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Chapter III
ASCENSION.
Basaltic lavas.
Numerous craters truncated on the same side.
Singular structure of volcanic bombs.
Aeriform explosions.
Ejected granitic fragments.
Trachytic rocks.
Singular veins.
Jasper, its manner of formation.
Concretions in pumiceous tuff.
Calcareous deposits and frondescent incrustations on the coast.
Remarkable laminated beds, alternating with, and passing into, obsidian.
Origin of obsidian.
Lamination of volcanic rocks.
(MAP 2: THE ISLAND OF ASCENSION.)
This island is situated in the Atlantic Ocean, in latitude 8 degrees S.,
longitude 14 degrees W. It has the form of an irregular triangle (see Map
2), each side being about six miles in length. Its highest point is 2,870
feet ("Geographical Journal" volume 5 page 243.) above the level of the
sea. The whole is volcanic, and, from the absence of proofs to the
contrary, I believe of subaerial origin. The fundamental rock is everywhere
of a pale colour, generally compact, and of a feldspathic nature. In the
S.E. portion of the island, where the highest land is situated, well
characterised trachyte, and other congenerous rocks of that varying family,
occur. Nearly the entire circumference is covered up by black and rugged
streams of basaltic lava, with here and there a hill or single point of
rock (one of which near the sea-coast, north of the Fort, is only two or
three yards across) of the trachyte still remaining exposed.
BASALTIC ROCKS.
The overlying basaltic lava is in some parts extremely vesicular, in others
little so; it is of a black colour, but sometimes contains crystals of
glassy feldspar, and seldom much olivine. These streams appear to have
possessed singularly little fluidity; their side walls and lower ends being
very steep, and even as much as between twenty and thirty feet in height.
Their surface is extraordinarily rugged, and from a short distance appears
as if studded with small craters. These projections consist of broad,
irregularly conical, hillocks, traversed by fissures, and composed of the
same unequally scoriaceous basalt with the surrounding streams, but having
an obscure tendency to a columnar structure; they rise to a height between
ten and thirty feet above the general surface, and have been formed, as I
presume, by the heaping up of the viscid lava at points of greater
resistance. At the base of several of these hillocks, and occasionally
likewise on more level parts, solid ribs, composed of angulo-globular
masses of basalt, resembling in size and outline arched sewers or gutters
of brickwork, but not being hollow, project between two or three feet above
the surface of the streams; what their origin may have been, I do not know.
Many of the superficial fragments from these basaltic streams present
singularly convoluted forms; and some specimens could hardly be
distinguished from logs of dark-coloured wood without their bark.
Many of the basaltic streams can be traced, either to points of eruption at
the base of the great central mass of trachyte, or to separate, conical,
red-coloured hills, which are scattered over the northern and western
borders of the island. Standing on the central eminence, I counted between
twenty and thirty of these cones of eruption. The greater number of them
had their truncated summits cut off obliquely, and they all sloped towards
the S.E., whence the trade-wind blows. (M. Lesson in the "Zoology of the
Voyage of the 'Coquille'" page 490 has observed this fact. Mr. Hennah
("Geolog. Proceedings" 1835 page 189) further remarks that the most
extensive beds of ashes at Ascension invariably occur on the leeward side
of the island.) This structure no doubt has been caused by the ejected
fragments and ashes being always blown, during eruptions, in greater
quantity towards one side than towards the other. M. Moreau de Jonnes has
made a similar observation with respect to the volcanic orifices in the
West Indian Islands.
VOLCANIC BOMBS.
(FIGURE 3: FRAGMENT OF A SPHERICAL VOLCANIC BOMB, with the interior parts
coarsely cellular, coated by a concentric layer of compact lava, and this
again by a crust of finely cellular rock.
FIGURE 4: VOLCANIC BOMB OF OBSIDIAN FROM AUSTRALIA. The upper figure gives
a front view; the lower a side view of the same object.)
These occur in great numbers strewed on the ground, and some of them lie at
considerable distances from any points of eruption. They vary in size from
that of an apple to that of a man's body; they are either spherical or
pear-shaped, or with the hinder part (corresponding to the tail of a comet)
irregular, studded with projecting points, and even concave. Their surfaces
are rough, and fissured with branching cracks; their internal structure is
either irregularly scoriaceous and compact, or it presents a symmetrical
and very curious appearance. An irregular segment of a bomb of this latter
kind, of which I found several, is accurately represented in Figure 3. Its
size was about that of a man's head. The whole interior is coarsely
cellular; the cells averaging in diameter about the tenth of an inch; but
nearer the outside they gradually decrease in size. This part is succeeded
by a well-defined shell of compact lava, having a nearly uniform thickness
of about the third of an inch; and the shell is overlaid by a somewhat
thicker coating of finely cellular lava (the cells varying from the
fiftieth to the hundredth of an inch in diameter), which forms the external
surface: the line separating the shell of compact lava from the outer
scoriaceous crust is distinctly defined. This structure is very simply
explained, if we suppose a mass of viscid, scoriaceous matter, to be
projected with a rapid, rotatory motion through the air; for whilst the
external crust, from cooling, became solidified (in the state we now see
it), the centrifugal force, by relieving the pressure in the interior parts
of the bomb, would allow the heated vapours to expand their cells; but
these being driven by the same force against the already-hardened crust,
would become, the nearer they were to this part, smaller and smaller or
less expanded, until they became packed into a solid, concentric shell. As
we know that chips from a grindstone (Nichol "Architecture of the
Heavens.") can be flirted off, when made to revolve with sufficient
velocity, we need not doubt that the centrifugal force would have power to
modify the structure of a softened bomb, in the manner here supposed.
Geologists have remarked, that the external form of a bomb at once bespeaks
the history of its aerial course, and few now see that the internal
structure can speak, with almost equal plainness, of its rotatory movement.
M. Bory St. Vincent ("Voyage aux Quatre Isles d'Afrique" tome 1 page 222.)
has described some balls of lava from the Isle of Bourbon, which have a
closely similar structure. His explanation, however (if I understand it
rightly), is very different from that which I have given; for he supposes
that they have rolled, like snowballs, down the sides of the crater. M.
