Volcanic Islands

Chapter II

FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.

FERNANDO NORONHA.
Precipitous hill of phonolite.

TERCEIRA.
Trachytic rocks: their singular decomposition by steam of high temperature.

TAHITI.
Passage from wacke into trap; singular volcanic rock with the vesicles
half-filled with mesotype.

MAURITIUS.
Proofs of its recent elevation.
Structure of its more ancient mountains; similarity with St. Jago.

ST. PAUL'S ROCKS.
Not of volcanic origin.
Their singular mineralogical composition.


FERNANDO NORONHA.

During our short visit at this and the four following islands, I observed
very little worthy of description. Fernando Noronha is situated in the
Atlantic Ocean, in latitude 3 degrees 50 minutes S., and 230 miles distant
from the coast of South America. It consists of several islets, together
nine miles in length by three in breadth. The whole seems to be of volcanic
origin; although there is no appearance of any crater, or of any one
central eminence. The most remarkable feature is a hill 1,000 feet high, of
which the upper 400 feet consist of a precipitous, singularly shaped
pinnacle, formed of columnar phonolite, containing numerous crystals of
glassy feldspar, and a few needles of hornblende. From the highest
accessible point of this hill, I could distinguish in different parts of
the group several other conical hills, apparently of the same nature. At
St. Helena there are similar, great, conical, protuberant masses of
phonolite, nearly one thousand feet in height, which have been formed by
the injection of fluid feldspathic lava into yielding strata. If this hill
has had, as is probable, a similar origin, denudation has been here
effected on an enormous scale. Near the base of this hill, I observed beds
of white tuff, intersected by numerous dikes, some of amygdaloidal basalt
and others of trachyte; and beds of slaty phonolite with the planes of
cleavage directed N.W. and S.E. Parts of this rock, where the crystals were
scanty, closely resembled common clay-slate, altered by the contact of a
trap-dike. The lamination of rocks, which undoubtedly have once been fluid,
appears to me a subject well deserving attention. On the beach there were
numerous fragments of compact basalt, of which rock a distant facade of
columns seemed to be formed.

TERCEIRA IN THE AZORES.

The central parts of this island consist of irregularly rounded mountains
of no great elevation, composed of trachyte, which closely resembles in
general character the trachyte of Ascension, presently to be described.
This formation is in many parts overlaid, in the usual order of
superposition, by streams of basaltic lava, which near the coast compose
nearly the whole surface. The course which these streams have followed from
their craters, can often be followed by the eye. The town of Angra is
overlooked by a crateriform hill (Mount Brazil), entirely built of thin
strata of fine-grained, harsh, brown-coloured tuff. The upper beds are seen
to overlap the basaltic streams on which the town stands. This hill is
almost identical in structure and composition with numerous crateriformed
hills in the Galapagos Archipelago.

EFFECTS OF STEAM ON THE TRACHYTIC ROCKS.

In the central part of the island there is a spot, where steam is
constantly issuing in jets from the bottom of a small ravine-like hollow,
which has no exit, and which abuts against a range of trachytic mountains.
The steam is emitted from several irregular fissures: it is scentless, soon
blackens iron, and is of much too high temperature to be endured by the
hand. The manner in which the solid trachyte is changed on the borders of
these orifices is curious: first, the base becomes earthy, with red
freckles evidently due to the oxidation of particles of iron; then it
becomes soft; and lastly, even the crystals of glassy feldspar yield to the
dissolving agent. After the mass is converted into clay, the oxide of iron
seems to be entirely removed from some parts, which are left perfectly
white, whilst in other neighbouring parts, which are of the brightest red
colour, it seems to be deposited in greater quantity; some other masses are
marbled with two distinct colours. Portions of the white clay, now that
they are dry, cannot be distinguished by the eye from the finest prepared
chalk; and when placed between the teeth they feel equally soft-grained;
the inhabitants use this substance for white-washing their houses. The
cause of the iron being dissolved in one part, and close by being again
deposited, is obscure; but the fact has been observed in several other
places. (Spallanzani, Dolomieu, and Hoffman have described similar cases in
the Italian volcanic islands. Dolomieu says the iron at the Panza Islands
is redeposited in the form of veins (page 86 "Memoire sur les Isles
Ponces"). These authors likewise believe that the steam deposits silica: it
is now experimentally known that vapour of a high temperature is able to
dissolve silica.) In some half-decayed specimens, I found small, globular
aggregations of yellow hyalite, resembling gum-arabic, which no doubt had
been deposited by the steam.

