Volcanic Islands
by
Charles Darwin
Part 3 out of 3
probably they are present in many other places.) Their closer connection,
however, in this Archipelago, with ordinary volcanic action, is shown by
the pools of solidified basalt, with which they are sometimes filled up.
It at first appears very singular, that all the craters formed of tuff have
their southern sides, either quite broken down and wholly removed, or much
lower than the other sides. I saw and received accounts of twenty-eight of
these craters; of these, twelve form separate islets (These consist of the
three Crossman Islets, the largest of which is 600 feet in height;
Enchanted Island; Gardner Island (760 feet high); Champion Island (331 feet
high); Enderby Island; Brattle Island; two islets near Indefatigable
Island; and one near James Island. A second crater near James Island (with
a salt lake in its centre) has its southern side only about twenty feet
high, whilst the other parts of the circumference are about three hundred
feet in height.), and now exist as mere crescents quite open to the south,
with occasionally a few points of rock marking their former circumference:
of the remaining sixteen, some form promontories, and others stand at a
little distance inland from the shore; but all have their southern sides
either the lowest, or quite broken down. Two, however, of the sixteen had
their northern sides also low, whilst their eastern and western sides were
perfect. I did not see, or hear of, a single exception to the rule, of
these craters being broken down or low on the side, which faces a point of
the horizon between S.E. and S.W. This rule does not apply to craters
composed of lava and scoriae. The explanation is simple: at this
Archipelago, the waves from the trade-wind, and the swell propagated from
the distant parts of the open ocean, coincide in direction (which is not
the case in many parts of the Pacific), and with their united forces attack
the southern sides of all the islands; and consequently the southern slope,
even when entirely formed of hard basaltic rock, is invariably steeper than
the northern slope. As the tuff-craters are composed of a soft material,
and as probably all, or nearly all, have at some period stood immersed in
the sea, we need not wonder that they should invariably exhibit on their
exposed sides the effects of this great denuding power. Judging from the
worn condition of many of these craters, it is probable that some have been
entirely washed away. As there is no reason to suppose, that the craters
formed of scoriae and lava were erupted whilst standing in the sea, we can
see why the rule does not apply to them. At Ascension, it was shown that
the mouths of the craters, which are there all of terrestrial origin, have
been affected by the trade-wind; and this same power might here, also, aid
in making the windward and exposed sides of some of the craters originally
the lowest.
MINERALOGICAL COMPOSITION OF THE ROCKS.
In the northern islands, the basaltic lavas seem generally to contain more
albite than they do in the southern half of the Archipelago; but almost all
the streams contain some. The albite is not unfrequently associated with
olivine. I did not observe in any specimen distinguishable crystals of
hornblende or augite; I except the fused grains in the ejected fragments,
and in the pinnacle of the little crater, above described. I did not meet
with a single specimen of true trachyte; though some of the paler lavas,
when abounding with large crystals of the harsh and glassy albite, resemble
in some degree this rock; but in every case the basis fuses into a black
enamel. Beds of ashes and far-ejected scoriae, as previously stated, are
almost absent; nor did I see a fragment of obsidian or of pumice. Von Buch
believes that the absence of pumice on Mount Etna is consequent on the
feldspar being of the Labrador variety ("Description des Isles Canaries"
page 328.); if the presence of pumice depends on the constitution of the
feldspar, it is remarkable, that it should be absent in this archipelago,
and abundant in the Cordillera of South America, in both of which regions
the feldspar is of the albitic variety. Owing to the absence of ashes, and
the general indecomposable character of the lava in this Archipelago, the
islands are slowly clothed with a poor vegetation, and the scenery has a
desolate and frightful aspect.
ELEVATION OF THE LAND.
Proofs of the rising of the land are scanty and imperfect. At Chatham
Island, I noticed some great blocks of lava, cemented by calcareous matter,
containing recent shells; but they occurred at the height of only a few
feet above high-water mark. One of the officers gave me some fragments of
shells, which he found embedded several hundred feet above the sea, in the
tuff of two craters, distant from each other. It is possible, that these
fragments may have been carried up to their present height in an eruption
of mud; but as, in one instance, they were associated with broken oyster-
shells, almost forming a layer, it is more probable that the tuff was
uplifted with the shells in mass. The specimens are so imperfect that they
can be recognised only as belonging to recent marine genera. On Charles
Island, I observed a line of great rounded blocks, piled on the summit of a
vertical cliff, at the height of fifteen feet above the line, where the sea
now acts during the heaviest gales. This appeared, at first, good evidence
in favour of the elevation of the land; but it was quite deceptive, for I
afterwards saw on an adjoining part of this same coast, and heard from eye-
witnesses, that wherever a recent stream of lava forms a smooth inclined
plane, entering the sea, the waves during gales have the power of ROLLING
UP ROUNDED blocks to a great height, above the line of their ordinary
action. As the little cliff in the foregoing case is formed by a stream of
lava, which, before being worn back, must have entered the sea with a
gently sloping surface, it is possible or rather it is probable, that the
rounded boulders, now lying on its summit, are merely the remnants of those
which had been ROLLED UP during storms to their present height.
DIRECTION OF THE FISSURES OF ERUPTION.
The volcanic orifices in this group cannot be considered as
indiscriminately scattered. Three great craters on Albermarle Island form a
well-marked line, extending N.W. by N. and S.E. by S. Narborough Island,
and the great crater on the rectangular projection of Albemarle Island,
form a second parallel line. To the east, Hood's Island, and the islands
and rocks between it and James Island, form another nearly parallel line,
which, when prolonged, includes Culpepper and Wenman Islands, lying seventy
miles to the north. The other islands lying further eastward, form a less
regular fourth line. Several of these islands, and the vents on Albemarle
Island, are so placed, that they likewise fall on a set of rudely parallel
lines, intersecting the former lines at right angles; so that the principal
craters appear to lie on the points where two sets of fissures cross each
other. The islands themselves, with the exception of Albemarle Island, are
not elongated in the same direction with the lines on which they stand. The
direction of these islands is nearly the same with that which prevails in
so remarkable a manner in the numerous archipelagoes of the great Pacific
Ocean. Finally, I may remark, that amongst the Galapagos Islands there is
no one dominant vent much higher than all the others, as may be observed in
many volcanic archipelagoes: the highest is the great mound on the south-
western extremity of Albemarle Island, which exceeds by barely a thousand
feet several other neighbouring craters.
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.
ON THE SEPARATION OF THE CONSTITUENT MINERALS OF LAVA, ACCORDING TO THEIR
SPECIFIC GRAVITIES.
One side of Fresh-water Bay, in James Island, is formed by the wreck of a
large crater, mentioned in the last chapter, of which the interior has been
filled up by a pool of basalt, about two hundred feet in thickness. This
basalt is of a grey colour, and contains many crystals of glassy albite,
which become much more numerous in the lower, scoriaceous part. This is
contrary to what might have been expected, for if the crystals had been
originally disseminated in equal numbers, the greater intumescence of this
lower scoriaceous part would have made them appear fewer in number. Von
Buch has described a stream of obsidian on the Peak of Teneriffe, in which
the crystals of feldspar become more and more numerous, as the depth or
thickness increases, so that near the lower surface of the stream the lava
even resembles a primary rock. ("Description des Isles Canaries" pages 190
and 191.) Von Buch further states, that M. Dree, in his experiments in
melting lava, found that the crystals of feldspar always tended to
precipitate themselves to the bottom of the crucible. In these cases, I
presume there can be no doubt that the crystals sink from their weight. (In
a mass of molten iron, it is found ("Edinburgh New Philosophical Journal"
volume 24 page 66) that the substances, which have a closer affinity for
oxygen than iron has, rise from the interior of the mass to the surface.
But a similar cause can hardly apply to the separation of the crystals of
these lava-streams. The cooling of the surface of lava seems, in some
cases, to have affected its composition; for Dufrenoy ("Mem. pour servir"
tome 4 page 271) found that the interior parts of a stream near Naples
contained two-thirds of a mineral which was acted on by acids, whilst the
surface consisted chiefly of a mineral unattackable by acids.) The specific
gravity of feldspar varies from 2.4 to 2.58, whilst obsidian seems commonly
to be from 2.3 to 2.4; and in a fluidified state its specific gravity would
probably be less, which would facilitate the sinking of the crystals of
feldspar. (I have taken the specific gravities of the simple minerals from
Von Kobell, one of the latest and best authorities, and of the rocks from
various authorities. Obsidian, according to Phillips, is 2.35; and Jameson
says it never exceeds 2.4; but a specimen from Ascension, weighed by
myself, was 2.42.) At James Island, the crystals of albite, though no doubt
of less weight than the grey basalt, in the parts where compact, might
easily be of greater specific gravity than the scoriaceous mass, formed of
melted lava and bubbles of heated gas.
The sinking of crystals through a viscid substance like molten rock, as is
unequivocally shown to have been the case in the experiments of M. Dree, is
worthy of further consideration, as throwing light on the separation of the
trachytic and basaltic series of lavas. Mr. P. Scrope has speculated on
this subject; but he does not seem to have been aware of any positive
facts, such as those above given; and he has overlooked one very necessary
element, as it appears to me, in the phenomenon--namely, the existence of
either the lighter or heavier mineral in globules or in crystals. In a
substance of imperfect fluidity, like molten rock, it is hardly credible,
that the separate, infinitely small atoms, whether of feldspar, augite, or
of any other mineral, would have power from their slightly different
gravities to overcome the friction caused by their movement; but if the
atoms of any one of these minerals became, whilst the others remained
fluid, united into crystals or granules, it is easy to perceive that from
the lessened friction, their sinking or floating power would be greatly
increased. On the other hand, if all the minerals became granulated at the
same time, it is scarcely possible, from their mutual resistance, that any
separation could take place. A valuable, practical discovery, illustrating
the effect of the granulation of one element in a fluid mass, in aiding its
separation, has lately been made: when lead containing a small proportion
of silver, is constantly stirred whilst cooling, it becomes granulated, and
the grains of imperfect crystals of nearly pure lead sink to the bottom,
leaving a residue of melted metal much richer in silver; whereas if the
mixture be left undisturbed, although kept fluid for a length of time, the
two metals show no signs of separating. (A full and interesting account of
this discovery, by Mr. Pattinson, was read before the British Association
in September 1838. In some alloys, according to Turner "Chemistry" page
210, the heaviest metal sinks, and it appears that this takes place whilst
both metals are fluid. Where there is a considerable difference in gravity,
as between iron and the slag formed during the fusion of the ore, we need
not be surprised at the atoms separating, without either substance being
granulated.) The sole use of the stirring seems to be, the formation of
detached granules. The specific gravity of silver is 10.4, and of lead
11.35: the granulated lead, which sinks, is never absolutely pure, and the
residual fluid metal contains, when richest, only 1/119 part of silver. As
the difference in specific gravity, caused by the different proportions of
the two metals, is so exceedingly small, the separation is probably aided
in a great degree by the difference in gravity between the lead, when
granular though still hot, and when fluid.
