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.