As there is no escape for the rain-water, which trickles down the sides of
the ravine-like hollow, whence the steam issues, it must all percolate
downwards through the fissures at its bottom. Some of the inhabitants
informed me that it was on record that flames (some luminous appearance?)
had originally proceeded from these cracks, and that the flames had been
succeeded by the steam; but I was not able to ascertain how long this was
ago, or anything certain on the subject. When viewing the spot, I imagined
that the injection of a large mass of rock. like the cone of phonolite at
Fernando Noronha, in a semi-fluid state, by arching the surface might have
caused a wedge-shaped hollow with cracks at the bottom, and that the rain-
water percolating to the neighbourhood of the heated mass, would during
many succeeding years be driven back in the form of steam.

TAHITI (OTAHEITE).

I visited only a part of the north-western side of this island, and this
part is entirely composed of volcanic rocks. Near the coast there are
several varieties of basalt, some abounding with large crystals of augite
and tarnished olivine, others compact and earthy,--some slightly vesicular,
and others occasionally amygdaloidal. These rocks are generally much
decomposed, and to my surprise, I found in several sections that it was
impossible to distinguish, even approximately, the line of separation
between the decayed lava and the alternating beds of tuff. Since the
specimens have become dry, it is rather more easy to distinguish the
decomposed igneous rocks from the sedimentary tuffs. This gradation in
character between rocks having such widely different origins, may I think
be explained by the yielding under pressure of the softened sides of the
vesicular cavities, which in many volcanic rocks occupy a large proportion
of their bulk. As the vesicles generally increase in size and number in the
upper parts of a stream of lava, so would the effects of their compression
increase; the yielding, moreover, of each lower vesicle must tend to
disturb all the softened matter above it. Hence we might expect to trace a
perfect gradation from an unaltered crystalline rock to one in which all
the particles (although originally forming part of the same solid mass) had
undergone mechanical displacement; and such particles could hardly be
distinguished from others of similar composition, which had been deposited
as sediment. As lavas are sometimes laminated in their upper parts even
horizontal lines, appearing like those of aqueous deposition, could not in
all cases be relied on as a criterion of sedimentary origin. From these
considerations it is not surprising that formerly many geologists believed
in real transitions from aqueous deposits, through wacke, into igneous
traps.

In the valley of Tia-auru, the commonest rocks are basalts with much
olivine, and in some cases almost composed of large crystals of augite. I
picked up some specimens, with much glassy feldspar, approaching in
character to trachyte. There were also many large blocks of vesicular
basalt, with the cavities beautifully lined with chabasie (?), and
radiating bundles of mesotype. Some of these specimens presented a curious
appearance, owing to a number of the vesicles being half filled up with a
white, soft, earthy mesotypic mineral, which intumesced under the blowpipe
in a remarkable manner. As the upper surfaces in all the half-filled cells
are exactly parallel, it is evident that this substance has sunk to the
bottom of each cell from its weight. Sometimes, however, it entirely fills
the cells. Other cells are either quite filled, or lined, with small
crystals, apparently of chabasie; these crystals, also, frequently line the
upper half of the cells partly filled with the earthy mineral, as well as
the upper surface of this substance itself, in which case the two minerals
appear to blend into each other. I have never seen any other amygdaloid
with the cells half filled in the manner here described; and it is
difficult to imagine the causes which determined the earthy mineral to sink
from its gravity to the bottom of the cells, and the crystalline mineral to
adhere in a coating of equal thickness round the sides of the cells.
(MacCulloch, however, has described and given a plate of ("Geolog. Trans."
1st series volume 4 page 225) a trap rock, with cavities filled up
horizontally with quartz and chalcedony. The upper halves of these cavities
are often filled by layers, which follow each irregularity of the surface,
and by little depending stalactites of the same siliceous substances.)

The basic strata on the sides of the valley are gently inclined seaward,
and I nowhere observed any sign of disturbance; the strata are separated
from each other by thick, compact beds of conglomerate, in which the
fragments are large, some being rounded, but most angular. From the
character of these beds, from the compact and crystalline condition of most
of the lavas, and from the nature of the infiltrated minerals, I was led to
conjecture that they had originally flowed beneath the sea. This conclusion
agrees with the fact that the Rev. W. Ellis found marine remains at a
considerable height, which he believes were interstratified with volcanic
matter; as is likewise described to be the case by Messrs. Tyerman and
Bennett at Huaheine, an island in this same archipelago. Mr. Stutchbury
also discovered near the summit of one of the loftiest mountains of Tahiti,
at the height of several thousand feet, a stratum of semi-fossil coral.
None of these remains have been specifically examined. On the coast, where
masses of coral-rock would have afforded the clearest evidence, I looked in
vain for any signs of recent elevation. For references to the above
authorities, and for more detailed reasons for not believing that Tahiti
has been recently elevated, I must refer to the "Structure and Distribution
of Coral-Reefs."

MAURITIUS.

