with craters and lava-currents
in the Canaries and the Azores, and without craters and without
lava-currents, properly so-called, in the Euganean hills and the
Siebengebirge near Bonn;--and at other times the same study describes
to us volcanos arranged in single or double lines extending through
many hundred leagues in length, these lines being either parallel
to the direction of a great chain of mountains, as in Guatimala, in
Peru, and in Java, or cutting it transversely or at right angles,
as in tropical Mexico. In this land of the Aztecs the fire-emitting
trachytic mountains are the only ones which attain the elevation of the
lofty region of perpetual snow; they are ranged in the direction of
a parallel of latitude, and have probably been raised from a fissure
420 English geographical miles long, traversing the continent from the
Pacific to the Atlantic Ocean.
These assemblages of volcanos, whether in rounded groups or in double
lines, show in the most conclusive manner that the volcanic agencies
do not depend on small or restricted causes, in near proximity to the
surface of the earth, but that they are great phænomena of deep-seated
origin. The whole of the eastern part of the American continent, which
is poor in metals, is, in its present state, without fire-emitting
mountains, without masses of trachyte, and perhaps even without basalt
containing olivine. All the American volcanos are on the side of the
continent which is opposite to Asia, in the chain of the Andes which
runs nearly in the direction of a meridian, and extends over a length
of 7200 geographical miles.
The whole plateau or high-land of Quito, of which Pichincha, Cotopaxi,
and Tunguragua form the summits, is to be viewed as a single volcanic
furnace. The subterranean fire breaks forth sometimes through one
and sometimes through another of these openings, which it has been
customary to regard as separate and distinct volcanos. The progressive
march of the subterranean fire has been here directed for three
centuries from North to South. Even the earthquakes which occasion
such dreadful ravages in this part of the world afford remarkable
proofs of the existence of subterranean communications, not only
between countries where there are no volcanos (a fact which had long
been known), but also between fire-emitting openings situated at great
distances asunder. Thus in 1797 the volcano of Pasto, east of the
Guaytara River, emitted uninterruptedly for three months a lofty column
of smoke, which column disappeared at the instant when, at a distance
of 240 geographical miles, the great earthquake of Riobamba and the
immense eruption of mud called “Moya” took place, causing the death of
between thirty and forty thousand persons.
The sudden appearance of the Island of Sabrina near the Azores, on the
80th of January, 1811, was the precursor of the terrible earthquake
movements which, much farther to the west, shook almost incessantly,
from the month of May 1811 to June 1813, first the West Indian Islands,
then the plain of the Ohio and Mississipi, and lastly, the opposite
coast of Venezuela or Caraccas. Thirty days after the destruction of
the principal city of that province, the long tranquil volcano of
the Island of St. Vincent burst forth in an eruption. A remarkable
phenomenon accompanied this eruption: at the same moment when the
explosion took place, on the 30th of April, 1811, a loud subterranean
noise was heard in South America, which spread terror and dismay over
a district of 2200 (German) geographical square miles (35200 English
geographical square miles). The dwellers on the banks of the Apure
near the confluence of the Rio Nula, and the most distant inhabitants
of the sea coast of Venezuela, alike compared the sound to that of the
discharge of great pieces of ordnance. Now from the confluence of the
Nula with the Apure (by which latter river I arrived on the Orinoco)
to the volcano of St. Vincent is a distance in a straight line of
628 English geographical miles. The sound, which certainly was not
propagated through the air, must have proceeded from a deep-seated
subterranean cause; for its intensity was scarcely greater on the sea
coast nearest to the volcano where the eruption was taking place, than
in the interior of the country, in the basin of the Apure and the
Orinoco.
It would be unnecessary to multiply examples by citing other instances
which I have collected, but, to recall a phenomenon of European
historical importance, I will only farther mention the celebrated
earthquake of Lisbon. Simultaneously with that event, on the 1st
of November, 1755, not only were the Swiss lakes and the sea near
the coast of Sweden violently agitated, but even among the eastern
West Indian Islands, Martinique, Antigua, and Barbadoes, where the
tide never exceeds thirty inches, the sea suddenly rose more than
twenty feet. All these phenomena show the operation of subterranean
forces, acting either dynamically in earthquakes, in the tension and
agitation of the crust; or in volcanos, in the production and chemical
alteration of substances. They also show that these forces do not act
superficially, in the thin outermost crust of the globe, but from great
depths in the interior of our planet, through crevices or unfilled
veins, affecting simultaneously widely distant points of the earth’s
surface.
