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 cellularity, and of more or

less perfect degrees of glassiness. Tracing some of the less perfectly

glassy zones, they are seen to become studded with minute white

sphaerulites, which become more and more numerous, until at last they

unite and form a distinct layer: on the other hand, at Ascension, only

the brown sphaerulites unite and form layers; the white ones always

being irregularly disseminated. Some specimens at the Geological

Society, said to belong to an obsidian formation from Mexico, have an

earthy fracture, and are divided in the finest parallel laminae, by

specks of a black mineral, like the augitic or hornblendic specks in

the rocks at Ascension.) They all agree in the nodular or concretionary

character of the obsidian, and of the passage of these nodules into

layers. They all refer to the repeated alterations, often in undulatory

planes, of glassy, pearly, stony, and crystalline layers: the

crystalline layers, however, seem to be much more perfectly developed

at Ascension, than in the above-named countries. Humboldt compares some

of the stony beds, when viewed from a distance, to strata of a

schistose sandstone. Sphaerulites are described as occurring abundantly

in all cases; and they everywhere seem to mark the passage, from the

perfectly glassy to the stony and crystalline beds. Beudant’s account

(Beudant “Voyage” tome 3 page 373.) of his “perlite lithoide

globulaire” in every, even the most trifling particular, might have

been written for the little brown sphaerulitic globules of the rocks of

Ascension.

From the close similarity in so many respects, between the obsidian

formations of Hungary, Mexico, Peru, and of some of the Italian

islands, with that of Ascension, I can hardly doubt that in all these

cases, the obsidian and the sphaerulites owe their origin to a

concretionary aggregation of the silica, and of some of the other

constituent elements, taking place whilst the liquified mass cooled at

a certain required rate. It is, however, well-known, that in several

places, obsidian has flowed in streams like lava; for instance, at

Teneriffe, at the Lipari Islands, and at Iceland. (For Teneriffe see

von Buch “Descript. des Isles Canaries” pages 184 and 190; for the

Lipari Islands see Dolomieu “Voyage” page 34; for Iceland see Mackenzie

“Travels” page 369.) In these cases, the superficial parts are the most

perfectly glassy, the obsidian passing at the depth of a few feet into

an opaque stone. In an analysis by Vauquelin of a specimen of obsidian

from Hecla, which probably flowed as lava, the proportion of silica is

nearly the same as in the nodular or concretionary obsidian from

Mexico. It would be interesting to ascertain, whether the opaque

interior portions and the superficial glassy coating contained the same

proportional constituent parts: we know from M. Dufrenoy (“Memoires

pour servir a une Descript. Geolog. de la France” tome 4 page 371.)

that the exterior and interior parts of the same stream of lava

sometimes differ considerably in their composition. Even should the

whole body of the stream of obsidian turn out to be similarly composed

with nodular obsidian, it would only be necessary, in accordance with

the foregoing facts, to suppose that lava in these instances had been

erupted with its ingredients mixed in the same proportion, as in the

concretionary obsidian.

LAMINATION OF VOLCANIC ROCKS OF THE TRACHYTIC SERIES.

We have seen that, in several and widely distant countries, the strata

alternating with beds of obsidian, are highly laminated. The nodules,

also, both large and small, of the obsidian, are zoned with different

shades of colour; and I have seen a specimen from Mexico in Mr. Stokes’

collection, with its external surface weathered (MacCulloch states

“Classification of Rocks” page 531 that the exposed surfaces of the

pitchstone dikes in Arran are furrowed “with undulating lines,

resembling certain varieties of marbled paper, and which evidently

result from some corresponding difference of laminar structure.”) into

ridges and furrows, corresponding with the zones of different degrees

of glassiness: Humboldt (“Personal Narrative” volume 1 page 222.),

moreover, found on the Peak of Teneriffe, a stream of obsidian divided

by very thin, alternating, layers of pumice. Many other lavas of the

feldspathic series are laminated; thus, masses of common trachyte at

Ascension are divided by fine earthy lines, along which the rock

splits, separating thin layers of slightly different shades of colour;

