TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.
The sinking of crystals in fluid lava. Specific gravity of the
constituent parts of trachyte and of basalt, and their consequent
separation. Obsidian. Apparent non-separation of the elements of
plutonic rocks. Origin of trap-dikes in the plutonic series.
Distribution of volcanic islands; their prevalence in the great oceans.
They are generally arranged in lines. The central volcanoes of Von Buch
doubtful. Volcanic islands bordering continents. Antiquity of volcanic
islands, and their elevation in mass. Eruptions on parallel lines of
fissure within the same geological period.
ON THE SEPARATION OF THE CONSTITUENT MINERALS OF LAVA, ACCORDING TO
THEIR SPECIFIC GRAVITIES.
One side of Fresh-water Bay, in James Island, is formed by the wreck of
a large crater, mentioned in the last chapter, of which the interior
has been filled up by a pool of basalt, about two hundred feet in
thickness. This basalt is of a grey colour, and contains many crystals
of glassy albite, which become much more numerous in the lower,
scoriaceous part. This is contrary to what might have been expected,
for if the crystals had been originally disseminated in equal numbers,
the greater intumescence of this lower scoriaceous part would have made
them appear fewer in number. Von Buch has described a stream of
obsidian on the Peak of Teneriffe, in which the crystals of feldspar
become more and more numerous, as the depth or thickness increases, so
that near the lower surface of the stream the lava even resembles a
primary rock. (“Description des Isles Canaries” pages 190 and 191.) Von
Buch further states, that M. Dree, in his experiments in melting lava,
found that the crystals of feldspar always tended to precipitate
themselves to the bottom of the crucible. In these cases, I presume
there can be no doubt that the crystals sink from their weight. (In a
mass of molten iron, it is found (“Edinburgh New Philosophical Journal”
volume 24 page 66) that the substances, which have a closer affinity
for oxygen than iron has, rise from the interior of the mass to the
surface. But a similar cause can hardly apply to the separation of the
crystals of these lava-streams. The cooling of the surface of lava
seems, in some cases, to have affected its composition; for Dufrenoy
(“Mem. pour servir” tome 4 page 271) found that the interior parts of a
stream near Naples contained two-thirds of a mineral which was acted on
by acids, whilst the surface consisted chiefly of a mineral
unattackable by acids.) The specific gravity of feldspar varies from
2.4 to 2.58, whilst obsidian seems commonly to be from 2.3 to 2.4; and
in a fluidified state its specific gravity would probably be less,
which would facilitate the sinking of the crystals of feldspar. (I have
taken the specific gravities of the simple minerals from Von Kobell,
one of the latest and best authorities, and of the rocks from various
authorities. Obsidian, according to Phillips, is 2.35; and Jameson says
it never exceeds 2.4; but a specimen from Ascension, weighed by myself,
was 2.42.) At James Island, the crystals of albite, though no doubt of
less weight than the grey basalt, in the parts where compact, might
easily be of greater specific gravity than the scoriaceous mass, formed
of melted lava and bubbles of heated gas.
The sinking of crystals through a viscid substance like molten rock, as
is unequivocally shown to have been the case in the experiments of M.
