Volcanic Islands
by
Charles Darwin
Part 1 out of 3
This etext was prepared by Sue Asscher
VOLCANIC ISLANDS
BY
CHARLES DARWIN
EDITORIAL NOTE.
Although in some respects more technical in their subjects and style than
Darwin's "Journal," the books here reprinted will never lose their value
and interest for the originality of the observations they contain. Many
parts of them are admirably adapted for giving an insight into problems
regarding the structure and changes of the earth's surface, and in fact
they form a charming introduction to physical geology and physiography in
their application to special domains. The books themselves cannot be
obtained for many times the price of the present volume, and both the
general reader, who desires to know more of Darwin's work, and the student
of geology, who naturally wishes to know how a master mind reasoned on most
important geological subjects, will be glad of the opportunity of
possessing them in a convenient and cheap form.
The three introductions, which my friend Professor Judd has kindly
furnished, give critical and historical information which makes this
edition of special value.
G.T.B.
VOLCANIC ISLANDS.
TABLE OF CONTENTS.
CRITICAL INTRODUCTION.
CHAPTER I.--ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
Rocks of the lowest series.--A calcareous sedimentary deposit, with recent
shells, altered by the contact of superincumbent lava, its horizontality
and extent.--Subsequent volcanic eruptions, associated with calcareous
matter in an earthy and fibrous form, and often enclosed within the
separate cells of the scoriae.--Ancient and obliterated orifices of
eruption of small size.--Difficulty of tracing over a bare plain recent
streams of lava.--Inland hills of more ancient volcanic rock.--Decomposed
olivine in large masses.--Feldspathic rocks beneath the upper crystalline
basaltic strata.--Uniform structure and form of the more ancient volcanic
hills.--Form of the valleys near the coast.--Conglomerate now forming on
the sea beach.
CHAPTER II.--FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.
FERNANDO NORONHA.--Precipitous hill of phonolite.
TERCEIRA.--Trachytic rocks: their singular decomposition by steam of high
temperature.
TAHITI.--Passage from wacke into trap; singular volcanic rock with the
vesicles half-filled with mesotype.
MAURITIUS.--Proofs of its recent elevation.--Structure of its more ancient
mountains; similarity with St. Jago.
ST. PAUL'S ROCKS.--Not of volcanic origin.--Their singular mineralogical
composition.
CHAPTER III.--ASCENSION.
Basaltic lavas.--Numerous craters truncated on the same side.--Singular
structure of volcanic bombs.--Aeriform explosions.--Ejected granite
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.
CHAPTER IV.--ST. HELENA.
Lavas of the feldspathic, basaltic, and submarine series.--Section of
Flagstaff Hill and of the Barn.--Dikes.--Turk's Cap and Prosperous Bays.--
Basaltic ring.--Central crateriform ridge, with an internal ledge and a
parapet.--Cones of phonolite.--Superficial beds of calcareous sandstone.--
Extinct land-shells.--Beds of detritus.--Elevation of the land.--
Denudation.--Craters of elevation.
CHAPTER V.--GALAPAGOS ARCHIPELAGO.
Chatham Island.--Craters composed of a peculiar kind of tuff.--Small
basaltic craters, with hollows at their bases.--Albemarle Island; fluid
lavas, their composition.--Craters of tuff; inclination of their exterior
diverging strata, and structure of their interior converging strata.--James
Island, segment of a small basaltic crater; fluidity and composition of its
lava-streams, and of its ejected fragments.--Concluding remarks on the
craters of tuff, and on the breached condition of their southern sides.--
Mineralogical composition of the rocks of the archipelago.--Elevation of
the land.--Direction of the fissures of eruption.
CHAPTER VI.--TRACHYTE AND BASALT.--DISTRIBUTION OF VOLCANIC ISLES.
The sinking of crystals in fluid lava.--Specific gravity of the constituent
parts of trachyte and of basalt, and their consequent separation.--
Obsidian.--Apparent non-separation of the elements of plutonic rocks.--
Origin of trap-dikes in the plutonic series.--Distribution of volcanic
islands; their prevalence in the great oceans.--They are generally arranged
in lines.--The central volcanoes of Von Buch doubtful.--Volcanic islands
bordering continents.--Antiquity of volcanic islands, and their elevation
in mass.--Eruptions on parallel lines of fissure within the same geological
period.
CHAPTER VII.--AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.
New South Wales.--Sandstone formation.--Embedded pseudo-fragments of
shale.--Stratification.--Current-cleavage.--Great valleys.--Van Diemen's
Land.--Palaeozoic formation.--Newer formation with volcanic rocks.--
Travertin with leaves of extinct plants.--Elevation of the land.--New
Zealand.--King George's Sound.--Superficial ferruginous beds.--Superficial
calcareous deposits, with casts of branches; its origin from drifted
particles of shells and corals.--Their extent.--Cape of Good Hope.--
Junction of the granite and clay-slate.--Sandstone formation.
INDEX.
GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.
CRITICAL INTRODUCTION.
The preparation of the series of works published under the general title
"Geology of the Voyage of the 'Beagle'" occupied a great part of Darwin's
time during the ten years that followed his return to England. The second
volume of the series, entitled "Geological Observations on Volcanic
Islands, with Brief Notices on the Geology of Australia and the Cape of
Good Hope," made its appearance in 1844. The materials for this volume were
collected in part during the outward voyage, when the "Beagle" called at
St. Jago in the Cape de Verde Islands, and St. Paul's Rocks, and at
Fernando Noronha, but mainly during the homeward cruise; then it was that
the Galapagos Islands were surveyed, the Low Archipelago passed through,
and Tahiti visited; after making calls at the Bay of Islands, in New
Zealand, and also at Sydney, Hobart Town and King George's Sound in
Australia, the "Beagle" sailed across the Indian Ocean to the little group
of the Keeling or Cocos Islands, which Darwin has rendered famous by his
observations, and thence to Mauritius; calling at the Cape of Good Hope on
her way, the ship then proceeded successively to St. Helena and Ascension,
and revisited the Cape de Verde Islands before finally reaching England.
Although Darwin was thus able to gratify his curiosity by visits to a great
number of very interesting volcanic districts, the voyage opened for him
with a bitter disappointment. He had been reading Humboldt's "Personal
Narrative" during his last year's residence in Cambridge, and had copied
out from it long passages about Teneriffe. He was actually making inquiries
as to the best means of visiting that island, when the offer was made to
him to accompany Captain Fitzroy in the "Beagle. " His friend Henslow too,
on parting with him, had given him the advice to procure and read the
recently published first volume of the "Principles of Geology," though he
warned him against accepting the views advocated by its author. During the
time the "Beagle" was beating backwards and forwards when the voyage
commenced, Darwin, although hardly ever able to leave his berth, was
employing all the opportunities which the terrible sea-sickness left him,
in studying Humboldt and Lyell. We may therefore form an idea of his
feelings when, on the ship reaching Santa Cruz, and the Peak of Teneriffe
making its appearance among the clouds, they were suddenly informed that an
outbreak of cholera would prevent any landing!
Ample compensation for this disappointment was found, however, when the
ship reached Porta Praya in St. Jago, the largest of the Cape de Verde
Islands. Here he spent three most delightful weeks, and really commenced
his work as a geologist and naturalist. Writing to his father he says,
"Geologising in a volcanic country is most delightful; besides the interest
attached to itself, it leads you into most beautiful and retired spots.
Nobody but a person fond of Natural History can imagine the pleasure of
strolling under cocoa-nuts in a thicket of bananas and coffee-plants, and
an endless number of wild flowers. And this island, that has given me so
much instruction and delight, is reckoned the most uninteresting place that
we perhaps shall touch at during our voyage. It certainly is generally very
barren, but the valleys are more exquisitely beautiful, from the very
contrast. It is utterly useless to say anything about the scenery; it would
be as profitable to explain to a blind man colours, as to a person who has
not been out of Europe, the total dissimilarity of a tropical view.
Whenever I enjoy anything, I always look forward to writing it down, either
in my log-book (which increases in bulk), or in a letter; so you must
excuse raptures, and those raptures badly expressed. I find my collections
are increasing wonderfully, and from Rio I think I shall be obliged to send
a cargo home."
The indelible impression made on Darwin's mind by this first visit to a
volcanic island, is borne witness to by a remarkable passage in the
"Autobiography" written by him in 1876. "The geology of St. Jago is very
striking, yet simple; a stream of lava formerly flowed over the bed of the
sea, formed of triturated recent shells and corals, which it has baked into
a hard white rock. Since then the whole island has been upheaved. But the
line of white rock revealed to me a new and important fact, namely that
there had been afterwards subsidence round the craters which had since been
in action, and had poured forth lava. It then first dawned on me that I
might perhaps write a book on the geology of the various countries visited,
and this made me thrill with delight. That was a memorable hour to me, and
how distinctly I can call to mind the low cliff of lava beneath which I
rested, with the sun glaring hot, a few strange desert plants growing near
and with living corals in the tidal pools at my feet."
Only five years before, when listening to poor Professor Jameson's lectures
on the effete Wernerianism, which at that time did duty for geological
teaching, Darwin had found them "incredibly dull," and he declared that
"the sole effect they produced on me was a determination never so long as I
lived to read a book on Geology, or in any way to study the science."
What a contrast we find in the expressions which he makes use of in
referring to Geological Science, in his letters written home from the
"Beagle!" After alluding to the delight of collecting and studying marine
animals, he exclaims, "But Geology carries the day!" Writing to Henslow he
says, "I am quite charmed with Geology, but, like the wise animal between
two bundles of hay, I do not know which to like best; the old crystalline
group of rocks, or the softer and more fossiliferous beds." And just as the
long voyage is about to come to a close he again writes, "I find in Geology
a never-failing interest; as it has been remarked, it creates the same
grand ideas respecting this world which Astronomy does for the Universe."