the greater number, also, of the embedded crystals of glassy feldspar

are placed lengthways in the same direction. Mr. P. Scrope (“Geological

Transactions” volume 2 second series page 195.) has described a

remarkable columnar trachyte in the Panza Islands, which seems to have

been injected into an overlying mass of trachytic conglomerate: it is

striped with zones, often of extreme tenuity, of different textures and

colours; the harder and darker zones appearing to contain a larger

proportion of silica. In another part of the island, there are layers

of pearlstone and pitchstone, which in many respects resemble those of

Ascension. The zones in the columnar trachyte are generally contorted;

they extend uninterruptedly for a great length in a vertical direction,

and apparently parallel to the walls of the dike-like mass. Von Buch

(“Description des Iles Canaries” page 184.) has described at Teneriffe,

a stream of lava containing innumerable thin, plate-like crystals of

feldspar, which are arranged like white threads, one behind the other,

and which mostly follow the same direction. Dolomieu (“Voyage aux Isles

de Lipari” pages 35 and 85.) also states, that the grey lavas of the

modern cone of Vulcano, which have a vitreous texture, are streaked

with parallel white lines: he further describes a solid pumice-stone

which possesses a fissile structure, like that of certain micaceous

schists. Phonolite, which I may observe is often, if not always, an

injected rock, also, often has a fissile structure; this is generally

due to the parallel position of the embedded crystals of feldspar, but

sometimes, as at Fernando Noronha, seems to be nearly independent of

their presence. (In this case, and in that of the fissile pumice-stone,

the structure is very different from that in the foregoing cases, where

the laminae consist of alternate layers of different composition or

texture. In some sedimentary formations, however, which apparently are

homogeneous and fissile, as in glossy clay-slate, there is reason to

believe, according to D’Aubuisson, that the laminae are really due to

excessively thin, alternating, layers of mica.) From these facts we

see, that various rocks of the feldspathic series have either a

laminated or fissile structure, and that it occurs both in masses which

have injected into overlying strata, and in others which have flowed as

streams of lava.

The laminae of the beds, alternating with the obsidian at Ascension,

dip at a high angle under the mountain, at the base of which they are

situated; and they do not appear as if they had been inclined by

violence. A high inclination is common to these beds in Mexico, Peru,

and in some of the Italian islands (See Phillips “Mineralogy” for the

Italian Islands page 136. For Mexico and Peru see Humboldt “Essai

Geognostique.” Mr. Edwards also describes the high inclination of the

obsidian rocks of the Cerro del Navaja in Mexico in the “Proc. of the

Geolog. Soc.” June 1838.): on the other hand, in Hungary, the layers

are horizontal; the laminae, also, of some of the lava-streams above

referred to, as far as I can understand the descriptions given of them,

appear to be highly inclined or vertical. I doubt whether in any of

these cases, the laminae have been tilted into their present position;

and in some instances, as in that of the trachyte described by Mr.

Scrope, it is almost certain that they have been originally formed with

a high inclination. In many of these cases, there is evidence that the

mass of liquified rock has moved in the direction of the laminae. At

Ascension, many of the air-cells have a drawn out appearance, and are

crossed by coarse semi-glassy fibres, in the direction of the laminae;

and some of the layers, separating the sphaerulitic globules, have a

scored appearance, as if produced by the grating of the globules. I

have seen a specimen of zoned obsidian from Mexico, in Mr. Stokes’

collection, with the surfaces of the best-defined layers streaked or

furrowed with parallel lines; and these lines or streaks precisely

resembled those, produced on the surface of a mass of artificial glass

by its having been poured out of a vessel. Humboldt, also, has

described little cavities, which he compares to the tails of comets,

behind sphaerulites in laminated obsidian rocks from Mexico, and Mr.

