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.