In a body of liquified volcanic rock, left for some time without any
violent disturbance, we might expect, in accordance with the above facts,
that if one of the constituent minerals became aggregated into crystals or
granules, or had been enveloped in this state from some previously existing
mass, such crystals or granules would rise or sink, according to their
specific gravity. Now we have plain evidence of crystals being embedded in
many lavas, whilst the paste or basis has continued fluid. I need only
refer, as instances, to the several, great, pseudo-porphyritic streams at
the Galapagos Islands, and to the trachytic streams in many parts of the
world, in which we find crystals of feldspar bent and broken by the
movement of the surrounding, semi-fluid matter. Lavas are chiefly composed
of three varieties of feldspar, varying in specific gravity from 2.4 to
2.74; of hornblende and augite, varying from 3.0 to 3.4; of olivine,
varying from 3.3 to 3.4; and lastly, of oxides of iron, with specific
gravities from 4.8 to 5.2. Hence crystals of feldspar, enveloped in a mass
of liquified, but not highly vesicular lava, would tend to rise to the
upper parts; and crystals or granules of the other minerals, thus
enveloped, would tend to sink. We ought not, however, to expect any perfect
degree of separation in such viscid materials. Trachyte, which consists
chiefly of feldspar, with some hornblende and oxide of iron, has a specific
gravity of about 2.45; whilst basalt, composed chiefly of augite and
feldspar, often with much iron and olivine, has a gravity of about 3.0.
(Trachyte from Java was found by Von Buch to be 2.47; from Auvergne, by De
la Beche, it was 2.42; from Ascension, by myself, it was 2.42. Jameson and
other authors give to basalt a specific gravity of 3.0; but specimens from
Auvergne were found, by De la Beche, to be only 2.78; and from the Giant's
Causeway, to be 2.91.) Accordingly we find, that where both trachytic and
basaltic streams have proceeded from the same orifice, the trachytic
streams have generally been first erupted owing, as we must suppose, to the
molten lava of this series having accumulated in the upper parts of the
volcanic focus. This order of eruption has been observed by Beudant,
Scrope, and by other authors; three instances, also, have been given in
this volume. As the later eruptions, however, from most volcanic mountains,
burst through their basal parts, owing to the increased height and weight
of the internal column of molten rock, we see why, in most cases, only the
lower flanks of the central, trachytic masses, are enveloped by basaltic
streams. The separation of the ingredients of a mass of lava, would,
perhaps, sometimes take place within the body of a volcanic mountain, if
lofty and of great dimensions, instead of within the underground focus; in
which case, trachytic streams might be poured forth, almost
contemporaneously, or at short recurrent intervals, from its summit, and
basaltic streams from its base: this seems to have taken place at
Teneriffe. (Consult Von Buch's well-known and admirable "Description
Physique" of this island, which might serve as a model of descriptive
geology.) I need only further remark, that from violent disturbances the
separation of the two series, even under otherwise favourable conditions,
would naturally often be prevented, and likewise their usual order of
eruption be inverted. From the high degree of fluidity of most basaltic
lavas, these perhaps, alone, would in many cases reach the surface.
As we have seen that crystals of feldspar, in the instance described by Von
Buch, sink in obsidian, in accordance with their known greater specific
gravity, we might expect to find in every trachytic district, where
obsidian has flowed as lava, that it had proceeded from the upper or
highest orifices. This, according to Von Buch, holds good in a remarkable
manner both at the Lipari Islands and on the Peak of Teneriffe; at this
latter place obsidian has never flowed from a less height than 9,200 feet.
Obsidian, also, appears to have been erupted from the loftiest peaks of the
Peruvian Cordillera. I will only further observe, that the specific gravity
of quartz varies from 2.6 to 2.8; and therefore, that when present in a
volcanic focus, it would not tend to sink with the basaltic bases; and
this, perhaps, explains the frequent presence, and the abundance of this
mineral, in the lavas of the trachytic series, as observed in previous
parts of this volume.
An objection to the foregoing theory will, perhaps, be drawn from the
plutonic rocks not being separated into two evidently distinct series, of
different specific gravities; although, like the volcanic, they have been
liquified. In answer, it may first be remarked, that we have no evidence of
the atoms of any one of the constituent minerals in the plutonic series
having been aggregated, whilst the others remained fluid, which we have
endeavoured to show is an almost necessary condition of their separation;
on the contrary, the crystals have generally impressed each other with
their forms. (The crystalline paste of phonolite is frequently penetrated
by long needles of hornblende; from which it appears that the hornblende,
though the more fusible mineral, has crystallised before, or at the same
time with a more refractory substance. Phonolite, as far as my observations
serve, in every instance appears to be an injected rock, like those of the
plutonic series; hence probably, like these latter, it has generally been
cooled without repeated and violent disturbances. Those geologists who have
doubted whether granite could have been formed by igneous liquefaction,
because minerals of different degrees of fusibility impress each other with
their forms, could not have been aware of the fact of crystallised
hornblende penetrating phonolite, a rock undoubtedly of igneous origin. The
viscidity, which it is now known, that both feldspar and quartz retain at a
temperature much below their points of fusion, easily explains their mutual
impressment. Consult on this subject Mr. Horner's paper on Bonn "Geolog.
Transact." volume 4 page 439; and "L'Institut" with respect to quartz 1839
page 161.)
In the second place, the perfect tranquillity, under which it is probable
that the plutonic masses, buried at profound depths, have cooled, would,
most likely, be highly unfavourable to the separation of their constituent
minerals; for, if the attractive force, which during the progressive
cooling draws together the molecules of the different minerals, has power
sufficient to keep them together, the friction between such half-formed
crystals or pasty globules would effectually prevent the heavier ones from
sinking, or the lighter ones from rising. On the other hand, a small amount
of disturbance, which would probably occur in most volcanic foci, and which
we have seen does not prevent the separation of granules of lead from a
mixture of molten lead and silver, or crystals of feldspar from streams of
lava, by breaking and dissolving the less perfectly formed globules, would
permit the more perfect and therefore unbroken crystals, to sink or rise,
according to their specific gravity.
Although in plutonic rocks two distinct species, corresponding to the
trachytic and basaltic series, do not exist, I much suspect that a certain
amount of separation of their constituent parts has often taken place. I
suspect this from having observed how frequently dikes of greenstone and
basalt intersect widely extended formations of granite and the allied
metamorphic rocks. I have never examined a district in an extensive
granitic region without discovering dikes; I may instance the numerous
trap-dikes, in several districts of Brazil, Chile, and Australia, and at
the Cape of Good Hope: many dikes likewise occur in the great granitic
tracts of India, in the north of Europe, and in other countries. Whence,
then, has the greenstone and basalt, forming these dikes, come? Are we to
suppose, like some of the elder geologists, that a zone of trap is
uniformly spread out beneath the granitic series, which composes, as far as
we know, the foundations of the earth's crust? Is it not more probable,
that these dikes have been formed by fissures penetrating into partially
cooled rocks of the granitic and metamorphic series, and by their more
fluid parts, consisting chiefly of hornblende, oozing out, and being sucked
into such fissures? At Bahia, in Brazil, in a district composed of gneiss
and primitive greenstone, I saw many dikes, of a dark augitic (for one
crystal certainly was of this mineral) or hornblendic rock, which, as
several appearances clearly proved, either had been formed before the
surrounding mass had become solid, or had together with it been afterwards
thoroughly softened. (Portions of these dikes have been broken off, and are
now surrounded by the primary rocks, with their laminae conformably winding
round them. Dr. Hubbard also ("Silliman's Journal" volume 34 page 119), has
described an interlacement of trap-veins in the granite of the White
Mountains, which he thinks must have been formed when both rocks were
soft.) On both sides of one of these dikes, the gneiss was penetrated, to
the distance of several yards, by numerous, curvilinear threads or streaks
of dark matter, which resembled in form clouds of the class called cirrhi-
comae; some few of these threads could be traced to their junction with the
dike. When examining them, I doubted whether such hair-like and curvilinear
veins could have been injected, and I now suspect, that instead of having
been injected from the dike, they were its feeders. If the foregoing views
of the origin of trap-dikes in widely extended granitic regions far from
rocks of any other formation, be admitted as probable, we may further
admit, in the case of a great body of plutonic rock, being impelled by
repeated movements into the axis of a mountain-chain, that its more liquid
constituent parts might drain into deep and unseen abysses; afterwards,
perhaps, to be brought to the surface under the form, either of injected
masses of greenstone and augitic porphyry, or of basaltic eruptions. (Mr.
Phillips "Lardner's Encyclop." volume 2 page 115 quotes Von Buch's
statement, that augitic porphyry ranges parallel to, and is found
constantly at the base of, great chains of mountains. Humboldt, also, has
remarked the frequent occurrence of trap-rock, in a similar position; of
which fact I have observed many examples at the foot of the Chilian
Cordillera. The existence of granite in the axes of great mountain chains
is always probable, and I am tempted to suppose, that the laterally
injected masses of augitic porphyry and of trap, bear nearly the same
relation to the granitic axes which basaltic lavas bear to the central
trachytic masses, round the flanks of which they have so frequently been
erupted.) Much of the difficulty which geologists have experienced when
they have compared the composition of volcanic with plutonic formations,
will, I think, be removed, if we may believe that most plutonic masses have
been, to a certain extent, drained of those comparatively weighty and
easily liquified elements, which compose the trappean and basaltic series
of rocks.