Approaching this island on the northern or north-western side, a curved
chain of bold mountains, surmounted by rugged pinnacles, is seen to rise
from a smooth border of cultivated land, which gently slopes down to the
coast. At the first glance, one is tempted to believe that the sea lately
reached the base of these mountains, and upon examination, this view, at
least with respect to the inferior parts of the border, is found to be
perfectly correct. Several authors have described masses of upraised coral-
rock round the greater part of the circumference of the island. (Captain
Carmichael, in Hooker's "Bot. Misc." volume 2 page 301. Captain Lloyd has
lately, in the "Proceedings of the Geological Society" (volume 3 page 317),
described carefully some of these masses. In the "Voyage a l'Isle de
France, par un Officier du Roi," many interesting facts are given on this
subject. Consult also "Voyage aux Quatre Isles d'Afrique, par M. Bory St.
Vincent.") Between Tamarin Bay and the Great Black River I observed, in
company with Captain Lloyd, two hillocks of coral-rock, formed in their
lower part of hard calcareous sandstone, and in their upper of great
blocks, slightly aggregated, of Astraea and Madrepora, and of fragments of
basalt; they were divided into beds dipping seaward, in one case at an
angle of 8 degrees, and in the other at 18 degrees; they had a water-worn
appearance, and they rose abruptly from a smooth surface, strewed with
rolled debris of organic remains, to a height of about twenty feet. The
Officier du Roi, in his most interesting tour in 1768 round the island, has
described masses of upraised coral-rocks, still retaining that moat-like
structure (see my "Coral Reefs") which is characteristic of the living
reefs. On the coast northward of Port Louis, I found the lava concealed for
a considerable space inland by a conglomerate of corals and shells, like
those on the beach, but in parts consolidated by red ferruginous matter. M.
Bory St. Vincent has described similar calcareous beds over nearly the
whole of the plain of Pamplemousses. Near Port Louis, when turning over
some large stones, which lay in the bed of a stream at the head of a
protected creek, and at the height of some yards above the level of spring
tides, I found several shells of serpula still adhering to their under
sides.

The jagged mountains near Port Louis rise to a height of between two and
three thousand feet; they consist of strata of basalt, obscurely separated
from each other by firmly aggregated beds of fragmentary matter; and they
are intersected by a few vertical dikes. The basalt in some parts abounds
with large crystals of augite and olivine, and is generally compact. The
interior of the island forms a plain, raised probably about a thousand feet
above the level of the sea, and composed of streams of lava which have
flowed round and between the rugged basaltic mountains. These more recent
lavas are also basaltic, but less compact, and some of them abound with
feldspar, so that they even fuse into a pale coloured glass. On the banks
of the Great River, a section is exposed nearly five hundred feet deep,
worn through numerous thin sheets of the lava of this series, which are
separated from each other by beds of scoriae. They seem to have been of
subaerial formation, and to have flowed from several points of eruption on
the central platform, of which the Piton du Milieu is said to be the
principal one. There are also several volcanic cones, apparently of this
modern period, round the circumference of the island, especially at the
northern end, where they form separate islets.

The mountains composed of the more compact and crystalline basalt, form the
main skeleton of the island. M. Bailly ("Voyage aux Terres Australes" tome
1 page 54.) states that they all "se developpent autour d'elle comme une
ceinture d'immenses remparts, toutes affectant une pente plus ou moins
enclinee vers le rivage de la mer; tandis, au contraire, que vers le centre
de l'ile elles presentent une coupe abrupte, et souvent taillee a pic.
Toutes ces montagnes sont formees de couches paralleles inclinees du centre
de l'ile vers la mer." These statements have been disputed, though not in
detail, by M. Quoy, in the voyage of Freycinet. As far as my limited means
of observation went, I found them perfectly correct. (M. Lesson, in his
account of this island, in the "Voyage of the 'Coquille'," seems to follow
M. Bailly's views.) The mountains on the N.W. side of the island, which I
examined, namely, La Pouce, Peter Botts, Corps de Garde, Les Mamelles, and
apparently another farther southward, have precisely the external shape and
stratification described by M. Bailly. They form about a quarter of his
girdle of ramparts. Although these mountains now stand quite detached,
being separated from each other by breaches, even several miles in width,
through which deluges of lava have flowed from the interior of the island;
nevertheless, seeing their close general similarity, one must feel
convinced that they originally formed parts of one continuous mass. Judging
from the beautiful map of the Mauritius, published by the Admiralty from a
French MS., there is a range of mountains (M. Bamboo) on the opposite side
of the island, which correspond in height, relative position, and external
form, with those just described. Whether the girdle was ever complete may
well be doubted; but from M. Bailly's statements, and my own observations,
it may be safely concluded that mountains with precipitous inland flanks,
and composed of strata dipping outwards, once extended round a considerable
portion of the circumference of the island. The ring appears to have been
oval and of vast size; its shorter axis, measured across from the inner
sides of the mountains near Port Louis and those near Grand Port, being no
less than thirteen geographical miles in length. M. Bailly boldly supposes
that this enormous gulf, which has since been filled up to a great extent
by streams of modern lava, was formed by the sinking in of the whole upper
part of one great volcano.