Beudant ("Voyage en Hongrie" tome 2 page 214.), also, has described some
singular little balls of obsidian, never more than six or eight inches in
diameter, which he found strewed on the surface of the ground: their form
is always oval; sometimes they are much swollen in the middle, and even
spindle-shaped: their surface is regularly marked with concentric ridges
and furrows, all of which on the same ball are at right angles to one axis:
their interior is compact and glassy. M. Beudant supposes that masses of
lava, when soft, were shot into the air, with a rotatory movement round the
same axis, and that the form and superficial ridges of the bombs were thus
produced. Sir Thomas Mitchell has given me what at first appears to be the
half of a much flattened oval ball of obsidian; it has a singular
artificial-like appearance, which is well represented (of the natural size)
in Figure 4. It was found in its present state, on a great sandy plain
between the rivers Darling and Murray, in Australia, and at the distance of
several hundred miles from any known volcanic region. It seems to have been
embedded in some reddish tufaceous matter; and may have been transported
either by the aborigines or by natural means. The external saucer consists
of compact obsidian, of a bottle-green colour, and is filled with finely
cellular black lava, much less transparent and glassy than the obsidian.
The external surface is marked with four or five not quite perfect ridges,
which are represented rather too distinctly in Figure 4. Here, then, we
have the external structure described by M. Beudant, and the internal
cellular condition of the bombs from Ascension. The lip of the saucer is
slightly concave, exactly like the margin of a soup-plate, and its inner
edge overlaps a little the central cellular lava. This structure is so
symmetrical round the entire circumference, that one is forced to suppose
that the bomb burst during its rotatory course, before being quite
solidified, and that the lip and edges were thus slightly modified and
turned inwards. It may be remarked that the superficial ridges are in
planes, at right angles to an axis, transverse to the longer axis of the
flattened oval: to explain this circumstance, we may suppose that when the
bomb burst, the axis of rotation changed.
AERIFORM EXPLOSIONS.
The flanks of Green Mountain and the surrounding country are covered by a
great mass, some hundred feet in thickness, of loose fragments. The lower
beds generally consist of fine-grained, slightly consolidated tuffs (Some
of this peperino, or tuff, is sufficiently hard not to be broken by the
greatest force of the fingers.), and the upper beds of great loose
fragments, with alternating finer beds. (On the northern side of the Green
Mountain a thin seam, about an inch in thickness, of compact oxide of iron,
extends over a considerable area; it lies conformably in the lower part of
the stratified mass of ashes and fragments. This substance is of a reddish-
brown colour, with an almost metallic lustre; it is not magnetic, but
becomes so after having been heated under the blowpipe, by which it is
blackened and partly fused. This seam of compact stone, by intercepting the
little rain-water which falls on the island, gives rise to a small dripping
spring, first discovered by Dampier. It is the only fresh water on the
island, so that the possibility of its being inhabited has entirely
depended on the occurrence of this ferruginous layer.) One white ribbon-
like layer of decomposed, pumiceous breccia, was curiously bent into deep
unbroken curves, beneath each of the large fragments in the superincumbent
stratum. From the relative position of these beds, I presume that a narrow-
mouthed crater, standing nearly in the position of Green Mountain, like a
great air-gun, shot forth, before its final extinction, this vast
accumulation of loose matter. Subsequently to this event, considerable
dislocations have taken place, and an oval circus has been formed by
subsidence. This sunken space lies at the north-eastern foot of Green
Mountain, and is well represented in Map 2. Its longer axis, which is
connected with a N.E. and S.W. line of fissure, is three-fifths of a
nautical mile in length; its sides are nearly perpendicular, except in one
spot, and about four hundred feet in height; they consist, in the lower
part, of a pale basalt with feldspar, and in the upper part, of the tuff
and loose ejected fragments; the bottom is smooth and level, and under
almost any other climate a deep lake would have been formed here. From the
thickness of the bed of loose fragments, with which the surrounding country
is covered, the amount of aeriform matter necessary for their projection
must have been enormous; hence we may suppose it probable that after the
explosions vast subterranean caverns were left, and that the falling in of
the roof of one of these produced the hollow here described. At the
Galapagos Archipelago, pits of a similar character, but of a much smaller
size, frequently occur at the bases of small cones of eruption.
EJECTED GRANITIC FRAGMENTS.
In the neighbourhood of Green Mountain, fragments of extraneous rock are
not unfrequently found embedded in the midst of masses of scoriae.
Lieutenant Evans, to whose kindness I am indebted for much information,
gave me several specimens, and I found others myself. They nearly all have
a granitic structure, are brittle, harsh to the touch, and apparently of
altered colours.
FIRST, a white syenite, streaked and mottled with red; it consists of well-
crystallised feldspar, numerous grains of quartz, and brilliant, though
small, crystals of hornblende. The feldspar and hornblende in this and the
succeeding cases have been determined by the reflecting goniometer, and the
quartz by its action under the blowpipe. The feldspar in these ejected
fragments, like the glassy kind in the trachyte, is from its cleavage a
potash-feldspar.
SECONDLY, a brick-red mass of feldspar, quartz, and small dark patches of a
decayed mineral; one minute particle of which I was able to ascertain, by
its cleavage, to be hornblende.
THIRDLY, a mass of confusedly crystallised white feldspar, with little
nests of a dark-coloured mineral, often carious, externally rounded, having
a glossy fracture, but no distinct cleavage: from comparison with the
second specimen, I have no doubt that it is fused hornblende.
FOURTHLY, a rock, which at first appears a simple aggregation of distinct
and large-sized crystals of dusty-coloured Labrador feldspar (Professor
Miller has been so kind as to examine this mineral. He obtained two good
cleavages of 86 degrees 30 minutes and 86 degrees 50 minutes. The mean of
several, which I made, was 86 degrees 30 minutes. Professor Miller states
that these crystals, when reduced to a fine powder, are soluble in
hydrochloric acid, leaving some undissolved silex behind; the addition of
oxalate of ammonia gives a copious precipitate of lime. He further remarks,
that according to Von Kobell, anorthite (a mineral occurring in the ejected
fragments at Mount Somma) is always white and transparent, so that if this
be the case, these crystals from Ascension must be considered as Labrador
feldspar. Professor Miller adds, that he has seen an account, in Erdmann's
"Journal fur tecnische Chemie," of a mineral ejected from a volcano which
had the external characters of Labrador feldspar, but differed in the
analysis from that given by mineralogists of this mineral: the author
attributed this difference to an error in the analysis of Labrador
feldspar, which is very old.); but in their interstices there is some white
granular feldspar, abundant scales of mica, a little altered hornblende,
and, as I believe, no quartz. I have described these fragments in detail,
because it is rare to find granitic rocks ejected from volcanoes with their
MINERALS UNCHANGED, as is the case with the first specimen, and partially
with the second. (Daubeny, in his work on Volcanoes page 386, remarks that
this is the case; and Humboldt, in his "Personal Narrative" volume 1 page
236, says "In general, the masses of known primitive rocks, I mean those
which perfectly resemble our granites, gneiss, and mica-slate, are very
rare in lavas: the substances we generally denote by the name of granite,
thrown out by Vesuvius, are mixtures of nepheline, mica, and pyroxene.")