ON THE DISTRIBUTION OF VOLCANIC ISLANDS.
During my investigations on coral-reefs, I had occasion to consult the
works of many voyagers, and I was invariably struck with the fact, that
with rare exceptions, the innumerable islands scattered throughout the
Pacific, Indian, and Atlantic Oceans, were composed either of volcanic, or
of modern coral-rocks. It would be tedious to give a long catalogue of all
the volcanic islands; but the exceptions which I have found are easily
enumerated: in the Atlantic, we have St. Paul's Rock, described in this
volume, and the Falkland Islands, composed of quartz and clay-slate; but
these latter islands are of considerable size, and lie not very far from
the South American coast (Judging from Forster's imperfect observation,
perhaps Georgia is not volcanic. Dr. Allan is my informant with regard to
the Seychelles. I do not know of what formation Rodriguez, in the Indian
Ocean, is composed.): in the Indian Ocean, the Seychelles (situated in a
line prolonged from Madagascar) consist of granite and quartz: in the
Pacific Ocean, New Caledonia, an island of large size, belongs (as far as
is known) to the primary class. New Zealand, which contains much volcanic
rock and some active volcanoes, from its size cannot be classed with the
small islands, which we are now considering. The presence of a small
quantity of non-volcanic rock, as of clay-slate on three of the Azores
(This is stated on the authority of Count V. de Bedemar, with respect to
Flores and Graciosa (Charlsworth "Magazine of Nat. Hist." volume 1 page
557). St. Maria has no volcanic rock, according to Captain Boyd (Von Buch
"Descript." page 365). Chatham Island has been described by Dr. Dieffenbach
in the "Geographical Journal" 1841 page 201. As yet we have received only
imperfect notices on Kerguelen Land, from the Antarctic Expedition.), or of
tertiary limestone at Madeira, or of clay-slate at Chatham Island in the
Pacific, or of lignite at Kerguelen Land, ought not to exclude such islands
or archipelagoes, if formed chiefly of erupted matter, from the volcanic
class.
The composition of the numerous islands scattered through the great oceans
being with such rare exceptions volcanic, is evidently an extension of that
law, and the effect of those same causes, whether chemical or mechanical,
from which it results, that a vast majority of the volcanoes now in action
stand either as islands in the sea, or near its shores. This fact of the
ocean-islands being so generally volcanic is also interesting in relation
to the nature of the mountain-chains on our continents, which are
comparatively seldom volcanic; and yet we are led to suppose that where our
continents now stand an ocean once extended. Do volcanic eruptions, we may
ask, reach the surface more readily through fissures formed during the
first stages of the conversion of the bed of the ocean into a tract of
land?
Looking at the charts of the numerous volcanic archipelagoes, we see that
the islands are generally arranged either in single, double, or triple
rows, in lines which are frequently curved in a slight degree. (Professors
William and Henry Darwin Rogers have lately insisted much, in a memoir read
before the American Association, on the regularly curved lines of elevation
in parts of the Appalachian range.) Each separate island is either rounded,
or more generally elongated in the same direction with the group in which
it stands, but sometimes transversely to it. Some of the groups which are
not much elongated present little symmetry in their forms; M. Virlet
("Bulletin de la Soc. Geolog." tome 3 page 110.) states that this is the
case with the Grecian Archipelago: in such groups I suspect (for I am aware
how easy it is to deceive oneself on these points), that the vents are
generally arranged on one line, or on a set of short parallel lines,
intersecting at nearly right angles another line, or set of lines. The
Galapagos Archipelago offers an example of this structure, for most of the
islands and the chief orifices on the largest island are so grouped as to
fall on a set of lines ranging about N.W. by N., and on another set ranging
about W.S.W.: in the Canary Archipelago we have a simpler structure of the
same kind: in the Cape de Verde group, which appears to be the least
symmetrical of any oceanic volcanic archipelago, a N.W. and S.E. line
formed by several islands, if prolonged, would intersect at right angles a
curved line, on which the remaining islands are placed.
Von Buch ("Description des Isles Canaries" page 324.) has classed all
volcanoes under two heads, namely, CENTRAL VOLCANOES, round which numerous
eruptions have taken place on all sides, in a manner almost regular, and
VOLCANIC CHAINS. In the examples given of the first class, as far as
position is concerned, I can see no grounds for their being called
"central;" and the evidence of any difference in mineralogical nature
between CENTRAL VOLCANOES and VOLCANIC CHAINS appears slight. No doubt some
one island in most small volcanic archipelagoes is apt to be considerably
higher than the others; and in a similar manner, whatever the cause may be,
that on the same island one vent is generally higher than all the others.
Von Buch does not include in his class of volcanic chains small
archipelagoes, in which the islands are admitted by him, as at the Azores,
to be arranged in lines; but when viewing on a map of the world how perfect
a series exists from a few volcanic islands placed in a row to a train of
linear archipelagoes following each other in a straight line, and so on to
a great wall like the Cordillera of America, it is difficult to believe
that there exists any essential difference between short and long volcanic
chains. Von Buch (Idem page 393.) states that his volcanic chains surmount,
or are closely connected with, mountain-ranges of primary formation: but if
trains of linear archipelagoes are, in the course of time, by the long-
continued action of the elevatory and volcanic forces, converted into
mountain-ranges, it would naturally result that the inferior primary rocks
would often be uplifted and brought into view.
Some authors have remarked that volcanic islands occur scattered, though at
very unequal distances, along the shores of the great continents, as if in
some measure connected with them. In the case of Juan Fernandez, situated
330 miles from the coast of Chile, there was undoubtedly a connection
between the volcanic forces acting under this island and under the
continent, as was shown during the earthquake of 1835. The islands,
moreover, of some of the small volcanic groups which thus border
continents, are placed in lines, related to those along which the adjoining
shores of the continents trend; I may instance the lines of intersection at
the Galapagos, and at the Cape de Verde Archipelagoes, and the best marked
line of the Canary Islands. If these facts be not merely accidental, we see
that many scattered volcanic islands and small groups are related not only
by proximity, but in the direction of the fissures of eruption to the
neighbouring continents--a relation, which Von Buch considers,
characteristic of his great volcanic chains.
In volcanic archipelagoes, the orifices are seldom in activity on more than
one island at a time; and the greater eruptions usually recur only after
long intervals. Observing the number of craters, that are usually found on
each island of a group, and the vast amount of matter which has been
erupted from them, one is led to attribute a high antiquity even to those
groups, which appear, like the Galapagos, to be of comparatively recent
origin. This conclusion accords with the prodigious amount of degradation,
by the slow action of the sea, which their originally sloping coasts must
have suffered, when they are worn back, as is so often the case, into grand
precipices. We ought not, however, to suppose, in hardly any instance, that
the whole body of matter, forming a volcanic island, has been erupted at
the level, on which it now stands: the number of dikes, which seem
invariably to intersect the interior parts of every volcano, show, on the
principles explained by M. Elie de Beaumont, that the whole mass has been
uplifted and fissured. A connection, moreover, between volcanic eruptions
and contemporaneous elevations in mass has, I think, been shown to exist in
my work on Coral-Reefs, both from the frequent presence of upraised organic
remains, and from the structure of the accompanying coral-reefs. (A similar
conclusion is forced on us, by the phenomena, which accompanied the
earthquake of 1835, at Concepcion, and which are detailed in my paper
(volume 5 page 601) in the "Geological Transactions.") Finally, I may
remark, that in the same Archipelago, eruptions have taken place within the
historical period on more than one of the parallel lines of fissure: thus,
at the Galapagos Archipelago, eruptions have taken place from a vent on
Narborough Island, and from one on Albemarle Island, which vents do not
fall on the same line; at the Canary Islands, eruptions have taken place in
Teneriffe and Lanzarote; and at the Azores, on the three parallel lines of
Pico, St. Jorge, and Terceira. Believing that a mountain-axis differs
essentially from a volcano, only in plutonic rocks having been injected,
instead of volcanic matter having been ejected, this appears to me an
interesting circumstance; for we may infer from it as probable, that in the
elevation of a mountain-chain, two or more of the parallel lines forming it
may be upraised and injected 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.
Their origin from drifted particles of shells and corals.
Their extent.
Cape of Good Hope.
Junction of the granite and clay-slate.
Sandstone formation.
The "Beagle," in her homeward voyage, touched at New Zealand, Australia,
Van Diemen's Land, and the Cape of Good Hope. In order to confine the Third
Part of these Geological Observations to South America, I will here briefly
describe all that I observed at these places worthy of the attention of
geologists.
NEW SOUTH WALES.
My opportunities of observation consisted of a ride of ninety geographical
miles to Bathurst, in a W.N.W. direction from Sydney. The first thirty
miles from the coast passes over a sandstone country, broken up in many
places by trap-rocks, and separated by a bold escarpment overhanging the
river Nepean, from the great sandstone platform of the Blue Mountains. This
upper platform is 1,000 feet high at the edge of the escarpment, and rises
in a distance of twenty-five miles to between three and four thousand feet
above the level of the sea. At this distance the road descends to a country
rather less elevated, and composed in chief part of primary rocks. There is
much granite, in one part passing into a red porphyry with octagonal
crystals of quartz, and intersected in some places by trap-dikes. Near the
Downs of Bathurst I passed over much pale-brown, glossy clay-slate, with
the shattered laminae running north and south; I mention this fact, because
Captain King informs me that, in the country a hundred miles southward,
near Lake George, the mica-slate ranges so invariably north and south that
the inhabitants take advantage of it in finding their way through the
forests.