Dree, is worthy of further consideration, as throwing light on the
separation of the trachytic and basaltic series of lavas. Mr. P. Scrope
has speculated on this subject; but he does not seem to have been aware
of any positive facts, such as those above given; and he has overlooked
one very necessary element, as it appears to me, in the
phenomenon—namely, the existence of either the lighter or heavier
mineral in globules or in crystals. In a substance of imperfect
fluidity, like molten rock, it is hardly credible, that the separate,
infinitely small atoms, whether of feldspar, augite, or of any other
mineral, would have power from their slightly different gravities to
overcome the friction caused by their movement; but if the atoms of any
one of these minerals became, whilst the others remained fluid, united
into crystals or granules, it is easy to perceive that from the
lessened friction, their sinking or floating power would be greatly
increased. On the other hand, if all the minerals became granulated at
the same time, it is scarcely possible, from their mutual resistance,
that any separation could take place. A valuable, practical discovery,
illustrating the effect of the granulation of one element in a fluid
mass, in aiding its separation, has lately been made: when lead
containing a small proportion of silver, is constantly stirred whilst
cooling, it becomes granulated, and the grains of imperfect crystals of
nearly pure lead sink to the bottom, leaving a residue of melted metal
much richer in silver; whereas if the mixture be left undisturbed,
although kept fluid for a length of time, the two metals show no signs
of separating. (A full and interesting account of this discovery, by
Mr. Pattinson, was read before the British Association in September
1838. In some alloys, according to Turner “Chemistry” page 210, the
heaviest metal sinks, and it appears that this takes place whilst both
metals are fluid. Where there is a considerable difference in gravity,
as between iron and the slag formed during the fusion of the ore, we
need not be surprised at the atoms separating, without either substance
being granulated.) The sole use of the stirring seems to be, the
formation of detached granules. The specific gravity of silver is 10.4,
and of lead 11.35: the granulated lead, which sinks, is never
absolutely pure, and the residual fluid metal contains, when richest,
only 1/119 part of silver. As the difference in specific gravity,
caused by the different proportions of the two metals, is so
exceedingly small, the separation is probably aided in a great degree
by the difference in gravity between the lead, when granular though
still hot, and when fluid.
In a body of liquified volcanic rock, left for some time without any
violent disturbance, we might expect, in accordance with the above
facts, that if one of the constituent minerals became aggregated into
crystals or granules, or had been enveloped in this state from some
previously existing mass, such crystals or granules would rise or sink,
according to their specific gravity. Now we have plain evidence of
crystals being embedded in many lavas, whilst the paste or basis has
continued fluid. I need only refer, as instances, to the several,
great, pseudo-porphyritic streams at the Galapagos Islands, and to the
trachytic streams in many parts of the world, in which we find crystals
of feldspar bent and broken by the movement of the surrounding,
semi-fluid matter. Lavas are chiefly composed of three varieties of
feldspar, varying in specific gravity from 2.4 to 2.74; of hornblende
and augite, varying from 3.0 to 3.4; of olivine, varying from 3.3 to
3.4; and lastly, of oxides of iron, with specific gravities from 4.8 to
5.2. Hence crystals of feldspar, enveloped in a mass of liquified, but
not highly vesicular lava, would tend to rise to the upper parts; and
crystals or granules of the other minerals, thus enveloped, would tend
to sink. We ought not, however, to expect any perfect degree of
separation in such viscid materials. Trachyte, which consists chiefly
of feldspar, with some hornblende and oxide of iron, has a specific
gravity of about 2.45; whilst basalt, composed chiefly of augite and
feldspar, often with much iron and olivine, has a gravity of about 3.0.
(Trachyte from Java was found by Von Buch to be 2.47; from Auvergne, by
De la Beche, it was 2.42; from Ascension, by myself, it was 2.42.
Jameson and other authors give to basalt a specific gravity of 3.0; but
specimens from Auvergne were found, by De la Beche, to be only 2.78;
and from the Giant’s Causeway, to be 2.91.) Accordingly we find, that
where both trachytic and basaltic streams have proceeded from the same
orifice, the trachytic streams have generally been first erupted owing,
as we must suppose, to the molten lava of this series having
accumulated in the upper parts of the volcanic focus. This order of
eruption has been observed by Beudant, Scrope, and by other authors;
three instances, also, have been given in this volume. As the later
eruptions, however, from most volcanic mountains, burst through their
basal parts, owing to the increased height and weight of the internal
column of molten rock, we see why, in most cases, only the lower flanks
of the central, trachytic masses, are enveloped by basaltic streams.