In this passage Darwin doubtless refers to a remark of Sir John Herschel's
in his admirable "Preliminary Discourse on the Study of Natural
Philosophy,"--a book which exercised a most remarkable and beneficial
influence on the mind of the young naturalist.
If there cannot be any doubt as to the strong predilection in Darwin's mind
for geological studies, both during and after the memorable voyage, there
is equally little difficulty in perceiving the school of geological thought
which, in spite of the warnings of Sedgwick and Henslow, had obtained
complete ascendancy over his mind. He writes in 1876: "The very first place
which I examined, namely St. Jago in the Cape de Verde Islands, showed me
clearly the wonderful superiority of Lyell's manner of treating Geology,
compared with that of any other author, whose works I had with me, or ever
afterwards read." And again, "The science of Geology is enormously indebted
to Lyell--more so, as I believe, than to any other man who ever lived...I
am proud to remember that the first place, namely, St. Jago, in the Cape de
Verde Archipelago, in which I geologised, convinced me of the infinite
superiority of Lyell's views over those advocated in any other work known
to me."
The passages I have cited will serve to show the spirit in which Darwin
entered upon his geological studies, and the perusal of the following pages
will furnish abundant proofs of the enthusiasm, acumen, and caution with
which his researches were pursued.
Large collections of rocks and minerals were made by Darwin during his
researches, and sent home to Cambridge, to be kept under the care of his
faithful friend Henslow. After visiting his relations and friends, Darwin's
first care on his return to England was to unpack and examine these
collections. He accordingly, at the end of 1836, took lodgings for three
months in Fitzwilliam Street, Cambridge, so as to be near Henslow; and in
studying and determining his geological specimens received much valuable
aid from the eminent crystallographer and mineralogist, Professor William
Hallows Miller.
The actual writing of the volume upon volcanic islands was not commenced
till 1843, when Darwin had settled in the spot which became his home for
the rest of his life--the famous house at Down, in Kent. Writing to his
friend Mr. Fox, on March 28th, 1843, he says, "I am very slowly progressing
with a volume, or rather pamphlet, on the volcanic islands which we
visited: I manage only a couple of hours per day, and that not very
regularly. It is uphill work writing books, which cost money in publishing,
and which are not read even by geologists."
The work occupied Darwin during the whole of the year 1843, and was issued
in the spring of the following year, the actual time engaged in preparing
it being recorded in his diary as "from the summer of 1842 to January
1844;" but the author does not appear to have been by any means satisfied
with the result when the book was finished. He wrote to Lyell, "You have
pleased me much by saying that you intend looking through my 'Volcanic
Islands;' it cost me eighteen months!!! and I have heard of very few who
have read it. Now I shall feel, whatever little (and little it is) there is
confirmatory of old work, or new, will work its effect and not be lost." To
Sir Joseph Hooker he wrote, "I have just finished a little volume on the
volcanic islands which we visited. I do not know how far you care for dry
simple geology, but I hope you will let me send you a copy."
Every geologist knows how full of interest and suggestiveness is this book
of Darwin's on volcanic islands. Probably the scant satisfaction which its
author seemed to find in it may be traced to the effect of a contrast which
he felt between the memory of glowing delights he had experienced when,
hammer in hand, he roamed over new and interesting scenes, and the slow,
laborious, and less congenial task of re-writing and arranging his notes in
book-form.
In 1874, in writing an account of the ancient volcanoes of the Hebrides, I
had frequent occasion to quote Mr. Darwin's observations on the Atlantic
volcanoes, in illustration of the phenomena exhibited by the relics of
still older volcanoes in our own islands. Darwin, in writing to his old
friend Sir Charles Lyell upon the subject, says, "I was not a little
pleased to see my volcanic book quoted, for I thought it was completely
dead and forgotten."
Two years later the original publishers of this book and of that on South
America proposed to re-issue them. Darwin at first hesitated, for he seemed
to think there could be little of abiding interest in them; he consulted me
upon the subject in one of the conversations which I used to have with him
at that time, and I strongly urged upon him the reprint of the works. I was
much gratified when he gave way upon the point, and consented to their
appearing just as originally issued. In his preface he says, "Owing to the
great progress which Geology has made in recent times, my views on some few
points may be somewhat antiquated, but I have thought it best to leave them
as they originally appeared."
It may be interesting to indicate, as briefly as possible, the chief
geological problem upon which the publication of Darwin's "Volcanic
Islands" threw new and important light. The merit of the work consisted in
supplying interesting observations, which in some cases have proved of
crucial value in exploding prevalent fallacies; in calling attention to
phenomena and considerations that had been quite overlooked by geologists,
but have since exercised an important influence in moulding geological
speculation; and lastly in showing the importance which attaches to small
and seemingly insignificant causes, some of which afford a key to the
explanation of very curious geological problems.
Visiting as he did the districts in which Von Buch and others had found
what they thought to be evidence of the truth of "Elevation-craters,"
Darwin was able to show that the facts were capable of a totally different
interpretation. The views originally put forward by the old German
geologist and traveller, and almost universally accepted by his countrymen,
had met with much support from Elie de Beaumont and Dufrenoy, the leaders
of geological thought in France. They were, however, stoutly opposed by
Scrope and Lyell in this country, and by Constant Prevost and Virlet on the
other side of the channel. Darwin, in the work before us, shows how little
ground there is for the assumption that the great ring-craters of the
Atlantic islands have originated in gigantic blisters of the earth's
surface which, opening at the top, have given origin to the craters.
Admitting the influence of the injection of lava into the structure of the
volcanic cones, in increasing their bulk and elevation, he shows that, in
the main, the volcanoes are built up by repeated ejections causing an
accumulation of materials around the vent.
While, however, agreeing on the whole with Scrope and Lyell, as to the
explosive origin of ordinary volcanic craters, Darwin clearly saw that, in
some cases, great craters might be formed or enlarged, by the subsidence of
the floors after eruptions. The importance of this agency, to which too
little attention has been directed by geologists, has recently been shown
by Professor Dana, in his admirable work on Kilauea and the other great
volcanoes of the Hawaiian Archipelago.
The effects of subsidence at a volcanic centre in producing a downward dip
of the strata around it, was first pointed out by Darwin, as the result of
his earliest work in the Cape de Verde Islands. Striking illustrations of
the same principle have since been pointed out by M. Robert and others in
Iceland, by Mr. Heaphy in New Zealand, and by myself in the Western Isles
of Scotland.
Darwin again and again called attention to the evidence that volcanic vents
exhibit relations to one another which can only be explained by assuming
the existence of lines of fissure in the earth's crust, along which the
lavas have made their way to the surface. But he, at the same time, clearly
saw that there was no evidence of the occurrence of great deluges of lava
along such fissures; he showed how the most remarkable plateaux, composed
of successive lava sheets, might be built up by repeated and moderate
ejections from numerous isolated vents; and he expressly insists upon the
rapidity with which the cinder-cones around the orifices of ejection and
the evidences of successive outflows of lava would be obliterated by
denudation.
One of the most striking parts of the book is that in which he deals with
the effects of denudation in producing "basal wrecks" or worn down stumps
of volcanoes. He was enabled to examine a series of cases in which could be
traced every gradation, from perfect volcanic cones down to the solidified
plugs which had consolidated in the vents from which ejections had taken
place. Darwin's observations on these points have been of the greatest
value and assistance to all who have essayed to study the effects of
volcanic action during earlier periods of the earth's history. Like Lyell,
he was firmly persuaded of the continuity of geological history, and ever
delighted in finding indications, in the present order of nature, that the
phenomena of the past could be accounted for by means of causes which are
still in operation. Lyell's last work in the field was carried on about his
home in Forfarshire, and only a few months before his death he wrote to
Darwin: "All the work which I have done has confirmed me in the belief that
the only difference between Palaeozoic and recent volcanic rocks is no more
than we must allow for, by the enormous time to which the products of the
oldest volcanoes have been subjected to chemical changes."
Darwin was greatly impressed, as the result of his studies of volcanic
phenomena, followed by an examination of the great granite-masses of the
Andes, with the relations between the so-called Plutonic rocks and those of
undoubtedly volcanic origin. It was indeed a fortunate circumstance, that
after studying some excellent examples of recent volcanic rocks, he
proceeded to examine in South America many fine illustrations of the older
igneous rock-masses, and especially of the most highly crystalline types of
the same, and then on his way home had opportunities of reviving the
impression made upon him by the fresh and unaltered volcanic rocks. Some of
the general considerations suggested by these observations were discussed
in a paper read by him before the Geological Society, on March 7th, 1838,
under the title "On the Connection of Certain Volcanic Phenomena, and On
the Formation of Mountain-chains, and the Effect of Continental
Elevations." The exact bearing of these two classes of facts upon one
another are more fully discussed in his book on South American geology.
The proofs of recent elevation around many of the volcanic islands led
Darwin to conclude that volcanic areas were, as a rule, regions in which
upward movements were taking place, and he was naturally led to contrast
them with the areas in which, as he showed, the occurrence of atolls,
encircling reefs, and barrier-reefs afford indication of subsidence. In
this way he was able to map out the oceanic areas in different zones, along
which opposite kinds of movement were taking place. His conclusions on this
subject were full of novelty and suggestiveness.
Very clearly did Darwin recognise the importance of the fact that most of
the oceanic islands appear to be of volcanic origin, though he was careful
to point out the remarkable exceptions which somewhat invalidate the
generalisation. In his "Origin of Species" he has elaborated the idea and
suggested the theory of the permanence of ocean-basins, a suggestion which
has been adopted and pushed farther by subsequent authors, than we think
its originator would have approved. His caution and fairness of mind on
this and similar speculative questions was well-known to all who were in
the habit of discussing them with him.