The sandstone of the Blue Mountains is at least 1,200 feet thick, and in
some parts is apparently of greater thickness; it consists of small grains
of quartz, cemented by white earthy matter, and it abounds with ferruginous
veins. The lower beds sometimes alternate with shales and coal: at Wolgan I
found in carbonaceous shale leaves of the Glossopteris Brownii, a fern
which so frequently accompanies the coal of Australia. The sandstone
contains pebbles of quartz; and these generally increase in number and size
(seldom, however, exceeding an inch or two in diameter) in the upper beds:
I observed a similar circumstance in the grand sandstone formation at the
Cape of Good Hope. On the South American coast, where tertiary and supra-
tertiary beds have been extensively elevated, I repeatedly noticed that the
uppermost beds were formed of coarser materials than the lower: this
appears to indicate that, as the sea became shallower, the force of the
waves or currents increased. On the lower platform, however, between the
Blue Mountains and the coast, I observed that the upper beds of the
sandstone frequently passed into argillaceous shale,--the effect, probably,
of this lower space having been protected from strong currents during its
elevation. The sandstone of the Blue Mountains evidently having been of
mechanical origin, and not having suffered any metamorphic action, I was
surprised at observing that, in some specimens, nearly all the grains of
quartz were so perfectly crystallised with brilliant facets that they
evidently had not in their PRESENT form been aggregated in any previously
existing rock. (I have lately seen, in a paper by Smith (the father of
English geologists), in the "Magazine of Natural History," that the grains
of quartz in the millstone grit of England are often crystallised. Sir
David Brewster, in a paper read before the British Association, 1840,
states, that in old decomposed glass, the silex and metals separate into
concentric rings, and that the silex regains its crystalline structure, as
is shown by its action on light.) It is difficult to imagine how these
crystals could have been formed; one can hardly believe that they were
separately precipitated in their present crystallised state. Is it possible
that rounded grains of quartz may have been acted on by a fluid corroding
their surfaces, and depositing on them fresh silica? I may remark that, in
the sandstone formation of the Cape of Good Hope, it is evident that silica
has been profusely deposited from aqueous solution.
In several parts of the sandstone I noticed patches of shale which might at
the first glance have been mistaken for extraneous fragments; their
horizontal laminae, however, being parallel with those of the sandstone,
showed that they were the remnants of thin, continuous beds. One such
fragment (probably the section of a long narrow strip) seen in the face of
a cliff, was of greater vertical thickness than breadth, which proves that
this bed of shale must have been in some slight degree consolidated, after
having been deposited, and before being worn away by the currents. Each
patch of the shale shows, also, how slowly many of the successive layers of
sandstone were deposited. These pseudo-fragments of shale will perhaps
explain, in some cases, the origin of apparently extraneous fragments in
crystalline metamorphic rocks. I mention this, because I found near Rio de
Janeiro a well-defined angular fragment, seven yards long by two yards in
breadth, of gneiss containing garnets and mica in layers, enclosed in the
ordinary, stratified, porphyritic gneiss of the country. The laminae of the
fragment and of the surrounding matrix ran in exactly the same direction,
but they dipped at different angles. I do not wish to affirm that this
singular fragment (a solitary case, as far as I know) was originally
deposited in a layer, like the shale in the Blue Mountains, between the
strata of the porphyritic gneiss, before they were metamorphosed; but there
is sufficient analogy between the two cases to render such an explanation
possible.
STRATIFICATION OF THE ESCARPMENT.
The strata of the Blue Mountains appear to the eye horizontal; but they
probably have a similar inclination with the surface of the platform, which
slopes from the west towards the escarpment over the Nepean, at an angle of
one degree, or of one hundred feet in a mile. (This is stated on the
authority of Sir T. Mitchell in "Travels" volume 2 page 357.) The strata of
the escarpment dip almost conformably with its steeply inclined face, and
with so much regularity, that they appear as if thrown into their present
position; but on a more careful examination, they are seen to thicken and
to thin out, and in the upper part to be succeeded and almost capped by
horizontal beds. These appearances render it probable, that we here see an
original escarpment, not formed by the sea having eaten back into the
strata, but by the strata having originally extended only thus far. Those
who have been in the habit of examining accurate charts of sea-coasts,
where sediment is accumulating, will be aware, that the surfaces of the
banks thus formed, generally slope from the coast very gently towards a
certain line in the offing, beyond which the depth in most cases suddenly
becomes great. I may instance the great banks of sediment within the West
Indian Archipelago (I have described these very curious banks in the
Appendix to my volume on the structure of Coral-Reefs. I have ascertained
the inclination of the edges of the banks, from information given me by
Captain B. Allen, one of the surveyors, and by carefully measuring the
horizontal distances between the last sounding on the bank and the first in
the deep water. Widely extended banks in all parts of the West Indies have
the same general form of surface.), which terminate in submarine slopes,
inclined at angles of between thirty and forty degrees, and sometimes even
at more than forty degrees: every one knows how steep such a slope would
appear on the land. Banks of this nature, if uplifted, would probably have
nearly the same external form as the platform of the Blue Mountains, where
it abruptly terminates over the Nepean.
CURRENT-CLEAVAGE.
The strata of sandstone in the low coast country, and likewise on the Blue
Mountains, are often divided by cross or current laminae, which dip in
different directions, and frequently at an angle of forty-five degrees.
Most authors have attributed these cross layers to successive small
accumulations on an inclined surface; but from a careful examination in
some parts of the New Red Sandstone of England, I believe that such layers
generally form parts of a series of curves, like gigantic tidal ripples,
the tops of which have since been cut off, either by nearly horizontal
layers, or by another set of great ripples, the folds of which do not
exactly coincide with those below them. It is well-known to surveyors that
mud and sand are disturbed during storms at considerable depths, at least
from three hundred to four hundred and fifty feet (See Martin White on
"Soundings in the British Channel" pages 4 and 166.), so that the nature of
the bottom even becomes temporarily changed; the bottom, also, at a depth
between sixty and seventy feet, has been observed to be broadly rippled.
(M. Siau on the "Action of Waves" "Edin. New Phil. Journ." volume 31 page
245.) One may, therefore, be allowed to suspect, from the appearance just
mentioned in the New Red Sandstone, that at greater depths, the bed of the
ocean is heaped up during gales into great ripple-like furrows and
depressions, which are afterwards cut off by the currents during more
tranquil weather, and again furrowed during gales.
VALLEYS IN THE SANDSTONE PLATFORMS.
The grand valleys, by which the Blue Mountains and the other sandstone
platforms of this part of Australia are penetrated, and which long offered
an insuperable obstacle to the attempts of the most enterprising colonist
to reach the interior country, form the most striking feature in the
geology of New South Wales. They are of grand dimensions, and are bordered
by continuous links of lofty cliffs. It is not easy to conceive a more
magnificent spectacle, than is presented to a person walking on the summit-
plains, when without any notice he arrives at the brink of one of these
cliffs, which are so perpendicular, that he can strike with a stone (as I
have tried) the trees growing, at the depth of between one thousand and one
thousand five hundred feet below him; on both hands he sees headland beyond
headland of the receding line of cliff, and on the opposite side of the
valley, often at the distance of several miles, he beholds another line
rising up to the same height with that on which he stands, and formed of
the same horizontal strata of pale sandstone. The bottoms of these valleys
are moderately level, and the fall of the rivers flowing in them, according
to Sir T. Mitchell, is gentle. The main valleys often send into the
platform great baylike arms, which expand at their upper ends; and on the
other hand, the platform often sends promontories into the valley, and even
leaves in them great, almost insulated, masses. So continuous are the
bounding lines of cliff, that to descend into some of these valleys, it is
necessary to go round twenty miles; and into others, the surveyors have
only lately penetrated, and the colonists have not yet been able to drive
in their cattle. But the most remarkable point of structure in these
valleys, is, that although several miles wide in their upper parts, they
generally contract towards their mouths to such a degree as to become
impassable. The Surveyor-General, Sir T. Mitchell, in vain endeavoured,
first on foot and then by crawling between the great fallen fragments of
sandstone, to ascend through the gorge by which the river Grose joins the
Nepean ("Travels in Australia" volume 1 page 154.--I must express my
obligation to Sir T. Mitchell for several interesting personal
communications on the subject of these great valleys of New South Wales.);
yet the valley of the Grose in its upper part, as I saw, forms a
magnificent basin some miles in width, and is on all sides surrounded by
cliffs, the summits of which are believed to be nowhere less than 3,000
feet above the level of the sea. When cattle are driven into the valley of
the Wolgan by a path (which I descended) partly cut by the colonists, they
cannot escape; for this valley is in every other part surrounded by
perpendicular cliffs, and eight miles lower down, it contracts, from an
average width of half a mile, to a mere chasm impassable to man or beast.
Sir T. Mitchell states, that the great valley of the Cox river with all its
branches contracts, where it unites with the Nepean, into a gorge 2,200
yards wide, and about one thousand feet in depth. (Idem volume 2 page 358.)
Other similar cases might have been added.
The first impression, from seeing the correspondence of the horizontal
strata, on each side of these valleys and great amphitheatre-like
depressions, is that they have been in chief part hollowed out, like other
valleys, by aqueous erosion; but when one reflects on the enormous amount
of stone, which on this view must have been removed, in most of the above
cases through mere gorges or chasms, one is led to ask whether these spaces
may not have subsided. But considering the form of the irregularly
branching valleys, and of the narrow promontories, projecting into them
from the platforms, we are compelled to abandon this notion. To attribute
these hollows to alluvial action, would be preposterous; nor does the
drainage from the summit-level always fall, as I remarked near the
Weatherboard, into the head of these valleys, but into one side of their
bay-like recesses. Some of the inhabitants remarked to me, that they never
viewed one of these baylike recesses, with the headlands receding on both
hands, without being struck with their resemblance to a bold sea-coast.