The separation of the ingredients of a mass of lava, would, perhaps,
sometimes take place within the body of a volcanic mountain, if lofty
and of great dimensions, instead of within the underground focus; in
which case, trachytic streams might be poured forth, almost
contemporaneously, or at short recurrent intervals, from its summit,
and basaltic streams from its base: this seems to have taken place at
Teneriffe. (Consult Von Buch’s well-known and admirable “Description
Physique” of this island, which might serve as a model of descriptive
geology.) I need only further remark, that from violent disturbances
the separation of the two series, even under otherwise favourable
conditions, would naturally often be prevented, and likewise their
usual order of eruption be inverted. From the high degree of fluidity
of most basaltic lavas, these perhaps, alone, would in many cases reach
the surface.
As we have seen that crystals of feldspar, in the instance described by
Von Buch, sink in obsidian, in accordance with their known greater
specific gravity, we might expect to find in every trachytic district,
where obsidian has flowed as lava, that it had proceeded from the upper
or highest orifices. This, according to Von Buch, holds good in a
remarkable manner both at the Lipari Islands and on the Peak of
Teneriffe; at this latter place obsidian has never flowed from a less
height than 9,200 feet. Obsidian, also, appears to have been erupted
from the loftiest peaks of the Peruvian Cordillera. I will only further
observe, that the specific gravity of quartz varies from 2.6 to 2.8;
and therefore, that when present in a volcanic focus, it would not tend
to sink with the basaltic bases; and this, perhaps, explains the
frequent presence, and the abundance of this mineral, in the lavas of
the trachytic series, as observed in previous parts of this volume.
An objection to the foregoing theory will, perhaps, be drawn from the
plutonic rocks not being separated into two evidently distinct series,
of different specific gravities; although, like the volcanic, they have
been liquified. In answer, it may first be remarked, that we have no
evidence of the atoms of any one of the constituent minerals in the
plutonic series having been aggregated, whilst the others remained
fluid, which we have endeavoured to show is an almost necessary
condition of their separation; on the contrary, the crystals have
generally impressed each other with their forms. (The crystalline paste
of phonolite is frequently penetrated by long needles of hornblende;
from which it appears that the hornblende, though the more fusible
mineral, has crystallised before, or at the same time with a more
refractory substance. Phonolite, as far as my observations serve, in
every instance appears to be an injected rock, like those of the
plutonic series; hence probably, like these latter, it has generally
been cooled without repeated and violent disturbances. Those geologists
who have doubted whether granite could have been formed by igneous
liquefaction, because minerals of different degrees of fusibility
impress each other with their forms, could not have been aware of the
fact of crystallised hornblende penetrating phonolite, a rock
undoubtedly of igneous origin. The viscidity, which it is now known,
that both feldspar and quartz retain at a temperature much below their
points of fusion, easily explains their mutual impressment. Consult on
this subject Mr. Horner’s paper on Bonn “Geolog. Transact.” volume 4
page 439; and “L’Institut” with respect to quartz 1839 page 161.)
In the second place, the perfect tranquillity, under which it is
probable that the plutonic masses, buried at profound depths, have
cooled, would, most likely, be highly unfavourable to the separation of
their constituent minerals; for, if the attractive force, which during
the progressive cooling draws together the molecules of the different
minerals, has power sufficient to keep them together, the friction
between such half-formed crystals or pasty globules would effectually
prevent the heavier ones from sinking, or the lighter ones from rising.
On the other hand, a small amount of disturbance, which would probably
occur in most volcanic foci, and which we have seen does not prevent
the separation of granules of lead from a mixture of molten lead and
silver, or crystals of feldspar from streams of lava, by breaking and
dissolving the less perfectly formed globules, would permit the more
perfect and therefore unbroken crystals, to sink or rise, according to
their specific gravity.