Some years before the voyage of the "Beagle," Mr. Poulett Scrope had
pointed out the remarkable analogies that exist between certain igneous
rocks of banded structure, as seen in the Ponza Islands, and the foliated
crystalline schists. It does not appear that Darwin was acquainted with
this remarkable memoir, but quite independently he called attention to the
same phenomena when he came to study some very similar rocks which occur in
the island of Ascension. Coming fresh from the study of the great masses of
crystalline schist in the South American continent, he was struck by the
circumstance that in the undoubtedly igneous rocks of Ascension we find a
similar separation of the constituent minerals along parallel "folia."
These observations led Darwin to the same conclusion as that arrived at
some time before by Scrope--namely that when crystallisation takes place in
rock masses under the influence of great deforming stresses, a separation
and parallel arrangement of the constituent minerals will result. This is a
process which is now fully recognised as having been a potent factor in the
production of the metamorphic rock, and has been called by more recent
writers "dynamo-metamorphism."
In this, and in many similar discussions, in which exact mineralogical
knowledge was required, it is remarkable how successful Darwin was in
making out the true facts with regard to the rocks he studied by the simple
aid of a penknife and pocket-lens, supplemented by a few chemical tests and
the constant use of the blowpipe. Since his day, the method of study of
rocks by thin sections under the microscope has been devised, and has
become a most efficient aid in all petrographical inquiries. During the
voyage of H.M.S. "Challenger," many of the islands studied by Darwin have
been revisited and their rocks collected. The results of their study by one
of the greatest masters of the science of micropetrography--Professor
Renard of Brussels--have been recently published in one of the volumes of
"Reports on the 'Challenger' Expedition." While much that is new and
valuable has been contributed to geological science by these more recent
investigations, and many changes have been made in nomenclature and other
points of detail, it is interesting to find that all the chief facts
described by Darwin and his friend Professor Miller have stood the test of
time and further study, and remain as a monument of the acumen and accuracy
in minute observation of these pioneers in geological research.
JOHN W. JUDD.
CHAPTER I.--ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
Rocks of the lowest series.
A calcareous sedimentary deposit, with recent shells, altered by the
contact of superincumbent lava, its horizontality and extent.
Subsequent volcanic eruptions, associated with calcareous matter in an
earthy and fibrous form, and often enclosed within the separate cells of
the scoriae.
Ancient and obliterated orifices of eruption of small size.
Difficulty of tracing over a bare plain recent streams of lava.
Inland hills of more ancient volcanic rock.
Decomposed olivine in large masses.
Feldspathic rocks beneath the upper crystalline basaltic strata.
Uniform structure and form of the more ancient volcanic hills.
Form of the valleys near the coast.
Conglomerate now forming on the sea beach.
(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE VERDE ISLANDS.)
The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty
miles in length by about twelve in breadth. My observations, made during
two visits, were confined to the southern portion within the distance of a
few leagues from Porto Praya. The country, viewed from the sea, presents a
varied outline: smooth conical hills of a reddish colour (like Red Hill in
Figure 1 (Map 1). (The outline of the coast, the position of the villages,
streamlets, and of most of the hills in this woodcut, are copied from the
chart made on board H.M.S. "Leven." The square-topped hills (A, B, C, etc.)
are put in merely by eye, to illustrate my description.)), and others less
regular, flat-topped, and of a blackish colour (like A, B, C,) rise from
successive, step-formed plains of lava. At a distance, a chain of
mountains, many thousand feet in height, traverses the interior of the
island. There is no active volcano in St. Jago, and only one in the group,
namely at Fogo. The island since being inhabited has not suffered from
destructive earthquakes.
The lowest rocks exposed on the coast near Porto Praya, are highly
crystalline and compact; they appear to be of ancient, submarine, volcanic
origin; they are unconformably covered by a thin, irregular, calcareous
deposit, abounding with shells of a late tertiary period; and this again is
capped by a wide sheet of basaltic lava, which has flowed in successive
streams from the interior of the island, between the square-topped hills
marked A, B, C, etc. Still more recent streams of lava have been erupted
from the scattered cones, such as Red and Signal Post Hills. The upper
strata of the square-topped hills are intimately related in mineralogical
composition, and in other respects, with the lowest series of the coast-
rocks, with which they seem to be continuous.
MINERALOGICAL DESCRIPTION OF THE ROCKS OF THE LOWEST SERIES.
These rocks possess an extremely varying character; they consist of black,
brown, and grey, compact, basaltic bases, with numerous crystals of augite,
hornblende, olivine, mica, and sometimes glassy feldspar. A common variety
is almost entirely composed of crystals of augite with olivine. Mica, it is
known, seldom occurs where augite abounds; nor probably does the present
case offer a real exception, for the mica (at least in my best
characterised specimen, in which one nodule of this mineral is nearly half
an inch in length) is as perfectly rounded as a pebble in a conglomerate,
and evidently has not been crystallised in the base, in which it is now
enclosed, but has proceeded from the fusion of some pre-existing rock.
These compact lavas alternate with tuffs, amygdaloids, and wacke, and in
some places with coarse conglomerate. Some of the argillaceous wackes are
of a dark green colour, others, pale yellowish-green, and others nearly
white; I was surprised to find that some of the latter varieties, even
where whitest, fused into a jet black enamel, whilst some of the green
varieties afforded only a pale gray bead. Numerous dikes, consisting
chiefly of highly compact augitic rocks, and of gray amygdaloidal
varieties, intersect the strata, which have in several places been
dislocated with considerable violence, and thrown into highly inclined
positions. One line of disturbance crosses the northern end of Quail Island
(an islet in the Bay of Porto Praya), and can be followed to the mainland.
These disturbances took place before the deposition of the recent
sedimentary bed; and the surface, also, had previously been denuded to a
great extent, as is shown by many truncated dikes.
DESCRIPTION OF THE CALCAREOUS DEPOSIT OVERLYING THE FOREGOING VOLCANIC
ROCKS.
This stratum is very conspicuous from its white colour, and from the
extreme regularity with which it ranges in a horizontal line for some miles
along the coast. Its average height above the sea, measured from the upper
line of junction with the superincumbent basaltic lava, is about sixty
feet; and its thickness, although varying much from the inequalities of the
underlying formation, may be estimated at about twenty feet. It consists of
quite white calcareous matter, partly composed of organic debris, and
partly of a substance which may be aptly compared in appearance with
mortar. Fragments of rock and pebbles are scattered throughout this bed,
often forming, especially in the lower part, a conglomerate. Many of the
fragments of rock are whitewashed with a thin coating of calcareous matter.
At Quail Island, the calcareous deposit is replaced in its lowest part by a
soft, brown, earthy tuff, full of Turritellae; this is covered by a bed of
pebbles, passing into sandstone, and mixed with fragments of echini, claws
of crabs, and shells; the oyster-shells still adhering to the rock on which
they grew. Numerous white balls appearing like pisolitic concretions, from
the size of a walnut to that of an apple, are embedded in this deposit;
they usually have a small pebble in their centres. Although so like
concretions, a close examination convinced me that they were Nulliporae,
retaining their proper forms, but with their surfaces slightly abraded:
these bodies (plants as they are now generally considered to be) exhibit
under a microscope of ordinary power, no traces of organisation in their
internal structure. Mr. George R. Sowerby has been so good as to examine
the shells which I collected: there are fourteen species in a sufficiently
perfect condition for their characters to be made out with some degree of
certainty, and four which can be referred only to their genera. Of the
fourteen shells, of which a list is given in the Appendix, eleven are
recent species; one, though undescribed, is perhaps identical with a
species which I found living in the harbour of Porto Praya; the two
remaining species are unknown, and have been described by Mr. Sowerby.
Until the shells of this Archipelago and of the neighbouring coasts are
better known, it would be rash to assert that even these two latter shells
are extinct. The number of species which certainly belong to existing
kinds, although few in number, are sufficient to show that the deposit
belongs to a late tertiary period. From its mineralogical character, from
the number and size of the embedded fragments, and from the abundance of
Patellae, and other littoral shells, it is evident that the whole was
accumulated in a shallow sea, near an ancient coast-line.
EFFECTS PRODUCED BY THE FLOWING OF THE SUPERINCUMBENT BASALTIC LAVA OVER
THE CALCAREOUS DEPOSIT.
These effects are very curious. The calcareous matter is altered to the
depth of about a foot beneath the line of junction; and a most perfect
gradation can be traced, from loosely aggregated, small, particles of
shells, corallines, and Nulliporae, into a rock, in which not a trace of
mechanical origin can be discovered, even with a microscope. Where the
metamorphic change has been greatest, two varieties occur. The first is a
hard, compact, white, fine-grained rock, striped with a few parallel lines
of black volcanic particles, and resembling a sandstone, but which, upon
close examination, is seen to be crystallised throughout, with the
cleavages so perfect that they can be readily measured by the reflecting
goniometer. In specimens, where the change has been less complete, when
moistened and examined under a strong lens, the most interesting gradation
can be traced, some of the rounded particles retaining their proper forms,
and others insensibly melting into the granulo-crystalline paste. The
weathered surface of this stone, as is so frequently the case with ordinary
limestones, assumes a brick-red colour.