Scrope has described other cavities behind fragments embedded in his

laminated trachyte, and which he supposes to have been produced during

the movement of the mass. (“Geological Transactions” volume 2 second

series page 200 etc. These embedded fragments, in some instances,

consist of the laminated trachyte broken off and “enveloped in those

parts, which still remained liquid.” Beudant, also, frequently refers

in his great work on “Hungary” tome 3 page 386, to trachytic rocks,

irregularly spotted with fragments of the same varieties, which in

other parts form the parallel ribbons. In these cases, we must suppose,

that after part of the molten mass had assumed a laminated structure, a

fresh irruption of lava broke up the mass, and involved fragments, and

that subsequently the whole became relaminated.) From such facts, most

authors have attributed the lamination of these volcanic rocks to their

movement whilst liquified. Although it is easy to perceive, why each

separate air-cell, or each fibre in pumice-stone (Dolomieu “Voyage”

page 64.), should be drawn out in the direction of the moving mass; it

is by no means at first obvious why such air-cells and fibres should be

arranged by the movement, in the same planes, in laminae absolutely

straight and parallel to each other, and often of extreme tenuity; and

still less obvious is it, why such layers should come to be of slightly

different composition and of different textures.

In endeavouring to make out the cause of the lamination of these

igneous feldspathic rocks, let us return to the facts so minutely

described at Ascension. We there see, that some of the thinnest layers

are chiefly formed by numerous, exceedingly minute, though perfect,

crystals of different minerals; that other layers are formed by the

union of different kinds of concretionary globules, and that the layers

thus formed, often cannot be distinguished from the ordinary

feldspathic and pitchstone layers, composing a large portion of the

entire mass. The fibrous radiating structure of the sphaerulites seems,

judging from many analogous cases, to connect the concretionary and

crystalline forces: the separate crystals, also, of feldspar all lie in

the same parallel planes. (The formation, indeed, of a large crystal of

any mineral in a rock of mixed composition implies an aggregation of

the requisite atoms, allied to concretionary action. The cause of the

crystals of feldspar in these rocks of Ascension, being all placed

lengthways, is probably the same with that which elongates and flattens

all the brown sphaerulitic globules (which behave like feldspar under

the blowpipe) in this same direction.) These allied forces, therefore,

have played an important part in the lamination of the mass, but they

cannot be considered the primary force; for the several kinds of

nodules, both the smallest and largest, are internally zoned with

excessively fine shades of colour, parallel to the lamination of the

whole; and many of them are, also, externally marked in the same

direction with parallel ridges and furrows, which have not been

produced by weathering.

Some of the finest streaks of colour in the stony layers, alternating

with the obsidian, can be distinctly seen to be due to an incipient

crystallisation of the constituent minerals. The extent to which the

minerals have crystallised can, also, be distinctly seen to be

connected with the greater or less size, and with the number, of the

minute, flattened, crenulated air-cavities or fissures. Numerous facts,

as in the case of geodes, and of cavities in silicified wood, in

primary rocks, and in veins, show that crystallisation is much favoured

by space. Hence, I conclude, that, if in a mass of cooling volcanic

rock, any cause produced in parallel planes a number of minute fissures

or zones of less tension (which from the pent-up vapours would often be

expanded into crenulated air-cavities), the crystallisation of the

constituent parts, and probably the formation of concretions, would be

superinduced or much favoured in such planes; and thus, a laminated

structure of the kind we are here considering would be generated.

That some cause does produce parallel zones of less tension in volcanic

rocks, during their consolidation, we must admit in the case of the

thin alternate layers of obsidian and pumice described by Humboldt, and

of the small, flattened, crenulated air-cells in the laminated rocks of

Ascension; for on no other principle can we conceive why the confined

vapours should through their expansion form air-cells or fibres in

separate, parallel planes, instead of irregularly throughout the mass.

In Mr. Stokes’ collection, I have seen a beautiful example of this

structure, in a specimen of obsidian from Mexico, which is shaded and

zoned, like the finest agate, with numerous, straight, parallel layers,

more or less opaque and white, or almost perfectly glassy; the degree

of opacity and glassiness depending on the number of microscopically

minute, flattened air-cells; in this case, it is scarcely possible to

doubt but that the mass, to which the fragment belonged, must have been

subjected to some, probably prolonged, action, causing the tension

slightly to vary in the successive planes.