Although in plutonic rocks two distinct species, corresponding to the
trachytic and basaltic series, do not exist, I much suspect that a
certain amount of separation of their constituent parts has often taken
place. I suspect this from having observed how frequently dikes of
greenstone and basalt intersect widely extended formations of granite
and the allied metamorphic rocks. I have never examined a district in
an extensive granitic region without discovering dikes; I may instance
the numerous trap-dikes, in several districts of Brazil, Chile, and
Australia, and at the Cape of Good Hope: many dikes likewise occur in
the great granitic tracts of India, in the north of Europe, and in
other countries. Whence, then, has the greenstone and basalt, forming
these dikes, come? Are we to suppose, like some of the elder
geologists, that a zone of trap is uniformly spread out beneath the
granitic series, which composes, as far as we know, the foundations of
the earth’s crust? Is it not more probable, that these dikes have been
formed by fissures penetrating into partially cooled rocks of the
granitic and metamorphic series, and by their more fluid parts,
consisting chiefly of hornblende, oozing out, and being sucked into
such fissures? At Bahia, in Brazil, in a district composed of gneiss
and primitive greenstone, I saw many dikes, of a dark augitic (for one
crystal certainly was of this mineral) or hornblendic rock, which, as
several appearances clearly proved, either had been formed before the
surrounding mass had become solid, or had together with it been
afterwards thoroughly softened. (Portions of these dikes have been
broken off, and are now surrounded by the primary rocks, with their
laminae conformably winding round them. Dr. Hubbard also (“Silliman’s
Journal” volume 34 page 119), has described an interlacement of
trap-veins in the granite of the White Mountains, which he thinks must
have been formed when both rocks were soft.) On both sides of one of
these dikes, the gneiss was penetrated, to the distance of several
yards, by numerous, curvilinear threads or streaks of dark matter,
which resembled in form clouds of the class called cirrhi- comae; some
few of these threads could be traced to their junction with the dike.
When examining them, I doubted whether such hair-like and curvilinear
veins could have been injected, and I now suspect, that instead of
having been injected from the dike, they were its feeders. If the
foregoing views of the origin of trap-dikes in widely extended granitic
regions far from rocks of any other formation, be admitted as probable,
we may further admit, in the case of a great body of plutonic rock,
being impelled by repeated movements into the axis of a mountain-chain,
that its more liquid constituent parts might drain into deep and unseen
abysses; afterwards, perhaps, to be brought to the surface under the
form, either of injected masses of greenstone and augitic porphyry, or
of basaltic eruptions. (Mr. Phillips “Lardner’s Encyclop.” volume 2
page 115 quotes Von Buch’s statement, that augitic porphyry ranges
parallel to, and is found constantly at the base of, great chains of
mountains. Humboldt, also, has remarked the frequent occurrence of
trap-rock, in a similar position; of which fact I have observed many
examples at the foot of the Chilian Cordillera. The existence of
granite in the axes of great mountain chains is always probable, and I
am tempted to suppose, that the laterally injected masses of augitic
porphyry and of trap, bear nearly the same relation to the granitic
axes which basaltic lavas bear to the central trachytic masses, round
the flanks of which they have so frequently been erupted.) Much of the
difficulty which geologists have experienced when they have compared
the composition of volcanic with plutonic formations, will, I think, be
removed, if we may believe that most plutonic masses have been, to a
certain extent, drained of those comparatively weighty and easily
liquified elements, which compose the trappean and basaltic series of
rocks.
ON THE DISTRIBUTION OF VOLCANIC ISLANDS.