This is certainly the case; moreover, the numerous fine harbours, with
their widely branching arms, on the present coast of New South Wales, which
are generally connected with the sea by a narrow mouth, from one mile to a
quarter of a mile in width, passing through the sandstone coast-cliffs,
present a likeness, though on a miniature scale, to the great valleys of
the interior. But then immediately occurs the startling difficulty, why has
the sea worn out these great, though circumscribed, depressions on a wide
platform, and left mere gorges, through which the whole vast amount of
triturated matter must have been carried away? The only light I can throw
on this enigma, is by showing that banks appear to be forming in some seas
of the most irregular forms, and that the sides of such banks are so steep
(as before stated) that a comparatively small amount of subsequent erosion
would form them into cliffs: that the waves have power to form high and
precipitous cliffs, even in landlocked harbours, I have observed in many
parts of South America. In the Red Sea, banks with an extremely irregular
outline and composed of sediment, are penetrated by the most singularly
shaped creeks with narrow mouths: this is likewise the case, though on a
larger scale, with the Bahama Banks. Such banks, I have been led to
suppose, have been formed by currents heaping sediment on an irregular
bottom. (See the "Appendix" to the Part on Coral-Reefs. The fact of the sea
heaping up mud round a submarine nucleus, is worthy of the notice of
geologists: for outlyers of the same composition with the coast banks are
thus formed; and these, if upheaved and worn into cliffs, would naturally
be thought to have been once connected together.) That in some cases, the
sea, instead of spreading out sediment in a uniform sheet, heaps it round
submarine rocks and islands, it is hardly possible to doubt, after having
examined the charts of the West Indies. To apply these ideas to the
sandstone platforms of New South Wales, I imagine that the strata might
have been heaped on an irregular bottom by the action of strong currents,
and of the undulations of an open sea; and that the valley-like spaces thus
left unfilled might, during a slow elevation of the land, have had their
steeply sloping flanks worn into cliffs; the worn-down sandstone being
removed, either at the time when the narrow gorges were cut by the
retreating sea, or subsequently by alluvial action.
VAN DIEMEN'S LAND.
The southern part of this island is mainly formed of mountains of
greenstone, which often assumes a syenitic character, and contains much
hypersthene. These mountains, in their lower half, are generally encased by
strata containing numerous small corals and some shells. These shells have
been examined by Mr. G.B. Sowerby, and have been described by him: they
consist of two species of Producta, and of six of Spirifera; two of these,
namely, P. rugata and S. rotundata, resemble, as far as their imperfect
condition allows of comparison, British mountain-limestone shells. Mr.
Lonsdale has had the kindness to examine the corals; they consist of six
undescribed species, belonging to three genera. Species of these genera
occur in the Silurian, Devonian, and Carboniferous strata of Europe. Mr.
Lonsdale remarks, that all these fossils have undoubtedly a Palaeozoic
character, and that probably they correspond in age to a division of the
system above the Silurian formations.
The strata containing these remains are singular from the extreme
variability of their mineralogical composition. Every intermediate form is
present, between flinty-slate, clay-slate passing into grey wacke, pure
limestone, sandstone, and porcellanic rock; and some of the beds can only
be described as composed of a siliceo-calcareo-clay-slate. The formation,
as far as I could judge, is at least a thousand feet in thickness: the
upper few hundred feet usually consist of a siliceous sandstone, containing
pebbles and no organic remains; the inferior strata, of which a pale flinty
slate is perhaps the most abundant, are the most variable; and these
chiefly abound with the remains. Between two beds of hard crystalline
limestone, near Newtown, a layer of white soft calcareous matter is
quarried, and is used for whitewashing houses. From information given to me
by Mr. Frankland, the Surveyor-General, it appears that this Palaeozoic
formation is found in different parts of the whole island; from the same
authority, I may add, that on the north-eastern coast and in Bass' Straits
primary rocks extensively occur.
The shores of Storm Bay are skirted, to the height of a few hundred feet,
by strata of sandstone, containing pebbles of the formation just described,
with its characteristic fossils, and therefore belonging to a subsequent
age. These strata of sandstone often pass into shale, and alternate with
layers of impure coal; they have in many places been violently disturbed.
Near Hobart Town, I observed one dike, nearly a hundred yards in width, on
one side of which the strata were tilted at an angle of 60 degrees, and on
the other they were in some parts vertical, and had been altered by the
effects of the heat. On the west side of Storm Bay, I found these strata
capped by streams of basaltic lava with olivine; and close by there was a
mass of brecciated scoriae, containing pebbles of lava, which probably
marks the place of an ancient submarine crater. Two of these streams of
basalt were separated from each other by a layer of argillaceous wacke,
which could be traced passing into partially altered scoriae. The wacke
contained numerous rounded grains of a soft, grass-green mineral, with a
waxy lustre, and translucent on its edges: under the blowpipe it instantly
blackened, and the points fused into a strongly magnetic, black enamel. In
these characters, it resembles those masses of decomposed olivine,
described at St. Jago in the Cape de Verde group; and I should have thought
that it had thus originated, had I not found a similar substance, in
cylindrical threads, within the cells of the vesicular basalt,--a state
under which olivine never appears; this substance, I believe, would be
classed as bole by mineralogists. (Chlorophaeite, described by Dr.
MacCulloch ("Western Islands" volume 1 page 504) as occurring in a basaltic
amygdaloid, differs from this substance, in remaining unchanged before the
blowpipe, and in blackening from exposure to the air. May we suppose that
olivine, in undergoing the remarkable change described at St. Jago, passes
through several states?)
TRAVERTIN WITH EXTINCT PLANTS.
Behind Hobart Town there is a small quarry of a hard travertin, the lower
strata of which abound with distinct impressions of leaves. Mr. Robert
Brown has had the kindness to look at my specimens, and he informed me that
there are four or five kinds, none of which he recognises as belonging to
existing species. The most remarkable leaf is palmate, like that of a fan-
palm, and no plant having leaves of this structure has hitherto been
discovered in Van Diemen's Land. The other leaves do not resemble the most
usual form of the Eucalyptus (of which tribe the existing forests are
chiefly composed), nor do they resemble that class of exceptions to the
common form of the leaves of the Eucalyptus, which occur in this island.
The travertin containing this remnant of a lost vegetation, is of a pale
yellow colour, hard, and in parts even crystalline; but not compact, and is
everywhere penetrated by minute, tortuous, cylindrical pores. It contains a
very few pebbles of quartz, and occasionally layers of chalcedonic nodules,
like those of chert in our Greensand. From the pureness of this calcareous
rock, it has been searched for in other places, but has never been found.
From this circumstance, and from the character of the deposit, it was
probably formed by a calcareous spring entering a small pool or narrow
creek. The strata have subsequently been tilted and fissured; and the
surface has been covered by a singular mass, with which, also, a large
fissure has been filled up, formed of balls of trap embedded in a mixture
of wacke and a white, earthy, alumino-calcareous substance. Hence it would
appear, as if a volcanic eruption had taken place on the borders of the
pool, in which the calcareous matter was depositing, and had broken it up
and drained it.
ELEVATION OF THE LAND.
Both the eastern and western shores of the bay, in the neighbourhood of
Hobart Town, are in most parts covered to the height of thirty feet above
the level of high-water mark, with broken shells, mingled with pebbles. The
colonists attribute these shells to the aborigines having carried them up
for food: undoubtedly, there are many large mounds, as was pointed out to
me by Mr. Frankland, which have been thus formed; but I think from the
numbers of the shells, from their frequent small size, from the manner in
which they are thinly scattered, and from some appearances in the form of
the land, that we must attribute the presence of the greater number to a
small elevation of the land. On the shore of Ralph Bay (opening into Storm
Bay) I observed a continuous beach about fifteen feet above high-water
mark, clothed with vegetation, and by digging into it, pebbles encrusted
with Serpulae were found: along the banks, also, of the river Derwent, I
found a bed of broken sea-shells above the surface of the river, and at a
point where the water is now much too fresh for sea-shells to live; but in
both these cases, it is just possible, that before certain spits of sand
and banks of mud in Storm Bay were accumulated, the tides might have risen
to the height where we now find the shells. ( It would appear that some
changes are now in progress in Ralph Bay, for I was assured by an
intelligent farmer, that oysters were formerly abundant in it, but that
about the year 1834 they had, without any apparent cause, disappeared. In
the "Transactions of the Maryland Academy" volume 1 part 1 page 28 there is
an account by Mr. Ducatel of vast beds of oysters and clams having been
destroyed by the gradual filling up of the shallow lagoons and channels, on
the shores of the southern United States. At Chiloe, in South America, I
heard of a similar loss, sustained by the inhabitants, in the disappearance
from one part of the coast of an edible species of Ascidia.)
Evidence more or less distinct of a change of level between the land and
water, has been detected on almost all the land on this side of the globe.
Captain Grey, and other travellers, have found in Southern Australia
upraised shells, belonging either to the recent, or to a late tertiary
period. The French naturalists in Baudin's expedition, found shells
similarly circumstanced on the S.W. coast of Australia. The Rev. W.B.
Clarke finds proofs of the elevation of the land, to the amount of 400
feet, at the Cape of Good Hope. ("Proceedings of the Geological Society"
volume 3 page 420.) In the neighbourhood of the Bay of Islands in New
Zealand, I observed that the shores were scattered to some height, as at
Van Diemen's Land, with sea-shells, which the colonists attribute to the
natives. (I will here give a catalogue of the rocks which I met with near
the Bay of Islands, in New Zealand:--1st, Much basaltic lava, and scoriform
rocks, forming distinct craters;--2nd, A castellated hill of horizontal
strata of flesh-coloured limestone, showing when fractured distinct
crystalline facets: the rain has acted on this rock in a remarkable manner,
corroding its surface into a miniature model of an Alpine country: I
observed here layers of chert and clay ironstone; and in the bed of a
stream, pebbles of clay-slate;--3rd, The shores of the Bay of Islands are
formed of a feldspathic rock, of a bluish-grey colour, often much
decomposed, with an angular fracture, and crossed by numerous ferruginous
seams, but without any distinct stratification or cleavage. Some varieties
are highly crystalline, and would at once be pronounced to be trap; others
strikingly resembled clay-slate, slightly altered by heat: I was unable to
form any decided opinion on this formation.) Whatever may have been the
origin of these shells, I cannot doubt, after having seen a section of the
valley of the Thames River (37 degrees S.), drawn by the Rev. W. Williams,
that the land has been there elevated: on the opposite sides of this great
valley, three step-like terraces, composed of an enormous accumulation of
rounded pebbles, exactly correspond with each other: the escarpment of each
terrace is about fifty feet in height. No one after having examined the
terraces in the valleys on the western shores of South America, which are
strewed with sea-shells, and have been formed during intervals of rest in
the slow elevation of the land, could doubt that the New Zealand terraces
have been similarly formed. I may add, that Dr. Dieffenbach, in his
description of the Chatham Islands ("Geographical Journal" volume 11 pages
202, 205.) (S.W. of New Zealand), states that it is manifest "that the sea
has left many places bare which were once covered by its waters."