One other large fragment, found in another spot, is deserving of notice; it
is a conglomerate, containing small fragments of granitic, cellular, and
jaspery rocks, and of hornstone porphyries, embedded in a base of wacke,
threaded by numerous thin layers of a concretionary pitchstone passing into
obsidian. These layers are parallel, slightly tortuous, and short; they
thin out at their ends, and resemble in form the layers of quartz in
gneiss. It is probable that these small embedded fragments were not
separately ejected, but were entangled in a fluid volcanic rock, allied to
obsidian; and we shall presently see that several varieties of this latter
series of rock assume a laminated structure.
TRACHYTIC SERIES OF ROCKS.
Those occupy the more elevated and central, and likewise the south-eastern,
parts of the island. The trachyte is generally of a pale brown colour,
stained with small darker patches; it contains broken and bent crystals of
glassy feldspar, grains of specular iron, and black microscopical points,
which latter, from being easily fused, and then becoming magnetic, I
presume are hornblende. The greater number of the hills, however, are
composed of a quite white, friable stone, appearing like a trachytic tuff.
Obsidian, hornstone, and several kinds of laminated feldspathic rocks, are
associated with the trachyte. There is no distinct stratification; nor
could I distinguish a crateriform structure in any of the hills of this
series. Considerable dislocations have taken place; and many fissures in
these rocks are yet left open, or are only partially filled with loose
fragments. Within the space (This space is nearly included by a line
sweeping round Green Mountain, and joining the hills, called the Weather
Port Signal, Holyhead, and that denominated (improperly in a geological
sense) "the Crater of an old volcano."), mainly formed of trachyte, some
basaltic streams have burst forth; and not far from the summit of Green
Mountain, there is one stream of quite black, vesicular basalt, containing
minute crystals of glassy feldspar, which have a rounded appearance.
The soft white stone above mentioned is remarkable from its singular
resemblance, when viewed in mass, to a sedimentary tuff: it was long before
I could persuade myself that such was not its origin; and other geologists
have been perplexed by closely similar formations in trachytic regions. In
two cases, this white earthy stone formed isolated hills; in a third, it
was associated with columnar and laminated trachyte; but I was unable to
trace an actual junction. It contains numerous crystals of glassy feldspar
and black microscopical specks, and is marked with small darker patches,
exactly as in the surrounding trachyte. Its basis, however, when viewed
under the microscope, is generally quite earthy; but sometimes it exhibits
a decidedly crystalline structure. On the hill marked "Crater of an old
volcano," it passes into a pale greenish-grey variety, differing only in
its colour, and in not being so earthy; the passage was in one case
effected insensibly; in another, it was formed by numerous, rounded and
angular, masses of the greenish variety, being embedded in the white
variety;--in this latter case, the appearance was very much like that of a
sedimentary deposit, torn up and abraded during the deposition of a
subsequent stratum. Both these varieties are traversed by innumerable
tortuous veins (presently to be described), which are totally unlike
injected dikes, or indeed any other veins which I have ever seen. Both
varieties include a few scattered fragments, large and small, of dark-
coloured scoriaceous rocks, the cells of some of which are partially filled
with the white earthy stone; they likewise include some huge blocks of a
cellular porphyry. (The porphyry is dark coloured; it contains numerous,
often fractured, crystals of white opaque feldspar, also decomposing
crystals of oxide of iron; its vesicles include masses of delicate, hair-
like, crystals, apparently of analcime.) These fragments project from the
weathered surface, and perfectly resemble fragments embedded in a true
sedimentary tuff. But as it is known that extraneous fragments of cellular
rock are sometimes included in columnar trachyte, in phonolite (D'Aubuisson
"Traite de Geognosie" tome 2 page 548.), and in other compact lavas, this
circumstance is not any real argument for the sedimentary origin of the
white earthy stone. (Dr. Daubeny on Volcanoes, page 180 seems to have been
led to believe that certain trachytic formations of Ischia and of the Puy
de Dome, which closely resemble these of Ascension, were of sedimentary
origin, chiefly from the frequent presence in them "of scoriform portions,
different in colour from the matrix." Dr. Daubeny adds, that on the other
hand, Brocchi, and other eminent geologists, have considered these beds as
earthy varieties of trachyte; he considers the subject deserving of further
attention.) The insensible passage of the greenish variety into the white
one, and likewise the more abrupt passage by fragments of the former being
embedded in the latter, might result from slight differences in the
composition of the same mass of molten stone, and from the abrading action
of one such part still fluid on another part already solidified. The
curiously formed veins have, I believe, been formed by siliceous matter
being subsequently segregated. But my chief reason for believing that these
soft earthy stones, with their extraneous fragments, are not of sedimentary
origin, is the extreme improbability of crystals of feldspar, black
microscopical specks, and small stains of a darker colour occurring in the
same proportional numbers in an aqueous deposit, and in masses of solid
trachyte. Moreover, as I have remarked, the microscope occasionally reveals
a crystalline structure in the apparently earthy basis. On the other hand,
the partial decomposition of such great masses of trachyte, forming whole
mountains, is undoubtedly a circumstance of not easy explanation.
VEINS IN THE EARTHY TRACHYTIC MASSES.
These veins are extraordinarily numerous, intersecting in the most
complicated manner both coloured varieties of the earthy trachyte: they are
best seen on the flanks of the "Crater of the old volcano." They contain
crystals of glassy feldspar, black microscopical specks and little dark
stains, precisely as in the surrounding rock; but the basis is very
different, being exceedingly hard, compact, somewhat brittle, and of rather
less easy fusibility. The veins vary much, and suddenly, from the tenth of
an inch to one inch in thickness; they often thin out, not only on their
edges, but in their central parts, thus leaving round, irregular apertures;
their surfaces are rugged. They are inclined at every possible angle with
the horizon, or are horizontal; they are generally curvilinear, and often
interbranch one with another. From their hardness they withstand
weathering, and projecting two or three feet above the ground, they
occasionally extend some yards in length; these plate-like veins, when
struck, emit a sound, almost like that of a drum, and they may be
distinctly seen to vibrate; their fragments, which are strewed on the
ground, clatter like pieces of iron when knocked against each other. They
often assume the most singular forms; I saw a pedestal of the earthy
trachyte, covered by a hemispherical portion of a vein, like a great
umbrella, sufficiently large to shelter two persons. I have never met with,
or seen described, any veins like these; but in form they resemble the
ferruginous seams, due to some process of segregation, occurring not
uncommonly in sandstones,--for instance, in the New Red sandstone of
England. Numerous veins of jasper and of siliceous sinter, occurring on the
summit of this same hill, show that there has been some abundant source of
silica, and as these plate-like veins differ from the trachyte only in
their greater hardness, brittleness, and less easy fusibility, it appears
probable that their origin is due to the segregation or infiltration of
siliceous matter, in the same manner as happens with the oxides of iron in
many sedimentary rocks.