During my investigations on coral-reefs, I had occasion to consult the
works of many voyagers, and I was invariably struck with the fact, that
with rare exceptions, the innumerable islands scattered throughout the
Pacific, Indian, and Atlantic Oceans, were composed either of volcanic,
or of modern coral-rocks. It would be tedious to give a long catalogue
of all the volcanic islands; but the exceptions which I have found are
easily enumerated: in the Atlantic, we have St. Paul’s Rock, described
in this volume, and the Falkland Islands, composed of quartz and
clay-slate; but these latter islands are of considerable size, and lie
not very far from the South American coast (Judging from Forster’s
imperfect observation, perhaps Georgia is not volcanic. Dr. Allan is my
informant with regard to the Seychelles. I do not know of what
formation Rodriguez, in the Indian Ocean, is composed.): in the Indian
Ocean, the Seychelles (situated in a line prolonged from Madagascar)
consist of granite and quartz: in the Pacific Ocean, New Caledonia, an
island of large size, belongs (as far as is known) to the primary
class. New Zealand, which contains much volcanic rock and some active
volcanoes, from its size cannot be classed with the small islands,
which we are now considering. The presence of a small quantity of
non-volcanic rock, as of clay-slate on three of the Azores (This is
stated on the authority of Count V. de Bedemar, with respect to Flores
and Graciosa (Charlsworth “Magazine of Nat. Hist.” volume 1 page 557).
St. Maria has no volcanic rock, according to Captain Boyd (Von Buch
“Descript.” page 365). Chatham Island has been described by Dr.
Dieffenbach in the “Geographical Journal” 1841 page 201. As yet we have
received only imperfect notices on Kerguelen Land, from the Antarctic
Expedition.), or of tertiary limestone at Madeira, or of clay-slate at
Chatham Island in the Pacific, or of lignite at Kerguelen Land, ought
not to exclude such islands or archipelagoes, if formed chiefly of
erupted matter, from the volcanic class.
The composition of the numerous islands scattered through the great
oceans being with such rare exceptions volcanic, is evidently an
extension of that law, and the effect of those same causes, whether
chemical or mechanical, from which it results, that a vast majority of
the volcanoes now in action stand either as islands in the sea, or near
its shores. This fact of the ocean-islands being so generally volcanic
is also interesting in relation to the nature of the mountain-chains on
our continents, which are comparatively seldom volcanic; and yet we are
led to suppose that where our continents now stand an ocean once
extended. Do volcanic eruptions, we may ask, reach the surface more
readily through fissures formed during the first stages of the
conversion of the bed of the ocean into a tract of land?
Looking at the charts of the numerous volcanic archipelagoes, we see
that the islands are generally arranged either in single, double, or
triple rows, in lines which are frequently curved in a slight degree.
(Professors William and Henry Darwin Rogers have lately insisted much,
in a memoir read before the American Association, on the regularly
curved lines of elevation in parts of the Appalachian range.) Each
separate island is either rounded, or more generally elongated in the
same direction with the group in which it stands, but sometimes
transversely to it. Some of the groups which are not much elongated
present little symmetry in their forms; M. Virlet (“Bulletin de la Soc.
Geolog.” tome 3 page 110.) states that this is the case with the
Grecian Archipelago: in such groups I suspect (for I am aware how easy
it is to deceive oneself on these points), that the vents are generally
arranged on one line, or on a set of short parallel lines, intersecting
at nearly right angles another line, or set of lines. The Galapagos
Archipelago offers an example of this structure, for most of the
islands and the chief orifices on the largest island are so grouped as
to fall on a set of lines ranging about N.W. by N., and on another set
ranging about W.S.W.: in the Canary Archipelago we have a simpler
structure of the same kind: in the Cape de Verde group, which appears
to be the least symmetrical of any oceanic volcanic archipelago, a N.W.
and S.E. line formed by several islands, if prolonged, would intersect
at right angles a curved line, on which the remaining islands are
placed.