The second metamorphosed variety is likewise a hard rock, but without any
crystalline structure. It consists of a white, opaque, compact, calcareous
stone, thickly mottled with rounded, though regular, spots of a soft,
earthy, ochraceous substance. This earthy matter is of a pale yellowish-
brown colour, and appears to be a mixture of carbonate of lime with iron;
it effervesces with acids, is infusible, but blackens under the blowpipe,
and becomes magnetic. The rounded form of the minute patches of earthy
substance, and the steps in the progress of their perfect formation, which
can be followed in a suit of specimens, clearly show that they are due
either to some power of aggregation in the earthy particles amongst
themselves, or more probably to a strong attraction between the atoms of
the carbonate of line, and consequently to the segregation of the earthy
extraneous matter. I was much interested by this fact, because I have often
seen quartz rocks (for instance, in the Falkland Islands, and in the lower
Silurian strata of the Stiper-stones in Shropshire), mottled in a precisely
analogous manner, with little spots of a white, earthy substance (earthy
feldspar?); and these rocks, there was good reason to suppose, had
undergone the action of heat,--a view which thus receives confirmation.
This spotted structure may possibly afford some indication in
distinguishing those formations of quartz, which owe their present
structure to igneous action, from those produced by the agency of water
alone; a source of doubt, which I should think from my own experience, that
most geologists, when examining arenaceo-quartzose districts must have
experienced.
The lowest and most scoriaceous part of the lava, in rolling over the
sedimentary deposit at the bottom of the sea, has caught up large
quantities of calcareous matter, which now forms a snow-white, highly
crystalline basis to a breccia, including small pieces of black, glossy
scoriae. A little above this, where the lime is less abundant, and the lava
more compact, numerous little balls, composed of spicula of calcareous
spar, radiating from common centres, occupy the interstices. In one part of
Quail Island, the lime has thus been crystallised by the heat of the
superincumbent lava, where it is only thirteen feet in thickness; nor had
the lava been originally thicker, and since reduced by degradation, as
could be told from the degree of cellularity of its surface. I have already
observed that the sea must have been shallow in which the calcareous
deposit was accumulated. In this case, therefore, the carbonic acid gas has
been retained under a pressure, insignificant compared with that (a column
of water, 1,708 feet in height) originally supposed by Sir James Hall to be
requisite for this end: but since his experiments, it has been discovered
that pressure has less to do with the retention of carbonic acid gas, than
the nature of the circumjacent atmosphere; and hence, as is stated to be
the case by Mr. Faraday, masses of limestone are sometimes fused and
crystallised even in common limekilns. (I am much indebted to Mr. E.W.
Brayley in having given me the following references to papers on this
subject: Faraday in the "Edinburgh New Philosophical Journal" volume 15
page 398; Gay-Lussac in "Annales de Chem. et Phys." tome 63 page 219
translated in the "London and Edinburgh Philosophical Magazine" volume 10
page 496.) Carbonate of lime can be heated to almost any degree, according
to Faraday, in an atmosphere of carbonic acid gas, without being
decomposed; and Gay-Lussac found that fragments of limestone, placed in a
tube and heated to a degree, not sufficient by itself to cause their
decomposition, yet immediately evolved their carbonic acid, when a stream
of common air or steam was passed over them: Gay-Lussac attributes this to
the mechanical displacement of the nascent carbonic acid gas. The
calcareous matter beneath the lava, and especially that forming the
crystalline spicula between the interstices of the scoriae, although heated
in an atmosphere probably composed chiefly of steam, could not have been
subjected to the effects of a passing stream; and hence it is, perhaps,
that they have retained their carbonic acid, under a small amount of
pressure.
The fragments of scoriae, embedded in the crystalline calcareous basis, are
of a jet black colour, with a glossy fracture like pitchstone. Their
surfaces, however, are coated with a layer of a reddish-orange, translucent
substance, which can easily be scratched with a knife; hence they appear as
if overlaid by a thin layer of rosin. Some of the smaller fragments are
partially changed throughout into this substance: a change which appears
quite different from ordinary decomposition. At the Galapagos Archipelago
(as will be described in a future chapter), great beds are formed of
volcanic ashes and particles of scoriae, which have undergone a closely
similar change.
THE EXTENT AND HORIZONTALITY OF THE CALCAREOUS STRATUM.
(FIGURE 2: SIGNAL POST HILL. (Section with A low and C high.)
A.--Ancient volcanic rocks.
B.--Calcareous stratum.
C.--Upper basaltic lava.)
The upper line of surface of the calcareous stratum, which is so
conspicuous from being quite white and so nearly horizontal, ranges for
miles along the coast, at the height of about sixty feet above the sea. The
sheet of basalt, by which it is capped, is on an average eighty feet in
thickness. Westward of Porto Praya beyond Red Hill, the white stratum with
the superincumbent basalt is covered up by more recent streams. Northward
of Signal Post Hill, I could follow it with my eye, trending away for
several miles along the sea cliffs. The distance thus observed is about
seven miles; but I cannot doubt from its regularity that it extends much
farther. In some ravines at right angles to the coast, it is seen gently
dipping towards the sea, probably with the same inclination as when
deposited round the ancient shores of the island. I found only one inland
section, namely, at the base of the hill marked A, where, at the height of
some hundred feet, this bed was exposed; it here rested on the usual
compact augitic rock associated with wacke, and was covered by the
widespread sheet of modern basaltic lava. Some exceptions occur to the
horizontality of the white stratum: at Quail Island, its upper surface is
only forty feet above the level of the sea; here also the capping of lava
is only between twelve and fifteen feet in thickness; on the other hand, at
the north-east side of Porto Praya harbour, the calcareous stratum, as well
as the rock on which it rests, attain a height above the average level: the
inequality of level in these two cases is not, as I believe, owing to
unequal elevation, but to original irregularities at the bottom of the sea.
Of this fact, at Quail Island, there was clear evidence in the calcareous
deposit being in one part of much greater than the average thickness, and
in another part being entirely absent; in this latter case, the modern
basaltic lavas rested directly on those of more ancient origin.
Under Signal Post Hill, the white stratum dips into the sea in a remarkable
manner. This hill is conical, 450 feet in height, and retains some traces
of having had a crateriform structure; it is composed chiefly of matter
erupted posteriorly to the elevation of the great basaltic plain, but
partly of lava of apparently submarine origin and of considerable
antiquity. The surrounding plain, as well as the eastern flank of this
hill, has been worn into steep precipices, overhanging the sea. In these
precipices, the white calcareous stratum may be seen, at the height of
about seventy feet above the beach, running for some miles both northward
and southward of the hill, in a line appearing to be perfectly horizontal;
but for a space of a quarter of a mile directly under the hill, it dips
into the sea and disappears. On the south side the dip is gradual, on the
north side it is more abrupt, as is shown in Figure 2. As neither the
calcareous stratum, nor the superincumbent basaltic lava (as far as the
latter can be distinguished from the more modern ejections), appears to
thicken as it dips, I infer that these strata were not originally
accumulated in a trough, the centre of which afterwards became a point of
eruption; but that they have subsequently been disturbed and bent. We may
suppose either that Signal Post Hill subsided after its elevation with the
surrounding country, or that it never was uplifted to the same height with
it. This latter seems to me the most probable alternative, for during the
slow and equable elevation of this portion of the island, the subterranean
motive power, from expending part of its force in repeatedly erupting
volcanic matter from beneath this point, would, it is likely, have less
force to uplift it. Something of the same kind seems to have occurred near
Red Hill, for when tracing upwards the naked streams of lava from near
Porto Praya towards the interior of the island, I was strongly induced to
suspect, that since the lava had flowed, the slope of the land had been
slightly modified, either by a small subsidence near Red Hill, or by that
portion of the plain having been uplifted to a less height during the
elevation of the whole area.
THE BASALTIC LAVA, SUPERINCUMBENT ON THE CALCAREOUS DEPOSIT.
This lava is of a pale grey colour, fusing into a black enamel; its
fracture is rather earthy and concretionary; it contains olivine in small
grains. The central parts of the mass are compact, or at most crenulated
with a few minute cavities, and are often columnar. At Quail Island this
structure was assumed in a striking manner; the lava in one part being
divided into horizontal laminae, which became in another part split by
vertical fissures into five-sided plates; and these again, being piled on
each other, insensibly became soldered together, forming fine symmetrical
columns. The lower surface of the lava is vesicular, but sometimes only to
the thickness of a few inches; the upper surface, which is likewise
vesicular, is divided into balls, frequently as much as three feet in
diameter, made up of concentric layers. The mass is composed of more than
one stream; its total thickness being, on an average, about eighty feet:
the lower portion has certainly flowed beneath the sea, and probably
likewise the upper portion. The chief part of this lava has flowed from the
central districts, between the hills marked A, B, C, etc., in the woodcut-
map. The surface of the country, near the coast, is level and barren;
towards the interior, the land rises by successive terraces, of which four,
when viewed from a distance, could be distinctly counted.
VOLCANIC ERUPTIONS SUBSEQUENT TO THE ELEVATION OF THE COASTLAND; THE
EJECTED MATTER ASSOCIATED WITH EARTHY LIME.
These recent lavas have proceeded from those scattered, conical, reddish-
coloured hills, which rise abruptly from the plain-country near the coast.