SILICEOUS SINTER AND JASPER.
The siliceous sinter is either quite white, of little specific gravity, and
with a somewhat pearly fracture, passing into pinkish pearl quartz; or it
is yellowish white, with a harsh fracture, and it then contains an earthy
powder in small cavities. Both varieties occur, either in large irregular
masses in the altered trachyte, or in seams included in broad, vertical,
tortuous, irregular veins of a compact, harsh stone of a dull red colour,
appearing like a sandstone. This stone, however, is only altered trachyte;
and a nearly similar variety, but often honeycombed, sometimes adheres to
the projecting plate-like veins, described in the last paragraph. The
jasper is of an ochre yellow or red colour; it occurs in large irregular
masses, and sometimes in veins, both in the altered trachyte and in an
associated mass of scoriaceous basalt. The cells of the scoriaceous basalt
are lined or filled with fine, concentric layers of chalcedony, coated and
studded with bright-red oxide of iron. In this rock, especially in the
rather more compact parts, irregular angular patches of the red jasper are
included, the edges of which insensibly blend into the surrounding mass;
other patches occur having an intermediate character between perfect jasper
and the ferruginous, decomposed, basaltic base. In these patches, and
likewise in the large vein-like masses of jasper, there occur little
rounded cavities, of exactly the same size and form with the air-cells,
which in the scoriaceous basalt are filled and lined with layers of
chalcedony. Small fragments of the jasper, examined under the microscope,
seem to resemble the chalcedony with its colouring matter not separated
into layers, but mingled in the siliceous paste, together with some
impurities. I can understand these facts,--namely, the blending of the
jasper into the semi-decomposed basalt,--its occurrence in angular patches,
which clearly do not occupy pre-existing hollows in the rock,--and its
containing little vesicles filled with chalcedony, like those in the
scoriaceous lava,--only on the supposition that a fluid, probably the same
fluid which deposited the chalcedony in the air-cells, removed in those
parts where there were no cavities, the ingredients of the basaltic rock,
and left in their place silica and iron, and thus produced the jasper. In
some specimens of silicified wood, I have observed, that in the same manner
as in the basalt, the solid parts were converted into a dark-coloured
homogeneous stone, whereas the cavities formed by the larger sap-vessels
(which may be compared with the air-vesicles in the basaltic lava) and
other irregular hollows, apparently produced by decay, were filled with
concentric layers of chalcedony; in this case, there can be little doubt
that the same fluid deposited the homogeneous base and the chalcedonic
layers. After these considerations, I cannot doubt but that the jasper of
Ascension may be viewed as a volcanic rock silicified, in precisely the
same sense as this term is applied to wood, when silicified; we are equally
ignorant of the means by which every atom of wood, whilst in a perfect
state, is removed and replaced by atoms of silica, as we are of the means
by which the constituent parts of a volcanic rock could be thus acted on.
(Beudant "Voyage en Hongrie" tome 3 pages 502, 504 describes kidney-shaped
masses of jasper-opal, which either blend into the surrounding trachytic
conglomerate, or are embedded in it like chalk-flints; and he compares them
with the fragments of opalised wood, which are abundant in this same
formation. Beudant, however, appears to have viewed the process of their
formation rather as one of simple infiltration than of molecular exchange;
but the presence of a concretion, wholly different from the surrounding
matter, if not formed in a pre-existing hollow, clearly seems to me to
require, either a molecular or mechanical displacement of the atoms, which
occupied the space afterwards filled by it. The jasper-opal of Hungary
passes into chalcedony, and therefore in this case, as in that of
Ascension, jasper seems to be intimately related in origin with
chalcedony.) I was led to the careful examination of these rocks, and to
the conclusion here given, from having heard the Rev. Professor Henslow
express a similar opinion, regarding the origin in trap-rocks of many
chalcedonies and agates. Siliceous deposits seem to be very general, if not
of universal occurrence, in partially decomposed trachytic tuffs (Beudant
"Voyage Min." tome 3 page 507 enumerates cases in Hungary, Germany, Central
France, Italy, Greece, and Mexico.); and as these hills, according to the
view above given, consist of trachyte softened and altered in situ, the
presence of free silica in this case may be added as one more instance to
the list.
CONCRETIONS IN PUMICEOUS TUFF.
The hill, marked in Map 2 "Crater of an old volcano," has no claims to this
appellation, which I could discover, except in being surmounted by a
circular, very shallow, saucer-like summit, nearly half a mile in diameter.
This hollow has been nearly filled up with many successive sheets of ashes
and scoriae, of different colours, and slightly consolidated. Each
successive saucer-shaped layer crops out all round the margin, forming so
many rings of various colours, and giving to the hill a fantastic
appearance. The outer ring is broad, and of a white colour; hence it
resembles a course round which horses have been exercised, and has received
the name of the Devil's Riding School, by which it is most generally known.
These successive layers of ashes must have fallen over the whole
surrounding country, but they have all been blown away except in this one
hollow, in which probably moisture accumulated, either during an
extraordinary year when rain fell, or during the storms often accompanying
volcanic eruptions. One of the layers of a pinkish colour, and chiefly
derived from small, decomposed fragments of pumice, is remarkable, from
containing numerous concretions. These are generally spherical, from half
an inch to three inches in diameter; but they are occasionally cylindrical,
like those of iron-pyrites in the chalk of Europe. They consist of a very
tough, compact, pale-brown stone, with a smooth and even fracture. They are
divided into concentric layers by thin white partitions, resembling the
external superficies; six or eight of such layers are distinctly defined
near the outside; but those towards the inside generally become indistinct,
and blend into a homogeneous mass. I presume that these concentric layers
were formed by the shrinking of the concretion, as it became compact. The
interior part is generally fissured by minute cracks or septaria, which are
lined, both by black, metallic, and by other white and crystalline specks,
the nature of which I was unable to ascertain. Some of the larger
concretions consist of a mere spherical shell, filled with slightly
consolidated ashes. The concretions contain a small proportion of carbonate
of lime: a fragment placed under the blowpipe decrepitates, then whitens
and fuses into a blebby enamel, but does not become caustic. The
surrounding ashes do not contain any carbonate of lime; hence the
concretions have probably been formed, as is so often the case, by the
aggregation of this substance. I have not met with any account of similar
concretions; and considering their great toughness and compactness, their
occurrence in a bed, which probably has been subjected only to atmospheric
moisture, is remarkable.