The greater the variety of structure in volcanos, or in the elevations
which surround the channel through which the molten masses of the
interior of the earth reach its surface, the greater the importance of
submitting this structure to strict investigation and measurement. The
interest attaching to these measurements, which formed a particular
object of my researches in another quarter of the globe, is enhanced
by the consideration that at many points the magnitude to be measured
is found to be a variable quantity. The philosophical study of nature
endeavours, in the vicissitudes of phenomena, to connect the present
with the past.
If we desire to investigate either the fact of a periodical return,
or the law of progressive variations or changes in phenomena, it is
essential to obtain, by means of observations carefully made and
connected with determinate epochs, certain fixed points which may
afford a base for future numerical comparisons. If we only possessed
determinations made once in each period of a thousand years, of the
mean temperature of the atmosphere and of the earth in different
latitudes, or of the mean height of the barometer at the level of the
sea, we should know whether, and in what ratio, the temperature of
different climates had increased or decreased, or whether the height
of the atmosphere had undergone changes. Such points of comparison are
also needed for the inclination and declination of the magnetic needle,
as well as for the intensity of the magneto-electric forces, on which,
within the circle of this Academy, two excellent physicists, Seebeck
and Erman, have thrown so much light. As it is an honourable object for
the exertions of scientific societies to trace out perseveringly the
cosmical variations of temperature, atmospheric pressure, and magnetic
direction and intensity, so it is the duty of the geological traveller,
in determining the inequalities of the earth’s surface, to attend more
particularly to the variable height of volcanos. The endeavours made by
me for this object in the Mexican mountains, in respect to the Volcan
de Toluca, the Popocatepetl, the Cofre de Perote or Nauhcampatepetl,
and the Jorullo, and also the volcano of Pichincha in the Andes of
Quito, have been continued since my return to Europe at different
epochs on Vesuvius. Where complete trigonometric or barometric
measurements are wanting, accurate angles of altitude, taken at points
which are exactly determined, may be substituted for them; and for
a comparison of determinations made at different epochs, angles of
altitude so measured may even be often preferable to the complication
of circumstances which more complete operations may involve.
Saussure had measured Mount Vesuvius, in 1773, when the two margins
of the crater, the north-western and the south-eastern, appeared to
him be of equal height. He found their height above the level of the
sea 609 toises, 3894 English feet. The eruption of 1794 occasioned
a breaking down of the margin of the crater on the southern side,
and a consequent inequality between the height of the two edges
which the most unpractised eye does not fail to distinguish even at
a considerable distance. In 1805, Leopold von Buch, Gay-Lussac, and
myself, measured the height of Vesuvius three times, and found the
northern margin opposite to La Somma, (the Rocca del Palo), exactly as
given by Saussure, but the southern margin 75 toises, or 450 French
or 479 English feet, lower than he had found it in 1773. The whole
elevation of the volcano on the side of Torre del Greco (the side
towards which, for the last thirty years, the igneous action has,
as it were, been principally directed,) had at that time diminished
one-eighth. The height of the cone of ashes, as compared with the whole
height of the mountain, is in Vesuvius as 1 to 3; in Pichincha, as 1 to
10; and in the Peak of Teneriffe, as 1 to 22. In these three volcanic
mountains, the cone of ashes is therefore, relatively speaking, highest
in Vesuvius; probably because, being a low volcano, the action has been
principally by the summit.
A few months ago (in 1822) I was enabled not only to repeat my former
barometric measurements of the height of Vesuvius, but also, during
the course of three visits to the summit, to make a more complete
determination of all the edges of the crater[38]. These determinations
may not be without interest, since they include the long period of
great eruptions between 1805 and 1822, and constitute perhaps the only
known examination and measurement of a volcano at different epochs, in
which the different parts of the examination are all truly comparable
with each other. We learn from it that the margins of craters are a
phenomenon of far more permanent character than had been previously
inferred from passing observations, and this not only where (as in the
Peak of Teneriffe, and in all the volcanos of the chain of the Andes,)
they are visibly composed of trachyte, but also elsewhere. According to
my last determinations, the north-west edge of Vesuvius has, perhaps,
not altered at all since the time of Saussure, an interval of 49 years;
and the south-eastern side, on the side towards Bosche Tre Case, which,
in 1794, had become 400 French (426 English) feet lower, has since then
hardly altered 10 toises (60 French or 64 English feet).