KING GEORGE'S SOUND.
This settlement is situated at the south-western angle of the Australian
continent: the whole country is granitic, with the constituent minerals
sometimes obscurely arranged in straight or curved laminae. In these cases,
the rock would be called by Humboldt, gneiss-granite, and it is remarkable
that the form of the bare conical hills, appearing to be composed of great
folding layers, strikingly resembles, on a small scale, those composed of
gneiss-granite at Rio de Janeiro, and those described by Humboldt at
Venezuela. These plutonic rocks are, in many places, intersected by
trappean-dikes; in one place, I found ten parallel dikes ranging in an E.
and W. line; and not far off another set of eight dikes, composed of a
different variety of trap, ranging at right angles to the former ones. I
have observed in several primary districts, the occurrence of systems of
dikes parallel and close to each other.
SUPERFICIAL FERRUGINOUS BEDS.
The lower parts of the country are everywhere covered by a bed, following
the inequalities of the surface, of a honeycombed sandstone, abounding with
oxides of iron. Beds of nearly similar composition are common, I believe,
along the whole western coast of Australia, and on many of the East Indian
islands. At the Cape of Good Hope, at the base of the mountains formed of
granite and capped with sandstone, the ground is everywhere coated either
by a fine-grained, rubbly, ochraceous mass, like that at King George's
Sound, or by a coarser sandstone with fragments of quartz, and rendered
hard and heavy by an abundance of the hydrate of iron, which presents, when
freshly broken, a metallic lustre. Both these varieties have a very
irregular texture, including spaces either rounded or angular, full of
loose sand: from this cause the surface is always honeycombed. The oxide of
iron is most abundant on the edges of the cavities, where alone it affords
a metallic fracture. In these formations, as well as in many true
sedimentary deposits, it is evident that iron tends to become aggregated,
either in the form of a shell, or of a network. The origin of these
superficial beds, though sufficiently obscure, seems to be due to alluvial
action on detritus abounding with iron.
SUPERFICIAL CALCAREOUS DEPOSIT.
A calcareous deposit on the summit of Bald Head, containing branched
bodies, supposed by some authors to have been corals, has been celebrated
by the descriptions of many distinguished voyagers. (I visited this hill,
in company with Captain Fitzroy, and we came to a similar conclusion
regarding these branching bodies.) It folds round and conceals irregular
hummocks of granite, at the height of 600 feet above the level of the sea.
It varies much in thickness; where stratified, the beds are often inclined
at high angles, even as much as at thirty degrees, and they dip in all
directions. These beds are sometimes crossed by oblique and even-sided
laminae. The deposit consists either of a fine, white calcareous powder, in
which not a trace of structure can be discovered, or of exceedingly minute,
rounded grains, of brown, yellowish, and purplish colours; both varieties
being generally, but not always, mixed with small particles of quartz, and
being cemented into a more or less perfect stone. The rounded calcareous
grains, when heated in a slight degree, instantly lose their colours; in
this and in every other respect, closely resembling those minute, equal-
sized particles of shells and corals, which at St. Helena have been drifted
up the side of the mountains, and have thus been winnowed of all coarser
fragments. I cannot doubt that the coloured calcareous particles here have
had a similar origin. The impalpable powder has probably been derived from
the decay of the rounded particles; this certainly is possible, for on the
coast of Peru, I have traced LARGE UNBROKEN shells gradually falling into a
substance as fine as powdered chalk. Both of the above-mentioned varieties
of calcareous sandstone frequently alternate with, and blend into, thin
layers of a hard substalagmitic rock, which, even when the stone on each
side contains particles of quartz, is entirely free from them (I adopt this
term from Lieutenant Nelson's excellent paper on the Bermuda Islands
"Geolog. Trans." volume 5 page 106, for the hard, compact, cream- or brown-
coloured stone, without any crystalline structure, which so often
accompanies superficial calcareous accumulations. I have observed such
superficial beds, coated with substalagmitic rock, at the Cape of Good
Hope, in several parts of Chile, and over wide spaces in La Plata and
Patagonia. Some of these beds have been formed from decayed shells, but the
origin of the greater number is sufficiently obscure. The causes which
determine water to dissolve lime, and then soon to redeposit it, are not, I
think, known. The surface of the substalagmitic layers appears always to be
corroded by the rain-water. As all the above-mentioned countries have a
long dry season, compared with the rainy one, I should have thought that
the presence of the substalagmitic was connected with the climate, had not
Lieutenant Nelson found this substance forming under sea-water.
Disintegrated shell seems to be extremely soluble; of which I found good
evidence, in a curious rock at Coquimbo in Chile, which consisted of small,
pellucid, empty husks, cemented together. A series of specimens clearly
showed that these husks had originally contained small rounded particles of
shells, which had been enveloped and cemented together by calcareous matter
(as often happens on sea-beaches), and which subsequently had decayed, and
been dissolved by water, that must have penetrated through the calcareous
husks, without corroding them,--of which processes every stage could be
seen.): hence we must suppose that these layers, as well as certain vein-
like masses, have been formed by rain dissolving the calcareous matter and
re-precipitating it, as has happened at St. Helena. Each layer probably
marks a fresh surface, when the, now firmly cemented, particles existed as
loose sand. These layers are sometimes brecciated and re-cemented, as if
they had been broken by the slipping of the sand when soft. I did not find
a single fragment of a sea-shell; but bleached shells of the Helix melo, an
existing land species, abound in all the strata; and I likewise found
another Helix, and the case of an Oniscus.
The branches are absolutely undistinguishable in shape from the broken and
upright stumps of a thicket; their roots are often uncovered, and are seen
to diverge on all sides; here and there a branch lies prostrate. The
branches generally consist of the sandstone, rather firmer than the
surrounding matter, with the central parts filled, either with friable,
calcareous matter, or with a substalagmitic variety; this central part is
also frequently penetrated by linear crevices, sometimes, though rarely,
containing a trace of woody matter. These calcareous, branching bodies,
appear to have been formed by fine calcareous matter being washed into the
casts or cavities, left by the decay of branches and roots of thickets,
buried under drifted sand. The whole surface of the hill is now undergoing
disintegration, and hence the casts, which are compact and hard, are left
projecting. In calcareous sand at the Cape of Good Hope, I found the casts,
described by Abel, quite similar to these at Bald Head; but their centres
are often filled with black carbonaceous matter not yet removed. It is not
surprising, that the woody matter should have been almost entirely removed
from the casts on Bald Head; for it is certain, that many centuries must
have elapsed since the thickets were buried; at present, owing to the form
and height of the narrow promontory, no sand is drifted up, and the whole
surface, as I have remarked, is wearing away. We must, therefore, look back
to a period when the land stood lower, of which the French naturalists (See
M. Peron "Voyage" tome 1 page 204.) found evidence in upraised shells of
recent species, for the drifting on Bald Head of the calcareous and
quartzose sand, and the consequent embedment of the vegetable remains.
There was only one appearance which at first made me doubt concerning the
origin of the cast,--namely, that the finer roots from different stems
sometimes became united together into upright plates or veins; but when the
manner is borne in mind in which fine roots often fill up cracks in hard
earth, and that these roots would decay and leave hollows, as well as the
stems, there is no real difficulty in this case. Besides the calcareous
branches from the Cape of Good Hope, I have seen casts, of exactly the same
forms, from Madeira* and from Bermuda; at this latter place, the
surrounding calcareous rocks, judging from the specimens collected by
Lieutenant Nelson, are likewise similar, as is their subaerial formation.
Reflecting on the stratification of the deposit on Bald Head,--on the
irregularly alternating layers of substalagmitic rock,--on the uniformly
sized, and rounded particles, apparently of sea-shells and corals,--on the
abundance of land-shells throughout the mass,--and finally, on the absolute
resemblance of the calcareous casts, to the stumps, roots, and branches of
that kind of vegetation, which would grow on sand-hillocks, I think there
can be no reasonable doubt, notwithstanding the different opinion of some
authors, that a true view of their origin has been here given.
*(Dr. J. Macaulay has fully described ("Edinb. New Phil. Journ." volume 29
page 350) the casts from Madeira. He considers (differently from Mr. Smith
of Jordan Hill) these bodies to be corals, and the calcareous deposit to be
of subaqueous origin. His arguments chiefly rest (for his remarks on their
structure are vague) on the great quantity of the calcareous matter, and on
the casts containing animal matter, as shown by their evolving ammonia. Had
Dr. Macaulay seen the enormous masses of rolled particles of shells and
corals on the beach of Ascension, and especially on coral-reefs; and had he
reflected on the effects of long-continued, gentle winds, in drifting up
the finer particles, he would hardly have advanced the argument of
quantity, which is seldom trustworthy in geology. If the calcareous matter
has originated from disintegrated shells and corals, the presence of animal
matter is what might have been expected. Mr. Anderson analysed for Dr.
Macaulay part of a cast, and he found it composed of:--
Carbonate of lime......73.15
Silica.................11.90
Phosphate of lime.......8.81
Animal matter...........4.25
Sulphate of lime......a trace
98.11)
Calcareous deposits, like these of King George's Sound, are of vast extent
on the Australian shores. Dr. Fitton remarks, that "recent calcareous
breccia (by which term all these deposits are included) was found during
Baudin's voyage, over a space of no less than twenty-five degrees of
latitude and an equal extent of longitude, on the southern, western, and
north-western coasts." (For ample details on this formation consult Dr.
Fitton "Appendix to Captain King's Voyage." Dr. Fitton is inclined to
attribute a concretionary origin to the branching bodies: I may remark,
that I have seen in beds of sand in La Plata cylindrical stems which no
doubt thus originated; but they differed much in appearance from these at
Bald Head, and the other places above specified.) It appears also from M.