I ascended some of them, but will describe only one, namely, RED HILL,
which may serve as a type of its class, and is remarkable in some especial
respects. Its height is about six hundred feet; it is composed of bright
red, highly scoriaceous rock of a basaltic nature; on one side of its
summit there is a hollow, probably the last remnant of a crater. Several of
the other hills of this class, judging from their external forms, are
surmounted by much more perfect craters. When sailing along the coast, it
was evident that a considerable body of lava had flowed from Red Hill, over
a line of cliff about one hundred and twenty feet in height, into the sea:
this line of cliff is continuous with that forming the coast, and bounding
the plain on both sides of this hill; these streams, therefore, were
erupted, after the formation of the coast-cliffs, from Red Hill, when it
must have stood, as it now does, above the level of the sea. This
conclusion accords with the highly scoriaceous condition of all the rock on
it, appearing to be of subaerial formation: and this is important, as there
are some beds of calcareous matter near its summit, which might, at a hasty
glance, have been mistaken for a submarine deposit. These beds consist of
white, earthy, carbonate of lime, extremely friable so as to be crushed
with the least pressure; the most compact specimens not resisting the
strength of the fingers. Some of the masses are as white as quicklime, and
appear absolutely pure; but on examining them with a lens, minute particles
of scoriae can always be seen, and I could find none which, when dissolved
in acids, did not leave a residue of this nature. It is, moreover,
difficult to find a particle of the lime which does not change colour under
the blowpipe, most of them even becoming glazed. The scoriaceous fragments
and the calcareous matter are associated in the most irregular manner,
sometimes in obscure beds, but more generally as a confused breccia, the
lime in some parts and the scoriae in others being most abundant. Sir H. De
la Beche has been so kind as to have some of the purest specimens analysed,
with a view to discover, considering their volcanic origin, whether they
contained much magnesia; but only a small portion was found, such as is
present in most limestones.
Fragments of the scoriae embedded in the calcareous mass, when broken,
exhibit many of their cells lined and partly filled with a white, delicate,
excessively fragile, moss-like, or rather conferva-like, reticulation of
carbonate of lime. These fibres, examined under a lens of one-tenth of an
inch focal distance, appear cylindrical; they are rather above one-
thousandth of an inch in diameter; they are either simply branched, or more
commonly united into an irregular mass of network, with the meshes of very
unequal sizes and of unequal numbers of sides. Some of the fibres are
thickly covered with extremely minute spicula, occasionally aggregated into
little tuffs; and hence they have a hairy appearance. These spicula are of
the same diameter throughout their length; they are easily detached, so
that the object-glass of the microscope soon becomes scattered over with
them. Within the cells of many fragments of the scoria, the lime exhibits
this fibrous structure, but generally in a less perfect degree. These cells
do not appear to be connected with one another. There can be no doubt, as
will presently be shown, that the lime was erupted, mingled with the lava
in its fluid state, and therefore I have thought it worth while to describe
minutely this curious fibrous structure, of which I know nothing analogous.
From the earthy condition of the fibres, this structure does not appear to
be related to crystallisation.
Other fragments of the scoriaceous rock from this hill, when broken, are
often seen marked with short and irregular white streaks, which are owing
to a row of separate cells being partly, or quite, filled with white
calcareous powder. This structure immediately reminded me of the appearance
in badly kneaded dough, of balls and drawn-out streaks of flour, which have
remained unmixed with the paste; and I cannot doubt that small masses of
the lime, in the same manner remaining unmixed with the fluid lava, have
been drawn out when the whole was in motion. I carefully examined, by
trituration and solution in acids, pieces of the scoriae, taken from within
half-an-inch of those cells which were filled with the calcareous powder,
and they did not contain an atom of free lime. It is obvious that the lava
and lime have on a large scale been very imperfectly mingled; and where
small portions of the lime have been entangled within a piece of the viscid
lava, the cause of their now occupying, in the form of a powder or of a
fibrous reticulation, the vesicular cavities, is, I think, evidently due to
the confined gases having most readily expanded at the points where the
incoherent lime rendered the lava less adhesive.
A mile eastward of the town of Praya, there is a steep-sided gorge, about
one hundred and fifty yards in width, cutting through the basaltic plain
and underlying beds, but since filled up by a stream of more modern lava.
This lava is dark grey, and in most parts compact and rudely columnar; but
at a little distance from the coast, it includes in an irregular manner a
brecciated mass of red scoriae mingled with a considerable quantity of
white, friable, and in some parts, nearly pure earthy lime, like that on
the summit of Red Hill. This lava, with its entangled lime, has certainly
flowed in the form of a regular stream; and, judging from the shape of the
gorge, towards which the drainage of the country (feeble though it now be)
still is directed, and from the appearance of the bed of loose water-worn
blocks with their interstices unfilled, like those in the bed of a torrent,
on which the lava rests, we may conclude that the stream was of subaerial
origin. I was unable to trace it to its source, but, from its direction, it
seemed to have come from Signal Post Hill, distant one mile and a quarter,
which, like Red Hill, has been a point of eruption subsequent to the
elevation of the great basaltic plain. It accords with this view, that I
found on Signal Post Hill, a mass of earthy, calcareous matter of the same
nature, mingled with scoriae. I may here observe that part of the
calcareous matter forming the horizontal sedimentary bed, especially the
finer matter with which the embedded fragments of rock are whitewashed, has
probably been derived from similar volcanic eruptions, as well as from
triturated organic remains: the underlying, ancient, crystalline rocks,
also, are associated with much carbonate of lime, filling amygdaloidal
cavities, and forming irregular masses, the nature of which latter I was
unable to understand.
Considering the abundance of earthy lime near the summit of Red Hill, a
volcanic cone six hundred feet in height, of subaerial growth,--considering
the intimate manner in which minute particles and large masses of scoriae
are embedded in the masses of nearly pure lime, and on the other hand, the
manner in which small kernels and streaks of the calcareous powder are
included in solid pieces of the scoriae,--considering, also, the similar
occurrence of lime and scoriae within a stream of lava, also supposed, with
good reason, to have been of modern subaerial origin, and to have flowed
from a hill, where earthy lime also occurs: I think, considering these
facts, there can be no doubt that the lime has been erupted, mingled with
the molten lava. I am not aware that any similar case has been described:
it appears to me an interesting one, inasmuch as most geologists must have
speculated on the probable effects of a volcanic focus, bursting through
deep-seated beds of different mineralogical composition. The great
abundance of free silex in the trachytes of some countries (as described by
Beudant in Hungary, and by P. Scrope in the Panza Islands), perhaps solves
the inquiry with respect to deep-seated beds of quartz; and we probably
here see it answered, where the volcanic action has invaded subjacent
masses of limestone. One is naturally led to conjecture in what state the
now earthy carbonate of lime existed, when ejected with the intensely
heated lava: from the extreme cellularity of the scoriae on Red Hill, the
pressure cannot have been great, and as most volcanic eruptions are
accompanied by the emission of large quantities of steam and other gases,
we here have the most favourable conditions, according to the views at
present entertained by chemists, for the expulsion of the carbonic acid.
(Whilst deep beneath the surface, the carbonate of lime was, I presume, in
a fluid state. Hutton, it is known, thought that all amygdaloids were
produced by drops of molten limestone floating in the trap, like oil in
water: this no doubt is erroneous, but if the matter forming the summit of
Red Hill had been cooled under the pressure of a moderately deep sea, or
within the walls of a dike, we should, in all probability, have had a trap
rock associated with large masses of compact, crystalline, calcareous spar,
which, according to the views entertained by many geologists, would have
been wrongly attributed to subsequent infiltration.) Has the slow re-
absorption of this gas, it may be asked, given to the lime in the cells of
the lava, that peculiar fibrous structure, like that of an efflorescing
salt? Finally, I may remark on the great contrast in appearance between
this earthy lime, which must have been heated in a free atmosphere of steam
and other gases, while the white, crystalline, calcareous spar, produced by
a single thin sheet of lava (as at Quail Island) rolling over similar
earthy lime and the debris of organic remains, at the bottom of a shallow
sea.
SIGNAL POST HILL.
This hill has already been several times mentioned, especially with
reference to the remarkable manner in which the white calcareous stratum,
in other parts so horizontal (Figure 2), dips under it into the sea. It has
a broad summit, with obscure traces of a crateriform structure, and is
composed of basaltic rocks (Of these, one common variety is remarkable for
being full of small fragments of a dark jasper-red earthy mineral, which,
when examined carefully, shows an indistinct cleavage; the little fragments
are elongated in form, are soft, are magnetic before and after being
heated, and fuse with difficulty into a dull enamel. This mineral is
evidently closely related to the oxides of iron, but I cannot ascertain
what it exactly is. The rock containing this mineral is crenulated with
small angular cavities, which are lined and filled with yellowish crystals
of carbonate of lime.), some compact, others highly cellular with inclined
beds of loose scoriae, of which some are associated with earthy lime. Like
Red Hill, it has been the source of eruptions, subsequently to the
elevation of the surrounding basaltic plain; but unlike that hill, it has
undergone considerable denudation, and has been the seat of volcanic action
at a remote period, when beneath the sea. I judge of this latter
circumstance from finding on its inland flank the last remains of three
small points of eruption. These points are composed of glossy scoriae,
cemented by crystalline calcareous spar, exactly like the great submarine
calcareous deposit, where the heated lava has rolled over it: their
demolished state can, I think, be explained only by the denuding action of
the waves of the sea. I was guided to the first orifice by observing a
sheet of lava, about two hundred yards square, with steepish sides,
superimposed on the basaltic plain with no adjoining hillock, whence it
could have been erupted; and the only trace of a crater which I was able to
discover, consisted of some inclined beds of scoriae at one of its corners.
At the distance of fifty yards from a second level-topped patch of lava,
but of much smaller size, I found an irregular circular group of masses of
cemented, scoriaceous breccia, about six feet in height, which doubtless
had once formed the point of eruption. The third orifice is now marked only
by an irregular circle of cemented scoriae, about four yards in diameter,
and rising in its highest point scarcely three feet above the level of the
plain, the surface of which, close all round, exhibits its usual
appearance: here we have a horizontal basal section of a volcanic spiracle,
which, together with all its ejected matter, has been almost totally
obliterated.
The stream of lava, which fills the narrow gorge eastward of the town of
Praya, judging from its course, seems, as before remarked, to have come
from Signal Post Hill, and to have flowed over the plain, after its
elevation (The sides of this gorge, where the upper basaltic stratum is
intersected, are almost perpendicular. The lava, which has since filled it
up, is attached to these sides, almost as firmly as a dike is to its walls.