It is singular in how many respects those portions of St. Jago and of
Mauritius which I visited agree in their geological history. At both
islands, mountains of similar external form, stratification, and (at least
in their upper beds) composition, follow in a curved chain the coast-line.
These mountains in each case appear originally to have formed parts of one
continuous mass. The basaltic strata of which they are composed, from their
compact and crystalline structure, seem, when contrasted with the
neighbouring basaltic streams of subaerial formation, to have flowed
beneath the pressure of the sea, and to have been subsequently elevated. We
may suppose that the wide breaches between the mountains were in both cases
worn by the waves, during their gradual elevation--of which process, within
recent times, there is abundant evidence on the coast-land of both islands.
At both, vast streams of more recent basaltic lavas have flowed from the
interior of the island, round and between the ancient basaltic hills; at
both, moreover, recent cones of eruption are scattered around the
circumference of the island; but at neither have eruptions taken place
within the period of history. As remarked in the last chapter, it is
probable that these ancient basaltic mountains, which resemble (at least in
many respects) the basal and disturbed remnants of two gigantic volcanoes,
owe their present form, structure, and position, to the action of similar
causes.

ST. PAUL'S ROCKS.

This small island is situated in the Atlantic Ocean, nearly one degree
north of the equator, and 540 miles distant from South America, in 29
degrees 15 minutes west longitude. Its highest point is scarcely fifty feet
above the level of the sea; its outline is irregular, and its entire
circumference barely three-quarters of a mile. This little point of rock
rises abruptly out of the ocean; and, except on its western side, soundings
were not obtained, even at the short distance of a quarter of a mile from
its shore. It is not of volcanic origin; and this circumstance, which is
the most remarkable point in its history (as will hereafter be referred
to), properly ought to exclude it from the present volume. It is composed
of rocks, unlike any which I have met with, and which I cannot characterise
by any name, and must therefore describe.

The simplest, and one of the most abundant kinds, is a very compact, heavy,
greenish-black rock, having an angular, irregular fracture, with some
points just hard enough to scratch glass, and infusible. This variety
passes into others of paler green tints, less hard, but with a more
crystalline fracture, and translucent on their edges; and these are fusible
into a green enamel. Several other varieties are chiefly characterised by
containing innumerable threads of dark-green serpentine, and by having
calcareous matter in their interstices. These rocks have an obscure,
concretionary structure, and are full of variously coloured angular pseudo
fragments. These angular pseudo fragments consist of the first-described
dark green rock, of a brown softer kind, of serpentine, and of a yellowish
harsh stone, which, perhaps, is related to serpentine rock. There are other
vesicular, calcareo-ferruginous, soft stones. There is no distinct
stratification, but parts are imperfectly laminated; and the whole abounds
with innumerable veins, and vein-like masses, both small and large. Of
these vein-like masses, some calcareous ones, which contain minute
fragments of shells, are clearly of subsequent origin to the others.

A GLOSSY INCRUSTATION.

Extensive portions of these rocks are coated by a layer of a glossy
polished substance, with a pearly lustre and of a greyish white colour; it
follows all the inequalities of the surface, to which it is firmly
attached. When examined with a lens, it is found to consist of numerous
exceedingly thin layers, their aggregate thickness being about the tenth of
an inch. It is considerably harder than calcareous spar, but can be
scratched with a knife; under the blowpipe it scales off, decrepitates,
slightly blackens, emits a fetid odour, and becomes strongly alkaline: it
does not effervesce in acids. (In my "Journal" I have described this
substance; I then believed that it was an impure phosphate of lime.) I
presume this substance has been deposited by water draining from the birds'
dung, with which the rocks are covered. At Ascension, near a cavity in the
rocks which was filled with a laminated mass of infiltrated birds' dung, I
found some irregularly formed, stalactitical masses of apparently the same
nature. These masses, when broken, had an earthy texture; but on their
outsides, and especially at their extremities, they were formed of a pearly
substance, generally in little globules, like the enamel of teeth, but more
translucent, and so hard as just to scratch plate-glass. This substance
slightly blackens under the blowpipe, emits a bad smell, then becomes quite
white, swelling a little, and fuses into a dull white enamel; it does not
become alkaline; nor does it effervesce in acids. The whole mass had a
collapsed appearance, as if in the formation of the hard glossy crust the
whole had shrunk much. At the Abrolhos Islands on the coast of Brazil,
where also there is much birds' dung, I found a great quantity of a brown,
arborescent substance adhering to some trap-rock. In its arborescent form,
this substance singularly resembles some of the branched species of
Nullipora. Under the blowpipe, it behaves like the specimens from
Ascension; but it is less hard and glossy, and the surface has not the
shrunk appearance.