FORMATION OF CALCAREOUS ROCKS ON THE SEA-COAST.
On several of the sea-beaches, there are immense accumulations of small,
well-rounded particles of shells and corals, of white, yellowish, and pink
colours, interspersed with a few volcanic particles. At the depth of a few
feet, these are found cemented together into stone, of which the softer
varieties are used for building; there are other varieties, both coarse and
fine-grained, too hard for this purpose: and I saw one mass divided into
even layers half an inch in thickness, which were so compact that when
struck with a hammer they rang like flint. It is believed by the
inhabitants, that the particles become united in the course of a single
year. The union is effected by calcareous matter; and in the most compact
varieties, each rounded particle of shell and volcanic rock can be
distinctly seen to be enveloped in a husk of pellucid carbonate of lime.
Extremely few perfect shells are embedded in these agglutinated masses; and
I have examined even a large fragment under a microscope, without being
able to discover the least vestige of striae or other marks of external
form: this shows how long each particle must have been rolled about, before
its turn came to be embedded and cemented. (The eggs of the turtle being
buried by the parent, sometimes become enclosed in the solid rock. Mr.
Lyell has given a figure ("Principles of Geology" book 3 chapter 17) of
some eggs, containing the bones of young turtles, found thus entombed.) One
of the most compact varieties, when placed in acid, was entirely dissolved,
with the exception of some flocculent animal matter; its specific gravity
was 2.63. The specific gravity of ordinary limestone varies from 2.6 to
2.75; pure Carrara marble was found by Sir H. De la Beche to be 2.7.
("Researches in Theoretical Geology" page 12.) It is remarkable that these
rocks of Ascension, formed close to the surface, should be nearly as
compact as marble, which has undergone the action of heat and pressure in
the plutonic regions.
The great accumulation of loose calcareous particles, lying on the beach
near the Settlement, commences in the month of October, moving towards the
S.W., which, as I was informed by Lieutenant Evans, is caused by a change
in the prevailing direction of the currents. At this period the tidal
rocks, at the S.W. end of the beach, where the calcareous sand is
accumulating, and round which the currents sweep, become gradually coated
with a calcareous incrustation, half an inch in thickness. It is quite
white, compact, with some parts slightly spathose, and is firmly attached
to the rock. After a short time it gradually disappears, being either
redissolved, when the water is less charged with lime, or more probably is
mechanically abraded. Lieutenant Evans has observed these facts, during the
six years he has resided at Ascension. The incrustation varies in thickness
in different years: in 1831 it was unusually thick. When I was there in
July, there was no remnant of the incrustation; but on a point of basalt,
from which the quarrymen had lately removed a mass of the calcareous
freestone, the incrustation was perfectly preserved. Considering the
position of the tidal-rocks, and the period at which they become coated,
there can be no doubt that the movement and disturbance of the vast
accumulation of calcareous particles, many of them being partially
agglutinated together, cause the waves of the sea to be so highly charged
with carbonate of lime, that they deposit it on the first objects against
which they impinge. I have been informed by Lieutenant Holland, R.N., that
this incrustation is formed on many parts of the coast, on most of which, I
believe, there are likewise great masses of comminuted shells.
A FRONDESCENT CALCAREOUS INCRUSTATION.
(FIGURE 5. AN INCRUSTATION OF CALCAREOUS AND ANIMAL MATTER, coating the
tidal-rocks at Ascension.)
In many respects this is a singular deposit; it coats throughout the year
the tidal volcanic rocks, that project from the beaches composed of broken
shells. Its general appearance is well represented in Figure 5; but the
fronds or discs, of which it is composed, are generally so closely crowded
together as to touch. These fronds have their sinuous edges finely
crenulated, and they project over their pedestals or supports; their upper
surfaces are either slightly concave, or slightly convex; they are highly
polished, and of a dark grey or jet black colour; their form is irregular,
generally circular, and from the tenth of an inch to one inch and a half in
diameter; their thickness, or amount of their projection from the rock on
which they stand, varies much, about a quarter of an inch being perhaps
most usual. The fronds occasionally become more and more convex, until they
pass into botryoidal masses with their summits fissured; when in this
state, they are glossy and of an intense black, so as to resemble some
fused metallic substance. I have shown the incrustation, both in this
latter and in its ordinary state to several geologists, but not one could
conjecture its origin, except that perhaps it was of volcanic nature!
The substance forming the fronds has a very compact and often almost
crystalline fracture; the edges being translucent, and hard enough easily
to scratch calcareous spar. Under the blowpipe it immediately becomes
white, and emits a strong animal odour, like that from fresh shells. It is
chiefly composed of carbonate of lime; when placed in muriatic acid it
froths much, leaving a residue of sulphate of lime, and of an oxide of
iron, together with a black powder, which is not soluble in heated acids.
This latter substance seems to be carbonaceous, and is evidently the
colouring matter. The sulphate of lime is extraneous, and occurs in
distinct, excessively minute, lamellar plates, studded on the surface of
the fronds, and embedded between the fine layers of which they are
composed; when a fragment is heated in the blowpipe, these lamellae are
immediately rendered visible. The original outline of the fronds may often
be traced, either to a minute particle of shell fixed in a crevice of the
rock, or to several cemented together; these first become deeply corroded,
by the dissolving power of the waves, into sharp ridges, and then are
coated with successive layers of the glossy, grey, calcareous incrustation.
The inequalities of the primary support affect the outline of every
successive layer, in the same manner as may often be seen in bezoar-stones,
when an object like a nail forms the centre of aggregation. The crenulated
edges, however, of the frond appear to be due to the corroding power of the
surf on its own deposit, alternating with fresh depositions. On some smooth
basaltic rocks on the coast of St. Jago, I found an exceedingly thin layer
of brown calcareous matter, which under a lens presented a miniature
likeness of the crenulated and polished fronds of Ascension; in this case a
basis was not afforded by any projecting extraneous particles. Although the
incrustation at Ascension is persistent throughout the year; yet from the
abraded appearance of some parts, and from the fresh appearance of other
parts, the whole seems to undergo a round of decay and renovation, due
probably to changes in the form of the shifting beach, and consequently in
the action of the breakers: hence probably it is, that the incrustation
never acquires a great thickness. Considering the position of the encrusted
rocks in the midst of the calcareous beach, together with its composition,
I think there can be no doubt that its origin is due to the dissolution and
subsequent deposition of the matter composing the rounded particles of
shells and corals. (The selenite, as I have remarked is extraneous, and
must have been derived from the sea-water. It is an interesting
circumstance thus to find the waves of the ocean, sufficiently charged with
sulphate of lime, to deposit it on the rocks, against which they dash every
tide. Dr. Webster has described ("Voyage of the 'Chanticleer'" volume 2
page 319) beds of gypsum and salt, as much as two feet in thickness, left
by the evaporation of the spray on the rocks on the windward coast.