In most cases, where a stream of lava has flowed down a valley, it is
bounded on each side by loose scoriaceous masses.): the same observation
applies to a stream (possibly part of the same one) capping the sea cliffs,
a little eastward of the gorge. When I endeavoured to follow these streams
over the stony level plain, which is almost destitute of soil and
vegetation, I was much surprised to find, that although composed of hard
basaltic matter, and not having been exposed to marine denudation, all
distant traces of them soon became utterly lost. But I have since observed
at the Galapagos Archipelago, that it is often impossible to follow even
great deluges of quite recent lava across older streams, except by the size
of the bushes growing on them, or by the comparative states of glossiness
of their surfaces,--characters which a short lapse of time would be
sufficient quite to obscure. I may remark, that in a level country, with a
dry climate, and with the wind blowing always in one direction (as at the
Cape de Verde Archipelago), the effects of atmospheric degradation are
probably much greater than would at first be expected; for soil in this
case accumulates only in a few protected hollows, and being blown in one
direction, it is always travelling towards the sea in the form of the
finest dust, leaving the surface of the rocks bare, and exposed to the full
effects of renewed meteoric action.
INLAND HILLS OF MORE ANCIENT VOLCANIC ROCKS.
These hills are laid down by eye, and marked as A, B, C, etc., in Map 1.
They are related in mineralogical composition, and are probably directly
continuous with the lowest rocks exposed on the coast. These hills, viewed
from a distance, appear as if they had once formed part of an irregular
tableland, and from their corresponding structure and composition this
probably has been the case. They have flat, slightly inclined summits, and
are, on an average, about six hundred feet in height; they present their
steepest slope towards the interior of the island, from which point they
radiate outwards, and are separated from each other by broad and deep
valleys, through which the great streams of lava, forming the coast-plains,
have descended. Their inner and steeper escarpments are ranged in an
irregular curve, which rudely follows the line of the shore, two or three
miles inland from it. I ascended a few of these hills, and from others,
which I was able to examine with a telescope, I obtained specimens, through
the kindness of Mr. Kent, the assistant-surgeon of the "Beagle"; although
by these means I am acquainted with only a part of the range, five or six
miles in length, yet I scarcely hesitate, from their uniform structure, to
affirm that they are parts of one great formation, stretching round much of
the circumference of the island.
The upper and lower strata of these hills differ greatly in composition.
The upper are basaltic, generally compact, but sometimes scoriaceous and
amygdaloidal, with associated masses of wacke: where the basalt is compact,
it is either fine-grained or very coarsely crystallised; in the latter case
it passes into an augitic rock, containing much olivine; the olivine is
either colourless, or of the usual yellow and dull reddish shades. On some
of the hills, beds of calcareous matter, both in an earthy and in a
crystalline form, including fragments of glossy scoriae, are associated
with the basaltic strata. These strata differ from the streams of basaltic
lava forming the coast-plains, only in being more compact, and in the
crystals of augite, and in the grains of olivine being of much greater
size;--characters which, together with the appearance of the associated
calcareous beds, induce me to believe that they are of submarine formation.
Some considerable masses of wacke, which are associated with these basaltic
strata, and which likewise occur in the basal series on the coast,
especially at Quail Island, are curious. They consist of a pale yellowish-
green argillaceous substance, of a crumbling texture when dry, but unctuous
when moist: in its purest form, it is of a beautiful green tint, with
translucent edges, and occasionally with obscure traces of an original
cleavage. Under the blowpipe it fuses very readily into a dark grey, and
sometimes even black bead, which is slightly magnetic. From these
characters, I naturally thought that it was one of the pale species,
decomposed, of the genus augite;--a conclusion supported by the unaltered
rock being full of large separate crystals of black augite, and of balls
and irregular streaks of dark grey augitic rock. As the basalt ordinarily
consists of augite, and of olivine often tarnished and of a dull red
colour, I was led to examine the stages of decomposition of this latter
mineral, and I found, to my surprise, that I could trace a nearly perfect
gradation from unaltered olivine to the green wacke. Part of the same grain
under the blowpipe would in some instances behave like olivine, its colour
being only slightly changed, and part would give a black magnetic bead.
Hence I can have no doubt that the greenish wacke originally existed as
olivine; but great chemical changes must have been effected during the act
of decomposition thus to have altered a very hard, transparent, infusible
mineral, into a soft, unctuous, easily melted, argillaceous substance.
(D'Aubuisson "Traite de Geognosie" tome 2 page 569 mentions, on the
authority of M. Marcel de Serres, masses of green earth near Montpellier,
which are supposed to be due to the decomposition of olivine. I do not,
however, find, that the action of this mineral under the blowpipe being
entirely altered, as it becomes decomposed, has been noticed; and the
knowledge of this fact is important, as at first it appears highly
improbable that a hard, transparent, refractory mineral should be changed
into a soft, easily fused clay, like this of St. Jago. I shall hereafter
describe a green substance, forming threads within the cells of some
vesicular basaltic rocks in Van Diemen's Land, which behave under the
blowpipe like the green wacke of St. Jago; but its occurrence in
cylindrical threads, shows it cannot have resulted from the decomposition
of olivine, a mineral always existing in the form of grains or crystals.)
The basal strata of these hills, as well as some neighbouring, separate,
bare, rounded hillocks, consist of compact, fine-grained, non-crystalline
(or so slightly as scarcely to be perceptible), ferruginous, feldspathic
rocks, and generally in a state of semi-decomposition. Their fracture is
exceedingly irregular, and splintery; yet small fragments are often very
tough. They contain much ferruginous matter, either in the form of minute
grains with a metallic lustre, or of brown hair-like threads: the rock in
this latter case assuming a pseudo-brecciated structure. These rocks
sometimes contain mica and veins of agate. Their rusty brown or yellowish
colour is partly due to the oxides of iron, but chiefly to innumerable,
microscopically minute, black specks, which, when a fragment is heated, are
easily fused, and evidently are either hornblende or augite. These rocks,
therefore, although at first appearing like baked clay or some altered
sedimentary deposit, contain all the essential ingredients of trachyte;
from which they differ only in not being harsh, and in not containing
crystals of glassy feldspar. As is so often the case with trachytic
formation, no stratification is here apparent. A person would not readily
believe that these rocks could have flowed as lava; yet at St. Helena there
are well-characterised streams (as will be described in an ensuing chapter)
of nearly similar composition. Amidst the hillocks composed of these rocks,
I found in three places, smooth conical hills of phonolite, abounding with
fine crystals of glassy feldspar, and with needles of hornblende. These
cones of phonolite, I believe, bear the same relation to the surrounding
feldspathic strata which some masses of coarsely crystallised augitic rock,
in another part of the island, bear to the surrounding basalt, namely, that
both have been injected. The rocks of a feldspathic nature being anterior
in origin to the basaltic strata, which cap them, as well as to the
basaltic streams of the coast-plains, accords with the usual order of
succession of these two grand divisions of the volcanic series.
The strata of most of these hills in the upper part, where alone the planes
of division are distinguishable, are inclined at a small angle from the
interior of the island towards the sea-coast. The inclination is not the
same in each hill; in that marked A it is less than in B, D, or E; in C the
strata are scarcely deflected from a horizontal plane, and in F (as far as
I could judge without ascending it) they are slightly inclined in a reverse
direction, that is, inwards and towards the centre of the island.
Notwithstanding these differences of inclination, their correspondence in
external form, and in the composition both of their upper and lower parts,-
-their relative position in one curved line, with their steepest sides
turned inwards,--all seem to show that they originally formed parts of one
platform; which platform, as before remarked, probably extended round a
considerable portion of the circumference of the island. The upper strata
certainly flowed as lava, and probably beneath the sea, as perhaps did the
lower feldspathic masses: how then come these strata to hold their present
position, and whence were they erupted?
In the centre of the island there are lofty mountains, but they are
separated from the steep inland flanks of these hills by a wide space of
lower country: the interior mountains, moreover, seem to have been the
source of those great streams of basaltic lava which, contracting as they
pass between the bases of the hills in question, expand into the coast-
plains. (I saw very little of the inland parts of the island. Near the
village of St. Domingo, there are magnificent cliffs of rather coarsely
crystallised basaltic lava. Following the little stream in this valley,
about a mile above the village, the base of the great cliff was formed of a
compact fine-grained basalt, conformably covered by a bed of pebbles. Near
Fuentes, I met with pap-formed hills of the compact feldspathic series of
rocks.) Round the shores of St. Helena there is a rudely formed ring of
basaltic rocks, and at Mauritius there are remnants of another such a ring
round part, if not round the whole, of the island; here again the same
question immediately occurs, how came these masses to hold their present
position, and whence were they erupted? The same answer, whatever it may
be, probably applies in these three cases; and in a future chapter we shall
recur to this subject.
VALLEYS NEAR THE COAST.
These are broad, very flat, and generally bounded by low cliff-formed
sides. Portions of the basaltic plain are sometimes nearly or quite
isolated by them; of which fact, the space on which the town of Praya
stands offers an instance. The great valley west of the town has its bottom
filled up to a depth of more than twenty feet by well-rounded pebbles,
which in some parts are firmly cemented together by white calcareous
matter. There can be no doubt, from the form of these valleys, that they
were scooped out by the waves of the sea, during that equable elevation of
the land, of which the horizontal calcareous deposit, with its existing
species of marine remains, gives evidence. Considering how well shells have
been preserved in this stratum, it is singular that I could not find even a
single small fragment of shell in the conglomerate at the bottom of the
valleys. The bed of pebbles in the valley west of the town is intersected
by a second valley joining it as a tributary, but even this valley appears
much too wide and flat-bottomed to have been formed by the small quantity
of water, which falls only during one short wet season; for at other times
of the year these valleys are absolutely dry.