Several causes appear capable of producing zones of different tension,

in masses semi-liquified by heat. In a fragment of devitrified glass, I

have observed layers of sphaerulites which appeared, from the manner in

which they were abruptly bent, to have been produced by the simple

contraction of the mass in the vessel, in which it cooled. In certain

dikes on Mount Etna, described by M. Elie de Beaumont (“Mem. pour

servir” etc. tome 4 page 131.), as bordered by alternating bands of

scoriaceous and compact rock, one is led to suppose that the stretching

movement of the surrounding strata, which originally produced the

fissures, continued whilst the injected rock remained fluid. Guided,

however, by Professor Forbes’ (“Edinburgh New Phil. Journal” 1842 page

350.) clear description of the zoned structure of glacier-ice, far the

most probable explanation of the laminated structure of these

feldspathic rocks appears to be, that they have been stretched whilst

slowly flowing onwards in a pasty condition (I presume that this is

nearly the same explanation which Mr. Scrope had in his mind, when he

speaks (“Geolog. Transact.” volume 2 second series page 228) of the

ribboned structure of his trachytic rocks, having arisen, from “a

linear extension of the mass, while in a state of imperfect liquidity,

coupled with a concretionary process.”), in precisely the same manner

as Professor Forbes believes, that the ice of moving glaciers is

stretched and fissured. In both cases, the zones may be compared to

those in the finest agates; in both, they extend in the direction in

which the mass has flowed, and those exposed on the surface are

generally vertical: in the ice, the porous laminae are rendered

distinct by the subsequent congelation of infiltrated water, in the

stony feldspathic lavas, by subsequent crystalline and concretionary

action. The fragment of glassy obsidian in Mr. Stokes’ collection,

which is zoned with minute air-cells must strikingly resemble, judging

from Professor Forbes’ descriptions, a fragment of the zoned ice; and

if the rate of cooling and nature of the mass had been favourable to

its crystallisation or to concretionary action, we should here have had

the finest parallel zones of different composition and texture. In

glaciers, the lines of porous ice and of minute crevices seem to be due

to an incipient stretching, caused by the central parts of the frozen

stream moving faster than the sides and bottom, which are retarded by

friction: hence in glaciers of certain forms and towards the lower end

of most glaciers, the zones become horizontal. May we venture to

suppose that in the feldspathic lavas with horizontal laminae, we see

an analogous case? All geologists, who have examined trachytic regions,

have come to the conclusion, that the lavas of this series have

possessed an exceedingly imperfect fluidity; and as it is evident that

only matter thus characterised would be subject to become fissured and

to be formed into zones of different tensions, in the manner here

supposed, we probably see the reason why augitic lavas, which appear

generally to have possessed a high degree of fluidity, are not, like

the feldspathic lavas, divided into laminae of different composition

and texture. (Basaltic lavas, and many other rocks, are not

unfrequently divided into thick laminae or plates, of the same

composition, which are either straight or curved; these being crossed

by vertical lines of fissure, sometimes become united into columns.

This structure seems related, in its origin, to that by which many

rocks, both igneous and sedimentary, become traversed by parallel

systems of fissures.) Moreover, in the augitic series, there never

appears to be any tendency to concretionary action, which we have seen

plays an important part in the lamination of rocks, of the trachytic

series, or at least in rendering that structure apparent.

Whatever may be thought of the explanation here advanced of the

laminated structure of the rocks of the trachytic series, I venture to

call the attention of geologists to the simple fact, that in a body of

rock at Ascension, undoubtedly of volcanic origin, layers often of

extreme tenuity, quite straight, and parallel to each other, have been

produced;—some composed of distinct crystals of quartz and diopside,

mingled with amorphous augitic specks and granular feldspar,—others

entirely composed of these black augitic specks, with granules of oxide

of iron,—and lastly, others formed of crystalline feldspar, in a more

or less perfect state of purity, together with numerous crystals of

feldspar, placed lengthways. At this island, there is reason to

believe, and in some analogous cases, it is certainly known, that the

laminae have originally been formed with their present high

inclination. Facts of this nature are manifestly of importance, with

relation to the structural origin of that grand series of plutonic

rocks, which like the volcanic have undergone the action of heat, and

which consist of alternate layers of quartz, feldspar, mica and other

minerals.