Beautiful stalactites of selenite, resembling in form those of carbonate of
lime, are formed near these beds. Amorphous masses of gypsum, also, occur
in caverns in the interior of the island; and at Cross Hill (an old crater)
I saw a considerable quantity of salt oozing from a pile of scoriae. In
these latter cases, the salt and gypsum appear to be volcanic products.)
From this source it derives its animal matter, which is evidently the
colouring principle. The nature of the deposit, in its incipient stage, can
often be well seen upon a fragment of white shell, when jammed between two
of the fronds; it then appears exactly like the thinnest wash of a pale
grey varnish. Its darkness varies a little, but the jet blackness of some
of the fronds and of the botryoidal masses seems due to the translucency of
the successive grey layers. There is, however, this singular circumstance,
that when deposited on the under side of ledges of rock or in fissures, it
appears always to be of a pale, pearly grey colour, even when of
considerable thickness: hence one is led to suppose, that an abundance of
light is necessary to the development of the dark colour, in the same
manner as seems to be the case with the upper and exposed surfaces of the
shells of living mollusca, which are always dark, compared with their under
surfaces and with the parts habitually covered by the mantle of the animal.
In this circumstance,--in the immediate loss of colour and in the odour
emitted under the blowpipe,--in the degree of hardness and translucency of
the edges,--and in the beautiful polish of the surface (From the fact
described in my "Journal of Researches" of a coating of oxide of iron,
deposited by a streamlet on the rocks in its bed (like a nearly similar
coating at the great cataracts of the Orinoco and Nile), becoming finely
polished where the surf acts, I presume that the surf in this instance,
also, is the polishing agent.), rivalling when in a fresh state that of the
finest Oliva, there is a striking analogy between this inorganic
incrustation and the shells of living molluscous animals. (In the section
descriptive of St. Paul's Rocks, I have described a glossy, pearly
substance, which coats the rocks, and an allied stalactitical incrustation
from Ascension, the crust of which resembles the enamel of teeth, but is
hard enough to scratch plate-glass. Both these substances contain animal
matter, and seem to have been derived from water in filtering through
birds' dung.) This appears to me to be an interesting physiological fact.
(Mr. Horner and Sir David Brewster have described "Philosophical
Transactions" 1836 page 65 a singular "artificial substance, resembling
shell." It is deposited in fine, transparent, highly polished, brown-
coloured laminae, possessing peculiar optical properties, on the inside of
a vessel, in which cloth, first prepared with glue and then with lime, is
made to revolve rapidly in water. It is much softer, more transparent, and
contains more animal matter, than the natural incrustation at Ascension;
but we here again see the strong tendency which carbonate of lime and
animal matter evince to form a solid substance allied to shell.)
SINGULAR LAMINATED BEDS ALTERNATING WITH AND PASSING INTO OBSIDIAN.
These beds occur within the trachytic district, at the western base of
Green Mountain, under which they dip at a high inclination. They are only
partially exposed, being covered up by modern ejections; from this cause, I
was unable to trace their junction with the trachyte, or to discover
whether they had flowed as a stream of lava, or had been injected amidst
the overlying strata. There are three principal beds of obsidian, of which
the thickest forms the base of the section. The alternating stony layers
appear to me eminently curious, and shall be first described, and
afterwards their passage into the obsidian. They have an extremely
diversified appearance; five principal varieties may be noticed, but these
insensibly blend into each other by endless gradations.
FIRST.
A pale grey, irregularly and coarsely laminated (This term is open to some
misinterpretation, as it may be applied both to rocks divided into laminae
of exactly the same composition, and to layers firmly attached to each
other, with no fissile tendency, but composed of different minerals, or of
different shades of colour. The term "laminated," in this chapter, is
applied in these latter senses; where a homogeneous rock splits, as in the
former sense, in a given direction, like clay-slate, I have used the term
"fissile."), harsh-feeling rock, resembling clay-slate which has been in
contact with a trap-dike, and with a fracture of about the same degree of
crystalline structure. This rock, as well as the following varieties,
easily fuses into a pale glass. The greater part is honeycombed with
irregular, angular, cavities, so that the whole has a curious appearance,
and some fragments resemble in a remarkable manner silicified logs of
decayed wood. This variety, especially where more compact, is often marked
with thin whitish streaks, which are either straight or wrap round, one
behind the other, the elongated carious hollows.
SECONDLY.
A bluish grey or pale brown, compact, heavy, homogeneous stone, with an
angular, uneven, earthy fracture; viewed, however, under a lens of high
power, the fracture is seen to be distinctly crystalline, and even separate
minerals can be distinguished.
THIRDLY.
A stone of the same kind with the last, but streaked with numerous,
parallel, slightly tortuous, white lines of the thickness of hairs. These
white lines are more crystalline than the parts between them; and the stone
splits along them: they frequently expand into exceedingly thin cavities,
which are often only just perceptible with a lens. The matter forming the
white lines becomes better crystallised in these cavities, and Professor
Miller was fortunate enough, after several trials, to ascertain that the
white crystals, which are the largest, were of quartz (Professor Miller
informs me that the crystals which he measured had the faces P, z, m of the
figure (147) given by Haidinger in his Translation of Mohs; and he adds,
that it is remarkable, that none of them had the slightest trace of faces r
of the regular six-sided prism.), and that the minute green transparent
needles were augite, or, as they would more generally be called, diopside:
besides these crystals, there are some minute, dark specks without a trace
of crystalline, and some fine, white, granular, crystalline matter which is
probably feldspar. Minute fragments of this rock are easily fusible.
FOURTHLY.