Peron, with whose observations and opinions on the origin of the calcareous
matter and branching casts mine entirely accord, that the deposit is
generally much more continuous than near King George's Sound. At Swan
River, Archdeacon Scott states that in one part it extends ten miles
inland. ("Proceedings of the Geolog. Soc." volume 1 page 320.) Captain
Wickham, moreover, informs me that during his late survey of the western
coast, the bottom of the sea, wherever the vessel anchored, was
ascertained, by crowbars being let down, to consist of white calcareous
matter. Hence it seems that along this coast, as at Bermuda and at Keeling
Atoll, submarine and subaerial deposits are contemporaneously in process of
formation, from the disintegration of marine organic bodies. The extent of
these deposits, considering their origin, is very striking; and they can be
compared in this respect only with the great coral-reefs of the Indian and
Pacific Oceans. In other parts of the world, for instance in South America,
there are SUPERFICIAL calcareous deposits of great extent, in which not a
trace of organic structure is discoverable; these observations would lead
to the inquiry, whether such deposits may not, also, have been formed from
disintegrated shells and corals.
CAPE OF GOOD HOPE.
After the accounts given by Barrow, Carmichael, Basil Hall, and W.B. Clarke
of the geology of this district, I shall confine myself to a few
observations on the junction of the three principal formations. The
fundamental rock is granite (In several places I observed in the granite,
small dark-coloured balls, composed of minute scales of black mica in a
tough basis. In another place, I found crystals of black schorl radiating
from a common centre. Dr. Andrew Smith found, in the interior parts of the
country, some beautiful specimens of granite, with silvery mica radiating
or rather branching, like moss, from central points. At the Geological
Society, there are specimens of granite with crystallised feldspar
branching and radiating in like manner.), overlaid by clay-slate: the
latter is generally hard, and glossy from containing minute scales of mica;
it alternates with, and passes into, beds of slightly crystalline,
feldspathic, slaty rock. This clay-slate is remarkable from being in some
places (as on the Lion's Rump) decomposed, even to the depth of twenty
feet, into a pale-coloured, sandstone-like rock, which has been mistaken, I
believe, by some observers, for a separate formation. I was guided by Dr.
Andrew Smith to a fine junction at Green Point between the granite and
clay-slate: the latter at the distance of a quarter of a mile from the
spot, where the granite appears on the beach (though, probably, the granite
is much nearer underground), becomes slightly more compact and crystalline.
At a less distance, some of the beds of clay-slate are of a homogeneous
texture, and obscurely striped with different zones of colour, whilst
others are obscurely spotted. Within a hundred yards of the first vein of
granite, the clay-slate consists of several varieties; some compact with a
tinge of purple, others glistening with numerous minute scales of mica and
imperfectly crystallised feldspar; some obscurely granular, others
porphyritic with small, elongated spots of a soft white mineral, which
being easily corroded, gives to this variety a vesicular appearance. Close
to the granite, the clay-slate is changed into a dark-coloured, laminated
rock, having a granular fracture, which is due to imperfect crystals of
feldspar, coated by minute, brilliant scales of mica.
The actual junction between the granitic and clay-slate districts extends
over a width of about two hundred yards, and consists of irregular masses
and of numerous dikes of granite, entangled and surrounded by the clay-
slate: most of the dikes range in a N.W. and S.E. line, parallel to the
cleavage of the slate. As we leave the junction, thin beds, and lastly,
mere films of the altered clay-slate are seen, quite isolated, as if
floating, in the coarsely crystallised granite; but although completely
detached, they all retain traces of the uniform N.W. and S.E. cleavage.
This fact has been observed in other similar cases, and has been advanced
by some eminent geologists (See M. Keilhau "Theory on Granite" translated
in the "Edinburgh New Philosophical Journal" volume 24 page 402.), as a
great difficulty on the ordinary theory, of granite having been injected
whilst liquified; but if we reflect on the probable state of the lower
surface of a laminated mass, like clay-slate, after having been violently
arched by a body of molten granite, we may conclude that it would be full
of fissures parallel to the planes of cleavage; and that these would be
filled with granite, so that wherever the fissures were close to each
other, mere parting layers or wedges of the slate would depend into the
granite. Should, therefore, the whole body of rock afterwards become worn
down and denuded, the lower ends of these dependent masses or wedges of
slate would be left quite isolated in the granite; yet they would retain
their proper lines of cleavage, from having been united, whilst the granite
was fluid, with a continuous covering of clay-slate.
Following, in company with Dr. A. Smith, the line of junction between the
granite and the slate, as it stretched inland, in a S.E. direction, we came
to a place, where the slate was converted into a fine-grained, perfectly
characterised gneiss, composed of yellow-brown granular feldspar, of
abundant black brilliant mica, and of few and thin laminae of quartz. From
the abundance of the mica in this gneiss, compared with the small quantity
and excessively minute scales, in which it exists in the glossy clay-slate,
we must conclude, that it has been here formed by the metamorphic action--a
circumstance doubted, under nearly similar circumstances, by some authors.
The laminae of the clay-slate are straight; and it was interesting to
observe, that as they assumed the character of gneiss, they became
undulatory with some of the smaller flexures angular, like the laminae of
many true metamorphic schists.
SANDSTONE FORMATION.
This formation makes the most imposing feature in the geology of Southern
Africa. The strata are in many parts horizontal, and attain a thickness of
about two thousand feet. The sandstone varies in character; it contains
little earthy matter, but is often stained with iron; some of the beds are
very fine-grained and quite white; others are as compact and homogeneous as
quartz rock. In some places I observed a breccia of quartz, with the
fragments almost dissolved in a siliceous paste. Broad veins of quartz,
often including large and perfect crystals, are very numerous; and it is
evident in nearly all the strata, that silica has been deposited from
solution in remarkable quantity. Many of the varieties of quartzite
appeared quite like metamorphic rocks; but from the upper strata being as
siliceous as the lower, and from the undisturbed junctions with the
granite, which in many places can be examined, I can hardly believe that
these sandstone-strata have been exposed to heat. (The Rev. W.B. Clarke,
however, states, to my surprise ("Geolog. Proceedings" volume 3 page 422),
that the sandstone in some parts is penetrated by granitic dikes: such
dikes must belong to an epoch altogether subsequent to that when the molten
granite acted on the clay-slate.) On the lines of junction between these
two great formations, I found in several places the granite decayed to the
depth of a few inches, and succeeded, either by a thin layer of ferruginous
shale, or by four or five inches in thickness of the re-cemented crystals
of the granite, on which the great pile of sandstone immediately rested.
Mr. Schomburgk has described ("Geographical Journal" volume 10 page 246.) a
great sandstone formation in Northern Brazil, resting on granite, and
resembling to a remarkable degree, in composition and in the external form
of the land, this formation of the Cape of Good Hope. The sandstones of the
great platforms of Eastern Australia, which also rest on granite, differ in
containing more earthy and less siliceous matter. No fossil remains have
been discovered in these three vast deposits. Finally, I may add that I did
not see any boulders of far-transported rocks at the Cape of Good Hope, or
on the eastern and western shores of Australia, or at Van Diemen's Land. In
the northern island of New Zealand, I noticed some large blocks of
greenstone, but whether their parent rock was far distant, I had no
opportunity of determining.
INDEX TO VOLCANIC ISLANDS.
Abel, M., on calcareous casts at the Cape of Good Hope.
Abingdon island.
Abrolhos islands, incrustation on.
Aeriform explosions at Ascension.
Albatross, driven from St. Helena.
Albemarle island.
Albite, at the Galapagos archipelago.
Amygdaloidal cells, half filled.
Amygdaloids, calcareous origin of.
Ascension, arborescent incrustation on rocks of.
-absence of dikes, freedom from volcanic action, and state of lava-streams.
Ascidia, extinction of.
Atlantic Ocean, new volcanic focus in.
Augite, fused.
Australia.
Azores.
Bahia in Brazil, dikes at.
Bailly, M., on the mountains of Mauritius.
Bald Head.
Banks' Cove.
Barn, The, St. Helena.
Basalt, specific gravity of.
Basaltic coast-mountains at Mauritius.
-at St. Helena.
-at St. Jago.
Beaumont, M. Elie de, on circular subsidences in lava.
-on dikes indicating elevation.
-on inclination of lava-streams.
-on laminated dikes.
Bermuda, calcareous rocks of.
Beudant, M., on bombs.
-on jasper.
-on laminated trachyte.
-on obsidian of Hungary.
-on silex in trachyte.
Bole.
Bombs, volcanic.
Bory St. Vincent, on bombs.
Boulders, absence in Australia and Cape of Good Hope.
Brattle island.
Brewster, Sir D., on a calcareo-animal substance.
-on decomposed glass.
Brown, Mr. R., on extinct plants from Van Diemen's land.
-on sphaerulitic bodies in silicified wood.
Buch, Von, on cavernous lava.
-on central volcanoes.
-on crystals sinking in obsidian.
-on laminated lava.
-on obsidian streams.
-on olivine in basalt.
-on superficial calcareous beds in the Canary islands.
Calcareous deposit at St. Jago affected by heat.
-fibrous matter, entangled in streaks in scoriae.
-freestone at Ascension.
-incrustations at Ascension.
-sandstone at St. Helena.
-superficial beds at King George's sound.
Cape of Good Hope.
Carbonic acid, expulsion of, by heat.
Carmichael, Capt., on glassy coatings to dikes.
Casts, calcareous, of branches.
Chalcedonic nodules.
Chalcedony in basalt and in silicified wood.
Chatham island.
Chlorophaeite.
Clarke, Rev. W., on the Cape of Good Hope.
Clay-slate, its decomposition and junction with granite at the Cape of Good
Hope.
Cleavage of clay-slate in Australia.
Cleavage, cross, in sandstone.
Coast denudation at St. Helena.
Columnar basalt.
"Comptes Rendus," account of volcanic phenomena in the Atlantic.
Concepcion, earthquake of.
Concretions in aqueous and igneous rocks compared.
-in tuff.
-of obsidian.
Conglomerate, recent, at St. Jago.
Coquimbo, curious rock of.
Corals, fossil, from Van Diemen's Land.
Crater, segment of, at the Galapagos.
-great central one at St. Helena.
-internal ledges round, and parapet on.
Craters, basaltic, at Ascension.
-form of, affected by the trade wind.
-of elevation.
-of tuff at Terceira.
-of tuff at the Galapagos archipelago.
-their breached state.
-small basaltic at St. Jago.
--at the Galapagos archipelago.
Crystallisation favoured by space.