RECENT CONGLOMERATE.
On the shores of Quail Island, I found fragments of brick, bolts of iron,
pebbles, and large fragments of basalt, united by a scanty base of impure
calcareous matter into a firm conglomerate. To show how exceedingly firm
this recent conglomerate is, I may mention, that I endeavoured with a heavy
geological hammer to knock out a thick bolt of iron, which was embedded a
little above low-water mark, but was quite unable to succeed.
CHAPTER II.--FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.
FERNANDO NORONHA.
Precipitous hill of phonolite.
TERCEIRA.
Trachytic rocks: their singular decomposition by steam of high temperature.
TAHITI.
Passage from wacke into trap; singular volcanic rock with the vesicles
half-filled with mesotype.
MAURITIUS.
Proofs of its recent elevation.
Structure of its more ancient mountains; similarity with St. Jago.
ST. PAUL'S ROCKS.
Not of volcanic origin.
Their singular mineralogical composition.
FERNANDO NORONHA.
During our short visit at this and the four following islands, I observed
very little worthy of description. Fernando Noronha is situated in the
Atlantic Ocean, in latitude 3 degrees 50 minutes S., and 230 miles distant
from the coast of South America. It consists of several islets, together
nine miles in length by three in breadth. The whole seems to be of volcanic
origin; although there is no appearance of any crater, or of any one
central eminence. The most remarkable feature is a hill 1,000 feet high, of
which the upper 400 feet consist of a precipitous, singularly shaped
pinnacle, formed of columnar phonolite, containing numerous crystals of
glassy feldspar, and a few needles of hornblende. From the highest
accessible point of this hill, I could distinguish in different parts of
the group several other conical hills, apparently of the same nature. At
St. Helena there are similar, great, conical, protuberant masses of
phonolite, nearly one thousand feet in height, which have been formed by
the injection of fluid feldspathic lava into yielding strata. If this hill
has had, as is probable, a similar origin, denudation has been here
effected on an enormous scale. Near the base of this hill, I observed beds
of white tuff, intersected by numerous dikes, some of amygdaloidal basalt
and others of trachyte; and beds of slaty phonolite with the planes of
cleavage directed N.W. and S.E. Parts of this rock, where the crystals were
scanty, closely resembled common clay-slate, altered by the contact of a
trap-dike. The lamination of rocks, which undoubtedly have once been fluid,
appears to me a subject well deserving attention. On the beach there were
numerous fragments of compact basalt, of which rock a distant facade of
columns seemed to be formed.
TERCEIRA IN THE AZORES.
The central parts of this island consist of irregularly rounded mountains
of no great elevation, composed of trachyte, which closely resembles in
general character the trachyte of Ascension, presently to be described.
This formation is in many parts overlaid, in the usual order of
superposition, by streams of basaltic lava, which near the coast compose
nearly the whole surface. The course which these streams have followed from
their craters, can often be followed by the eye. The town of Angra is
overlooked by a crateriform hill (Mount Brazil), entirely built of thin
strata of fine-grained, harsh, brown-coloured tuff. The upper beds are seen
to overlap the basaltic streams on which the town stands. This hill is
almost identical in structure and composition with numerous crateriformed
hills in the Galapagos Archipelago.
EFFECTS OF STEAM ON THE TRACHYTIC ROCKS.
In the central part of the island there is a spot, where steam is
constantly issuing in jets from the bottom of a small ravine-like hollow,
which has no exit, and which abuts against a range of trachytic mountains.
The steam is emitted from several irregular fissures: it is scentless, soon
blackens iron, and is of much too high temperature to be endured by the
hand. The manner in which the solid trachyte is changed on the borders of
these orifices is curious: first, the base becomes earthy, with red
freckles evidently due to the oxidation of particles of iron; then it
becomes soft; and lastly, even the crystals of glassy feldspar yield to the
dissolving agent. After the mass is converted into clay, the oxide of iron
seems to be entirely removed from some parts, which are left perfectly
white, whilst in other neighbouring parts, which are of the brightest red
colour, it seems to be deposited in greater quantity; some other masses are
marbled with two distinct colours. Portions of the white clay, now that
they are dry, cannot be distinguished by the eye from the finest prepared
chalk; and when placed between the teeth they feel equally soft-grained;
the inhabitants use this substance for white-washing their houses. The
cause of the iron being dissolved in one part, and close by being again
deposited, is obscure; but the fact has been observed in several other
places. (Spallanzani, Dolomieu, and Hoffman have described similar cases in
the Italian volcanic islands. Dolomieu says the iron at the Panza Islands
is redeposited in the form of veins (page 86 "Memoire sur les Isles
Ponces"). These authors likewise believe that the steam deposits silica: it
is now experimentally known that vapour of a high temperature is able to
dissolve silica.) In some half-decayed specimens, I found small, globular
aggregations of yellow hyalite, resembling gum-arabic, which no doubt had
been deposited by the steam.
As there is no escape for the rain-water, which trickles down the sides of
the ravine-like hollow, whence the steam issues, it must all percolate
downwards through the fissures at its bottom. Some of the inhabitants
informed me that it was on record that flames (some luminous appearance?)
had originally proceeded from these cracks, and that the flames had been
succeeded by the steam; but I was not able to ascertain how long this was
ago, or anything certain on the subject. When viewing the spot, I imagined
that the injection of a large mass of rock. like the cone of phonolite at
Fernando Noronha, in a semi-fluid state, by arching the surface might have
caused a wedge-shaped hollow with cracks at the bottom, and that the rain-
water percolating to the neighbourhood of the heated mass, would during
many succeeding years be driven back in the form of steam.
TAHITI (OTAHEITE).
I visited only a part of the north-western side of this island, and this
part is entirely composed of volcanic rocks. Near the coast there are
several varieties of basalt, some abounding with large crystals of augite
and tarnished olivine, others compact and earthy,--some slightly vesicular,
and others occasionally amygdaloidal. These rocks are generally much
decomposed, and to my surprise, I found in several sections that it was
impossible to distinguish, even approximately, the line of separation
between the decayed lava and the alternating beds of tuff. Since the
specimens have become dry, it is rather more easy to distinguish the
decomposed igneous rocks from the sedimentary tuffs. This gradation in
character between rocks having such widely different origins, may I think
be explained by the yielding under pressure of the softened sides of the
vesicular cavities, which in many volcanic rocks occupy a large proportion
of their bulk. As the vesicles generally increase in size and number in the
upper parts of a stream of lava, so would the effects of their compression
increase; the yielding, moreover, of each lower vesicle must tend to
disturb all the softened matter above it. Hence we might expect to trace a
perfect gradation from an unaltered crystalline rock to one in which all
the particles (although originally forming part of the same solid mass) had
undergone mechanical displacement; and such particles could hardly be
distinguished from others of similar composition, which had been deposited
as sediment. As lavas are sometimes laminated in their upper parts even
horizontal lines, appearing like those of aqueous deposition, could not in
all cases be relied on as a criterion of sedimentary origin. From these
considerations it is not surprising that formerly many geologists believed
in real transitions from aqueous deposits, through wacke, into igneous
traps.
In the valley of Tia-auru, the commonest rocks are basalts with much
olivine, and in some cases almost composed of large crystals of augite. I
picked up some specimens, with much glassy feldspar, approaching in
character to trachyte. There were also many large blocks of vesicular
basalt, with the cavities beautifully lined with chabasie (?), and
radiating bundles of mesotype. Some of these specimens presented a curious
appearance, owing to a number of the vesicles being half filled up with a
white, soft, earthy mesotypic mineral, which intumesced under the blowpipe
in a remarkable manner. As the upper surfaces in all the half-filled cells
are exactly parallel, it is evident that this substance has sunk to the
bottom of each cell from its weight. Sometimes, however, it entirely fills
the cells. Other cells are either quite filled, or lined, with small
crystals, apparently of chabasie; these crystals, also, frequently line the
upper half of the cells partly filled with the earthy mineral, as well as
the upper surface of this substance itself, in which case the two minerals
appear to blend into each other. I have never seen any other amygdaloid
with the cells half filled in the manner here described; and it is
difficult to imagine the causes which determined the earthy mineral to sink
from its gravity to the bottom of the cells, and the crystalline mineral to
adhere in a coating of equal thickness round the sides of the cells.
(MacCulloch, however, has described and given a plate of ("Geolog. Trans."
1st series volume 4 page 225) a trap rock, with cavities filled up
horizontally with quartz and chalcedony. The upper halves of these cavities
are often filled by layers, which follow each irregularity of the surface,
and by little depending stalactites of the same siliceous substances.)
The basic strata on the sides of the valley are gently inclined seaward,
and I nowhere observed any sign of disturbance; the strata are separated
from each other by thick, compact beds of conglomerate, in which the
fragments are large, some being rounded, but most angular. From the
character of these beds, from the compact and crystalline condition of most
of the lavas, and from the nature of the infiltrated minerals, I was led to
conjecture that they had originally flowed beneath the sea. This conclusion
agrees with the fact that the Rev. W. Ellis found marine remains at a
considerable height, which he believes were interstratified with volcanic
matter; as is likewise described to be the case by Messrs. Tyerman and
Bennett at Huaheine, an island in this same archipelago. Mr. Stutchbury
also discovered near the summit of one of the loftiest mountains of Tahiti,
at the height of several thousand feet, a stratum of semi-fossil coral.
None of these remains have been specifically examined. On the coast, where
masses of coral-rock would have afforded the clearest evidence, I looked in
vain for any signs of recent elevation. For references to the above
authorities, and for more detailed reasons for not believing that Tahiti
has been recently elevated, I must refer to the "Structure and Distribution
of Coral-Reefs."
MAURITIUS.