A compact crystalline rock, banded in straight lines with innumerable
layers of white and grey shades of colour, varying in width from the
thirtieth to the two-hundredth of an inch; these layers seem to be composed
chiefly of feldspar, and they contain numerous perfect crystals of glassy
feldspar, which are placed lengthways; they are also thickly studded with
microscopically minute, amorphous, black specks, which are placed in rows,
either standing separately, or more frequently united, two or three or
several together, into black lines, thinner than a hair. When a small
fragment is heated in the blowpipe, the black specks are easily fused into
black brilliant beads, which become magnetic,--characters that apply to no
common mineral except hornblende or augite. With the black specks there are
mingled some others of a red colour, which are magnetic before being
heated, and no doubt are oxide of iron. Round two little cavities, in a
specimen of this variety, I found the black specks aggregated into minute
crystals, appearing like those of augite or hornblende, but too dull and
small to be measured by the goniometer; in the specimen, also, I could
distinguish amidst the crystalline feldspar, grains, which had the aspect
of quartz. By trying with a parallel ruler, I found that the thin grey
layers and the black hair-like lines were absolutely straight and parallel
to each other. It is impossible to trace the gradation from the homogeneous
grey rocks to these striped varieties, or indeed the character of the
different layers in the same specimen, without feeling convinced that the
more or less perfect whiteness of the crystalline feldspathic matter
depends on the more or less perfect aggregation of diffused matter, into
the black and red specks of hornblende and oxide of iron.
FIFTHLY.
A compact heavy rock, not laminated, with an irregular, angular, highly
crystalline, fracture; it abounds with distinct crystals of glassy
feldspar, and the crystalline feldspathic base is mottled with a black
mineral, which on the weathered surface is seen to be aggregated into small
crystals, some perfect, but the greater number imperfect. I showed this
specimen to an experienced geologist, and asked him what it was; he
answered, as I think every one else would have done, that it was a
primitive greenstone. The weathered surface, also, of the banded variety in
Figure 4, strikingly resembles a worn fragment of finely laminated gneiss.
These five varieties, with many intermediate ones, pass and repass into
each other. As the compact varieties are quite subordinate to the others,
the whole may be considered as laminated or striped. The laminae, to sum up
their characteristics, are either quite straight, or slightly tortuous, or
convoluted; they are all parallel to each other, and to the intercalating
strata of obsidian; they are generally of extreme thinness; they consist
either of an apparently homogeneous, compact rock, striped with different
shades of grey and brown colours, or of crystalline feldspathic layers in a
more or less perfect state of purity, and of different thicknesses, with
distinct crystals of glassy feldspar placed lengthways, or of very thin
layers chiefly composed of minute crystals of quartz and augite, or
composed of black and red specks of an augitic mineral and of an oxide of
iron, either not crystallised or imperfectly so. After having fully
described the obsidian, I shall return to the subject of the lamination of
rocks of the trachytic series.
The passage of the foregoing beds into the strata of glassy obsidian is
effected in several ways: first, angulo-modular masses of obsidian, both
large and small, abruptly appear disseminated in a slaty, or in an
amorphous, pale-coloured, feldspathic rock, with a somewhat pearly
fracture. Secondly, small irregular nodules of the obsidian, either
standing separately, or united into thin layers, seldom more than the tenth
of an inch in thickness, alternate repeatedly with very thin layers of a
feldspathic rock, which is striped with the finest parallel zones of
colour, like an agate, and which sometimes passes into the nature of
pitchstone; the interstices between the nodules of obsidian are generally
filled by soft white matter, resembling pumiceous ashes. Thirdly, the whole
substance of the bounding rock suddenly passes into an angulo-concretionary
mass of obsidian. Such masses (as well as the small nodules) of obsidian
are of a pale green colour, and are generally streaked with different
shades of colour, parallel to the laminae of the surrounding rock; they
likewise generally contain minute white sphaerulites, of which half is
sometimes embedded in a zone of one shade of colour, and half in a zone of
another shade. The obsidian assumes its jet black colour and perfectly
conchoidal fracture, only when in large masses; but even in these, on
careful examination and on holding the specimens in different lights, I
could generally distinguish parallel streaks of different shades of
darkness.
(FIGURE 6. OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The upper
ones are externally marked with parallel ridges. The internal radiating
structure of the lower ones, is much too plainly represented.
FIGURE 7. A LAYER FORMED BY THE UNION OF MINUTE BROWN SPHAERULITES,
INTERSECTING TWO OTHER SIMILAR LAYERS: the whole represented of nearly the
natural size.)
One of the commonest transitional rocks deserves in several respects a
further description. It is of a very complicated nature, and consists of
numerous thin, slightly tortuous layers of a pale-coloured feldspathic
stone, often passing into an imperfect pitchstone, alternating with layers
formed of numberless little globules of two varieties of obsidian, and of
two kinds of sphaerulites, embedded in a soft or in a hard pearly base. The
sphaerulites are either white and translucent, or dark brown and opaque;
the former are quite spherical, of small size, and distinctly radiated from
their centre. The dark brown sphaerulites are less perfectly round, and
vary in diameter from the twentieth to the thirtieth of an inch; when
broken they exhibit towards their centres, which are whitish, an obscure
radiating structure; two of them when united sometimes have only one
central point of radiation; there is occasionally a trace of or a hollow
crevice in their centres. They stand either separately, or are united two
or three or many together into irregular groups, or more commonly into
layers, parallel to the stratification of the mass. This union in many
cases is so perfect, that the two sides of the layer thus formed, are quite
even; and these layers, as they become less brown and opaque, cannot be
distinguished from the alternating layers of the pale-coloured feldspathic
stone. The sphaerulites, when not united, are generally compressed in the
plane of the lamination of the mass; and in this same plane, they are often
marked internally, by zones of different shades of colour, and externally
by small ridges and furrows. In the upper part of Figure 6, the
sphaerulites with the parallel ridges and furrows are represented on an
enlarged scale, but they are not well executed; and in the lower part,
their usual manner of grouping is shown. In another specimen, a thin layer
formed of the brown sphaerulites closely united together, intersects, as
represented in Figure 7, a layer of similar composition; and after running
for a short space in a slightly curved line, again intersects it, and
likewise a second layer lying a little way beneath that first intersected.
The small nodules also of obsidian are sometimes externally marked with
ridges and furrows, parallel to the lamination of the mass, but always less
plainly than the sphaerulites. These obsidian nodules are generally
angular, with their edges blunted: they are often impressed with the form
of the adjoining sphaerulites, than which they are always larger; the
separate nodules seldom appear to have drawn each other out by exerting a
mutually attractive force. Had I not found in some cases, a distinct centre
of attraction in these nodules of obsidian, I should have been led to have
considered them as residuary matter, left during the formation of the
pearlstone, in which they are embedded, and of the sphaerulitic globules.