Von Buch (“Description des Isles Canaries” page 324.) has classed all
volcanoes under two heads, namely, CENTRAL VOLCANOES, round which
numerous eruptions have taken place on all sides, in a manner almost
regular, and VOLCANIC CHAINS. In the examples given of the first class,
as far as position is concerned, I can see no grounds for their being
called “central;” and the evidence of any difference in mineralogical
nature between CENTRAL VOLCANOES and VOLCANIC CHAINS appears slight. No
doubt some one island in most small volcanic archipelagoes is apt to be
considerably higher than the others; and in a similar manner, whatever
the cause may be, that on the same island one vent is generally higher
than all the others. Von Buch does not include in his class of volcanic
chains small archipelagoes, in which the islands are admitted by him,
as at the Azores, to be arranged in lines; but when viewing on a map of
the world how perfect a series exists from a few volcanic islands
placed in a row to a train of linear archipelagoes following each other
in a straight line, and so on to a great wall like the Cordillera of
America, it is difficult to believe that there exists any essential
difference between short and long volcanic chains. Von Buch (Idem page
393.) states that his volcanic chains surmount, or are closely
connected with, mountain-ranges of primary formation: but if trains of
linear archipelagoes are, in the course of time, by the long- continued
action of the elevatory and volcanic forces, converted into
mountain-ranges, it would naturally result that the inferior primary
rocks would often be uplifted and brought into view.
Some authors have remarked that volcanic islands occur scattered,
though at very unequal distances, along the shores of the great
continents, as if in some measure connected with them. In the case of
Juan Fernandez, situated 330 miles from the coast of Chile, there was
undoubtedly a connection between the volcanic forces acting under this
island and under the continent, as was shown during the earthquake of
1835. The islands, moreover, of some of the small volcanic groups which
thus border continents, are placed in lines, related to those along
which the adjoining shores of the continents trend; I may instance the
lines of intersection at the Galapagos, and at the Cape de Verde
Archipelagoes, and the best marked line of the Canary Islands. If these
facts be not merely accidental, we see that many scattered volcanic
islands and small groups are related not only by proximity, but in the
direction of the fissures of eruption to the neighbouring continents—a
relation, which Von Buch considers, characteristic of his great
volcanic chains.
In volcanic archipelagoes, the orifices are seldom in activity on more
than one island at a time; and the greater eruptions usually recur only
after long intervals. Observing the number of craters, that are usually
found on each island of a group, and the vast amount of matter which
has been erupted from them, one is led to attribute a high antiquity
even to those groups, which appear, like the Galapagos, to be of
comparatively recent origin. This conclusion accords with the
prodigious amount of degradation, by the slow action of the sea, which
their originally sloping coasts must have suffered, when they are worn
back, as is so often the case, into grand precipices. We ought not,
however, to suppose, in hardly any instance, that the whole body of
matter, forming a volcanic island, has been erupted at the level, on
which it now stands: the number of dikes, which seem invariably to
intersect the interior parts of every volcano, show, on the principles
explained by M. Elie de Beaumont, that the whole mass has been uplifted
and fissured. A connection, moreover, between volcanic eruptions and
contemporaneous elevations in mass has, I think, been shown to exist in
my work on Coral-Reefs, both from the frequent presence of upraised
organic remains, and from the structure of the accompanying
coral-reefs. (A similar conclusion is forced on us, by the phenomena,
which accompanied the earthquake of 1835, at Concepcion, and which are
detailed in my paper (volume 5 page 601) in the “Geological
Transactions.”) Finally, I may remark, that in the same Archipelago,
eruptions have taken place within the historical period on more than
one of the parallel lines of fissure: thus, at the Galapagos
Archipelago, eruptions have taken place from a vent on Narborough
Island, and from one on Albemarle Island, which vents do not fall on
the same line; at the Canary Islands, eruptions have taken place in
Teneriffe and Lanzarote; and at the Azores, on the three parallel lines
of Pico, St. Jorge, and Terceira. Believing that a mountain-axis
differs essentially from a volcano, only in plutonic rocks having been
injected, instead of volcanic matter having been ejected, this appears
to me an interesting circumstance; for we may infer from it as
probable, that in the elevation of a mountain-chain, two or more of the
parallel lines forming it may be upraised and injected within the same
geological period.