Approaching this island on the northern or north-western side, a curved
chain of bold mountains, surmounted by rugged pinnacles, is seen to rise
from a smooth border of cultivated land, which gently slopes down to the
coast. At the first glance, one is tempted to believe that the sea lately
reached the base of these mountains, and upon examination, this view, at
least with respect to the inferior parts of the border, is found to be
perfectly correct. Several authors have described masses of upraised coral-
rock round the greater part of the circumference of the island. (Captain
Carmichael, in Hooker's "Bot. Misc." volume 2 page 301. Captain Lloyd has
lately, in the "Proceedings of the Geological Society" (volume 3 page 317),
described carefully some of these masses. In the "Voyage a l'Isle de
France, par un Officier du Roi," many interesting facts are given on this
subject. Consult also "Voyage aux Quatre Isles d'Afrique, par M. Bory St.
Vincent.") Between Tamarin Bay and the Great Black River I observed, in
company with Captain Lloyd, two hillocks of coral-rock, formed in their
lower part of hard calcareous sandstone, and in their upper of great
blocks, slightly aggregated, of Astraea and Madrepora, and of fragments of
basalt; they were divided into beds dipping seaward, in one case at an
angle of 8 degrees, and in the other at 18 degrees; they had a water-worn
appearance, and they rose abruptly from a smooth surface, strewed with
rolled debris of organic remains, to a height of about twenty feet. The
Officier du Roi, in his most interesting tour in 1768 round the island, has
described masses of upraised coral-rocks, still retaining that moat-like
structure (see my "Coral Reefs") which is characteristic of the living
reefs. On the coast northward of Port Louis, I found the lava concealed for
a considerable space inland by a conglomerate of corals and shells, like
those on the beach, but in parts consolidated by red ferruginous matter. M.
Bory St. Vincent has described similar calcareous beds over nearly the
whole of the plain of Pamplemousses. Near Port Louis, when turning over
some large stones, which lay in the bed of a stream at the head of a
protected creek, and at the height of some yards above the level of spring
tides, I found several shells of serpula still adhering to their under
sides.
The jagged mountains near Port Louis rise to a height of between two and
three thousand feet; they consist of strata of basalt, obscurely separated
from each other by firmly aggregated beds of fragmentary matter; and they
are intersected by a few vertical dikes. The basalt in some parts abounds
with large crystals of augite and olivine, and is generally compact. The
interior of the island forms a plain, raised probably about a thousand feet
above the level of the sea, and composed of streams of lava which have
flowed round and between the rugged basaltic mountains. These more recent
lavas are also basaltic, but less compact, and some of them abound with
feldspar, so that they even fuse into a pale coloured glass. On the banks
of the Great River, a section is exposed nearly five hundred feet deep,
worn through numerous thin sheets of the lava of this series, which are
separated from each other by beds of scoriae. They seem to have been of
subaerial formation, and to have flowed from several points of eruption on
the central platform, of which the Piton du Milieu is said to be the
principal one. There are also several volcanic cones, apparently of this
modern period, round the circumference of the island, especially at the
northern end, where they form separate islets.
The mountains composed of the more compact and crystalline basalt, form the
main skeleton of the island. M. Bailly ("Voyage aux Terres Australes" tome
1 page 54.) states that they all "se developpent autour d'elle comme une
ceinture d'immenses remparts, toutes affectant une pente plus ou moins
enclinee vers le rivage de la mer; tandis, au contraire, que vers le centre
de l'ile elles presentent une coupe abrupte, et souvent taillee a pic.
Toutes ces montagnes sont formees de couches paralleles inclinees du centre
de l'ile vers la mer." These statements have been disputed, though not in
detail, by M. Quoy, in the voyage of Freycinet. As far as my limited means
of observation went, I found them perfectly correct. (M. Lesson, in his
account of this island, in the "Voyage of the 'Coquille'," seems to follow
M. Bailly's views.) The mountains on the N.W. side of the island, which I
examined, namely, La Pouce, Peter Botts, Corps de Garde, Les Mamelles, and
apparently another farther southward, have precisely the external shape and
stratification described by M. Bailly. They form about a quarter of his
girdle of ramparts. Although these mountains now stand quite detached,
being separated from each other by breaches, even several miles in width,
through which deluges of lava have flowed from the interior of the island;
nevertheless, seeing their close general similarity, one must feel
convinced that they originally formed parts of one continuous mass. Judging
from the beautiful map of the Mauritius, published by the Admiralty from a
French MS., there is a range of mountains (M. Bamboo) on the opposite side
of the island, which correspond in height, relative position, and external
form, with those just described. Whether the girdle was ever complete may
well be doubted; but from M. Bailly's statements, and my own observations,
it may be safely concluded that mountains with precipitous inland flanks,
and composed of strata dipping outwards, once extended round a considerable
portion of the circumference of the island. The ring appears to have been
oval and of vast size; its shorter axis, measured across from the inner
sides of the mountains near Port Louis and those near Grand Port, being no
less than thirteen geographical miles in length. M. Bailly boldly supposes
that this enormous gulf, which has since been filled up to a great extent
by streams of modern lava, was formed by the sinking in of the whole upper
part of one great volcano.
It is singular in how many respects those portions of St. Jago and of
Mauritius which I visited agree in their geological history. At both
islands, mountains of similar external form, stratification, and (at least
in their upper beds) composition, follow in a curved chain the coast-line.
These mountains in each case appear originally to have formed parts of one
continuous mass. The basaltic strata of which they are composed, from their
compact and crystalline structure, seem, when contrasted with the
neighbouring basaltic streams of subaerial formation, to have flowed
beneath the pressure of the sea, and to have been subsequently elevated. We
may suppose that the wide breaches between the mountains were in both cases
worn by the waves, during their gradual elevation--of which process, within
recent times, there is abundant evidence on the coast-land of both islands.
At both, vast streams of more recent basaltic lavas have flowed from the
interior of the island, round and between the ancient basaltic hills; at
both, moreover, recent cones of eruption are scattered around the
circumference of the island; but at neither have eruptions taken place
within the period of history. As remarked in the last chapter, it is
probable that these ancient basaltic mountains, which resemble (at least in
many respects) the basal and disturbed remnants of two gigantic volcanoes,
owe their present form, structure, and position, to the action of similar
causes.
ST. PAUL'S ROCKS.
This small island is situated in the Atlantic Ocean, nearly one degree
north of the equator, and 540 miles distant from South America, in 29
degrees 15 minutes west longitude. Its highest point is scarcely fifty feet
above the level of the sea; its outline is irregular, and its entire
circumference barely three-quarters of a mile. This little point of rock
rises abruptly out of the ocean; and, except on its western side, soundings
were not obtained, even at the short distance of a quarter of a mile from
its shore. It is not of volcanic origin; and this circumstance, which is
the most remarkable point in its history (as will hereafter be referred
to), properly ought to exclude it from the present volume. It is composed
of rocks, unlike any which I have met with, and which I cannot characterise
by any name, and must therefore describe.
The simplest, and one of the most abundant kinds, is a very compact, heavy,
greenish-black rock, having an angular, irregular fracture, with some
points just hard enough to scratch glass, and infusible. This variety
passes into others of paler green tints, less hard, but with a more
crystalline fracture, and translucent on their edges; and these are fusible
into a green enamel. Several other varieties are chiefly characterised by
containing innumerable threads of dark-green serpentine, and by having
calcareous matter in their interstices. These rocks have an obscure,
concretionary structure, and are full of variously coloured angular pseudo
fragments. These angular pseudo fragments consist of the first-described
dark green rock, of a brown softer kind, of serpentine, and of a yellowish
harsh stone, which, perhaps, is related to serpentine rock. There are other
vesicular, calcareo-ferruginous, soft stones. There is no distinct
stratification, but parts are imperfectly laminated; and the whole abounds
with innumerable veins, and vein-like masses, both small and large. Of
these vein-like masses, some calcareous ones, which contain minute
fragments of shells, are clearly of subsequent origin to the others.
A GLOSSY INCRUSTATION.
Extensive portions of these rocks are coated by a layer of a glossy
polished substance, with a pearly lustre and of a greyish white colour; it
follows all the inequalities of the surface, to which it is firmly
attached. When examined with a lens, it is found to consist of numerous
exceedingly thin layers, their aggregate thickness being about the tenth of
an inch. It is considerably harder than calcareous spar, but can be
scratched with a knife; under the blowpipe it scales off, decrepitates,
slightly blackens, emits a fetid odour, and becomes strongly alkaline: it
does not effervesce in acids. (In my "Journal" I have described this
substance; I then believed that it was an impure phosphate of lime.) I
presume this substance has been deposited by water draining from the birds'
dung, with which the rocks are covered. At Ascension, near a cavity in the
rocks which was filled with a laminated mass of infiltrated birds' dung, I
found some irregularly formed, stalactitical masses of apparently the same
nature. These masses, when broken, had an earthy texture; but on their
outsides, and especially at their extremities, they were formed of a pearly
substance, generally in little globules, like the enamel of teeth, but more
translucent, and so hard as just to scratch plate-glass. This substance
slightly blackens under the blowpipe, emits a bad smell, then becomes quite
white, swelling a little, and fuses into a dull white enamel; it does not
become alkaline; nor does it effervesce in acids. The whole mass had a
collapsed appearance, as if in the formation of the hard glossy crust the
whole had shrunk much. At the Abrolhos Islands on the coast of Brazil,
where also there is much birds' dung, I found a great quantity of a brown,
arborescent substance adhering to some trap-rock. In its arborescent form,
this substance singularly resembles some of the branched species of
Nullipora. Under the blowpipe, it behaves like the specimens from
Ascension; but it is less hard and glossy, and the surface has not the
shrunk appearance.
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
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