Dartigues, M., on sphaerulites.
Daubeny, Dr., on a basin-formed island.
-on fragments in trachyte.
D'Aubuisson on hills of phonolite.
-on the composition of obsidian.
-on the lamination of clay-slate.
De la Beche, Sir H., on magnesia in erupted lime.
-on specific gravity of limestones.
Denudation of coast at St. Helena.
Diana's Peak, St. Helena.
Dieffenbach, Dr., on the Chatham Islands.
Dikes, truncated, on central crateriform ridge of St. Helena.
-at St. Helena; number of; coated by a glossy layer; uniform thickness of.
-great parallel ones at St. Helena.
-not observed at Ascension.
-of tuff.
-of trap in the plutonic series.
-remnants of, extending far into the sea round St. Helena.
Dislocations at Ascension.
-at St. Helena.
Distribution of volcanic islands.
Dolomieu, on decomposed trachyte.
-on laminated lava.
-on obsidian.
Dree, M., on crystals sinking in lava.
Dufrenoy, M., on the composition of the surface of certain lava-streams.
-on the inclination of tuff-strata.
Eggs of birds embedded at St. Helena.
-of turtle at Ascension.
Ejected fragments at Ascension.
-at the Galapagos archipelago.
Elevation of St. Helena.
-the Galapagos archipelago.
-Van Diemen's Land, Cape of Good Hope, New Zealand, Australia, and Chatham
island.
-of volcanic islands.
Ellis, Rev. W., on ledges within the great crater at Hawaii.
-on marine remains at Otaheite.
Eruption, fissures of.
Extinction of land-shells at St. Helena.
Faraday, Mr., on the expulsion of carbonic acid gas.
Feldspar, fusibility of.
-in radiating crystals.
-Labrador, ejected.
Feldspathic lavas.
-at St. Helena.
-rock, alternating with obsidian.
-lamination, and origin of.
Fernando Noronha.
Ferruginous superficial beds.
Fibrous calcareous matter at St. Jago.
Fissures of eruption.
Fitton, Dr., on calcareous breccia.
Flagstaff Hill, St. Helena.
Fleurian de Bellevue on sphaerulites.
Fluidity of lavas.
Forbes, Professor, on the structure of glaciers.
Fragments ejected at Ascension.
-at the Galapagos archipelago.
Freshwater Bay.
Fuerteventura (Feurteventura), calcareous beds of.
Galapagos archipelago.
-parapets round craters.
Gay Lussac, on the expulsion of carbonic acid gas.
Glaciers, their structure.
Glossiness of texture, origin of.
Gneiss, derived from clay-slate.
-with a great embedded fragment.
Gneiss-granite, form of hills of.
Good Hope, Cape of.
Gorges, narrow, at St. Helena.
Granite, junction with clay-slate, at the Cape of Good Hope.
Granitic ejected fragments.
Gravity, specific, of lavas.
Gypsum, at Ascension.
-in volcanic strata at St. Helena.
-on surface of the ground at ditto.
Hall, Sir J., on the expulsion of carbonic acid gas.
Heat, action of, on calcareous matter.
Hennah, Mr., on ashes at Ascension.
Henslow, Prof., on chalcedony.
Hoffmann, on decomposed trachyte.
Holland, Dr., on Iceland.
Horner, Mr., on a calcareo-animal substance.
-on fusibility of feldspar.
Hubbard, Dr., on dikes.
Humboldt on ejected fragments.
-on obsidian formations.
-on parapets round craters.
-on sphaerulites.
Hutton on amygdaloids.
Hyalite in decomposed trachyte.
Iceland, stratification of the circumferential hills.
Islands, volcanic, distribution of.
-their elevation.
Incrustation, on St. Paul's rocks.
Incrustations, calcareous, at Ascension.
Jago, St.
James island.
Jasper, origin of.
Jonnes, M. Moreau de, on craters affected by wind.
Juan Fernandez.
Keilhau, M., on granite.
Kicker Rock.
King George's sound.
Labrador feldspar, ejected.
Lakes at bases of volcanoes.
Lamination of volcanic rocks.
Land-shells, extinct, at St. Helena.
Lanzarote, calcareous beds of.
Lava, adhesion to sides of a gorge.
-feldspathic.
-with cells semi-amygdaloidal.
Lavas, specific gravity of.
Lava-streams blending together at St. Jago.
-composition of surface of.
-differences in the state of their surfaces.
-extreme thinness of.
-heaved up into hillocks at the Galapagos archipelago.
-their fluidity.
-with irregular hummocks at Ascension.
Lead, separation from silver.
Lesson, M., on craters at Ascension.
Leucite.
Lime, sulphate of, at Ascension.
Lonsdale, Mr., on fossil-corals from Van Diemen's land.
Lot, St. Helena.
Lyell, Mr., on craters of elevation.
-on embedded turtles' eggs.
-on glossy coating to dikes.
Macaulay, Dr., on calcareous casts at Madeira.
MacCulloch, Dr., on an amygdaloid.
-on chlorophaeite.
-on laminated pitchstone.
Mackenzie, Sir G., on cavernous lava-streams.
-on glossy coatings to dikes.
-on obsidian streams.
-on stratification in Iceland.
Madeira, calcareous casts at.
"Magazine, Nautical," account of volcanic phenomena in the Atlantic.
Marekanite.
Mauritius, crater of elevation of.
Mica, in rounded nodules.
-origin in metamorphic slate.
-radiating form of.
Miller, Prof., on ejected Labrador feldspar.
-on quartz crystals in obsidian beds.
Mitchell, Sir T., on bombs.
-on the Australian valleys.
Mud streams at the Galapagos archipelago.
Narborough island.
Nelson, Lieut., on the Bermuda islands.
New Caledonia.
New Red sandstone, cross cleavage of.
New South Wales.
New Zealand.
Nulliporae (fossil), resembling concretions.
Obsidian, absent at the Galapagos archipelago.
-bombs of.
-composition and origin of.
-crystals of feldspar sink in.
-its irruption from lofty craters.
-passage of beds into.
-specific gravity of.
-streams of.
Olivine decomposed at St. Jago.
-at Van Diemen's land.
-in the lavas at the Galapagos archipelago.
Oolitic structure of recent calcareous beds at St. Helena.
Otaheite.
Oysters, extinction of.
Panza islands, laminated trachyte of.
Pattinson, Mr., on the separation of lead and silver.
Paul's, St., rocks of.
Pearlstone.
Peperino.
Peron, M., on calcareous rocks of Australia.
Phonolite, hills of.
-laminated.
-with more fusible hornblende.
Pitchstone.
-dikes of.
Plants, extinct.
Plutonic rocks, separation of constituent parts of, by gravity.
Porto Praya.
Prevost, M. C., on rarity of great dislocations in volcanic islands.
Prosperous hill, St. Helena.
Pumice, absent at the Galapagos archipelago.
-laminated.
Puy de Dome, trachyte of.
Quail island, St. Jago.
Quartz, crystals of, in beds alternating with obsidian.
-crystallised in sandstone.
-fusibility of.
-rock, mottled from metamorphic action with earthy matter.
Red hill.
Resin-like altered scoriae.
Rio de Janeiro, gneiss of.
Robert, M., on strata of Iceland.
Rogers, Professor, on curved lines of elevation.
Salses, compared with tuff craters.
Salt deposited by the sea.
-in volcanic strata.
-lakes of, in craters.
Sandstone of Brazil.
-of the Cape of Good Hope.
-platforms of, in New South Wales.
Schorl, radiating.
Scrope, Mr. P., on laminated trachyte.
-on obsidian.
-on separation of trachyte and basalt.
-on silex in trachyte.
-on sphaerulites.
Seale, Mr., geognosy of St. Helena.
-on dikes.
-on embedded birds' bones.
Seale, on extinct shells of St. Helena.
Sedgwick, Professor, on concretions.
Septaria, in concretions in tuff.
Serpulae on upraised rocks.
Seychelles.
Shells, colour of, affected by light.
-from Van Diemen's land.
-land, extinct, at St. Helena.
-particles of, drifted by the wind at St. Helena.
Shelly matter deposited by the waves.
Siau, M., on ripples.
Signal Post Hill.
Silica, deposited by steam.
-large proportion of, in obsidian.
-specific gravity of.
Siliceous sinter.
Smith, Dr. A., on junction of granite and clay-slate.
Spallanzani on decomposed trachyte.
Specific gravity of recent calcareous rocks and of limestone.
-of lavas.
Sphaerulites in glass and in silicified wood.
-in obsidian.
Sowerby, Mr. G.B., on fossil-shells from Van Diemen's land.
-from St. Jago.
-land-shells from St. Helena.
St. Helena.
-crater of elevation of.
St. Jago, crater of elevation of.
-effects of calcareous matter on lava.
St. Paul's rocks.
Stokes, Mr., collections of sphaerulites and of obsidians.
Stony-top, Little.
-Great.
Stratification of sandstone in New South Wales.
Streams of obsidian.
Stutchbury, Mr., on marine remains at Otaheite.
Subsided space at Ascension.
Tahiti.
Talus, stratified, within tuff craters.
Terceira.
Tertiary deposit of St. Jago.
Trachyte, absent at the Galapagos archipelago.
-at Ascension.
-at Terceira.
-decomposition of, by steam.
-its lamination.
-its separation from basalt.
-softened at Ascension.
-specific gravity of.
-with singular veins.
Trap-dikes in the plutonic series.
-at King George's sound.
Travertin at Van Diemen's land.
Tropic-bird, now rare, at St. Helena.
Tuff, craters of.
-their breached state.
-peculiar kind of.
Turner, Mr., on the separation of molten metals.
Tyerman and Bennett on marine remains at Huaheine.
Valleys, gorge-like, at St. Helena.
-in New South Wales.
-in St. Jago.
Van Diemen's land.
Veins in trachyte.
-of jasper.
Vincent, Bory St., on bombs.
Volcanic bombs.
-island in process of formation in the Atlantic.
-islands, their distribution.
Wacke, its passage into lava.
Wackes, argillaceous.
Webster, Dr., on a basin-formed island.
-on gypsum at Ascension.
White, Martin, on soundings.
Wind, effects of, on the form of craters.
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