The sphaerulites and the little nodules of obsidian in these rocks so
closely resemble, in general form and structure, concretions in sedimentary
deposits, that one is at once tempted to attribute to them an analogous
origin. They resemble ordinary concretions in the following respects: in
their external form,--in the union of two or three, or of several, into an
irregular mass, or into an even-sided layer,--in the occasional
intersection of one such layer by another, as in the case of chalk-flints,-
-in the presence of two or three kinds of nodules, often close together, in
the same basis,--in their fibrous, radiating structure, with occasional
hollows in their centres,--in the co-existence of a laminary,
concretionary, and radiating structure, as is so well developed in the
concretions of magnesian limestone, described by Professor Sedgwick.
("Geological Transactions" volume 3 part 1 page 37.) Concretions in
sedimentary deposits, it is known, are due to the separation from the
surrounding mass of the whole or part of some mineral substance, and its
aggregation round certain points of attraction. Guided by this fact, I have
endeavoured to discover whether obsidian and the sphaerulites (to which may
be added marekanite and pearlstone, both of them occurring in nodular
concretions in the trachytic series) differ in their constituent parts,
from the minerals generally composing trachytic rocks. It appears from
three analyses, that obsidian contains on an average 76 per cent of silica;
from one analysis, that sphaerulites contain 79.12; from two, that
marekanite contains 79.25; and from two other analyses, that pearlstone
contains 75.62 of silica. (The foregoing analyses are taken from Beudant
"Traite de Mineralogie" tome 2 page 113; and one analysis of obsidian from
Phillips "Mineralogy.") Now, the constituent parts of trachyte, as far as
they can be distinguished consist of feldspar, containing 65.21 of silica;
or of albite, containing 69.09; of hornblende, containing 55.27 (These
analyses are taken from Von Kobell "Grundzuge der Mineralogie" 1838.), and
of oxide of iron: so that the foregoing glassy concretionary substances all
contain a larger proportion of silica than that occurring in ordinary
feldspathic or trachytic rocks. D'Aubuisson ("Traite de Geogn." tome 2 page
535.), also, has remarked on the large proportion of silica compared with
alumina, in six analyses of obsidian and pearlstone given in Brongniart's
"Mineralogy." Hence I conclude, that the foregoing concretions have been
formed by a process of aggregation, strictly analogous to that which takes
place in aqueous deposits, acting chiefly on the silica, but likewise on
some of the other elements of the surrounding mass, and thus producing the
different concretionary varieties. From the well-known effects of rapid
cooling (This is seen in the manufacture of common glass, and in Gregory
Watts's experiments on molten trap; also on the natural surfaces of lava-
streams, and on the side-walls of dikes.) in giving glassiness of texture,
it is probably necessary that the entire mass, in cases like that of
Ascension, should have cooled at a certain rate; but considering the
repeated and complicated alterations of nodules and thin layers of a glassy
texture with other layers quite stony or crystalline, all within the space
of a few feet or even inches, it is hardly possible that they could have
cooled at different rates, and thus have acquired their different textures.
The natural sphaerulites in these rocks very closely resemble those
produced in glass, when slowly cooled. (I do not know whether it is
generally known, that bodies having exactly the same appearance as
sphaerulites, sometimes occur in agates. Mr. Robert Brown showed me in an
agate, formed within a cavity in a piece of silicified wood, some little
specks, which were only just visible to the naked eye: these specks, when
placed by him under a lens of high power, presented a beautiful appearance:
they were perfectly circular, and consisted of the finest fibres of a brown
colour, radiating with great exactness from a common centre. These little
radiating stars are occasionally intersected, and portions are quite cut
off by the fine, ribbon-like zones of colour in the agate. In the obsidian
of Ascension, the halves of a sphaerulite often lie in different zones of
colour, but they are not cut off by them, as in the agate.) In some fine
specimens of partially devitrified glass, in the possession of Mr. Stokes,
the sphaerulites are united into straight layers with even sides, parallel
to each other, and to one of the outer surfaces, exactly as in the
obsidian. These layers sometimes interbranch and form loops; but I did not
see any case of actual intersection. They form the passage from the
perfectly glassy portions, to those nearly homogeneous and stony, with only
an obscure concretionary structure. In the same specimen, also,
sphaerulites differing slightly in colour and in structure, occur embedded
close together. Considering these facts, it is some confirmation of the
view above given of the concretionary origin of the obsidian and natural
sphaerulites, to find that M. Dartigues ("Journal de Physique" tome 59 1804
pages 10, 12.), in his curious paper on this subject, attributes the
production of sphaerulites in glass, to the different ingredients obeying
their own laws of attraction and becoming aggregated. He is led to believe
that this takes place, from the difficulty in remelting sphaerulitic glass,
without the whole be first thoroughly pounded and mixed together; and
likewise from the fact, that the change takes place most readily in glass
composed of many ingredients. In confirmation of M. Dartigues' view, I may
remark, that M. Fleuriau de Bellevue (Idem tome 60 1805 page 418.) found
that the sphaerulitic portions of devitrified glass were acted on both by
nitric acid and under the blowpipe, in a different manner from the compact
paste in which they were embedded.
COMPARISON OF THE OBSIDIAN BEDS AND ALTERNATING STRATA OF ASCENSION, WITH
THOSE OF OTHER COUNTRIES.
I have been struck with much surprise, how closely the excellent
description of the obsidian rocks of Hungary, given by Beudant ("Voyage en
Hongrie" tome 1 page 330; tome 2 pages 221 and 315; tome 3 pages 369, 371,
377, 381.), and that by Humboldt, of the same formation in Mexico and Peru
("Essai Geognostique" pages 176, 326, 328.), and likewise the descriptions
given by several authors (P. Scrope "Geological Transactions" volume 2
second series page 195. Consult also Dolomieu "Voyage aux Isles Lipari" and
D'Aubuisson "Traite de Geogn." tome 2 page 534.) of the trachytic regions
in the Italian islands, agree with my observations at Ascension. Many
passages might have been transferred without alteration from the works of
the above authors, and would have been applicable to this island. They all
agree in the laminated and stratified character of the whole series; and
Humboldt speaks of some of the beds of obsidian being ribboned like jasper.
(In Mr. Stokes' fine collection of obsidians from Mexico, I observe that
the sphaerulites are generally much larger than those of Ascension; they
are generally white, opaque, and are united into distinct layers: there are
many singular varieties, different from any at Ascension. The obsidians are
finely zoned, in quite straight or curved lines, with exceedingly slight
differences of tint, of cellula
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