GALAPAGOS ARCHIPELAGO.
Chatham Island. Craters composed of a peculiar kind of tuff. Small
basaltic craters, with hollows at their bases. Albemarle Island; fluid
lavas, their composition. Craters of tuff; inclination of their
exterior diverging strata, and structure of their interior converging
strata. James Island, segment of a small basaltic crater; fluidity and
composition of its lava-streams, and of its ejected fragments.
Concluding remarks on the craters of tuff, and on the breached
condition of their southern sides. Mineralogical composition of the
rocks of the archipelago. Elevation of the land. Direction of the
fissures of eruption.
(FIGURE 11. MAP 3. GALAPAGOS ARCHIPELAGO.
Showing Wenman, Abingdon, Bindloes, Tower, Narborough, Albemarle,
James, Indefatigable, Barrington, Chatham, Charles and Hood’s Islands.)
This archipelago is situated under the equator, at a distance of
between five and six hundred miles from the west coast of South
America. It consists of five principal islands, and of several small
ones, which together are equal in area, but not in extent of land, to
Sicily, conjointly with the Ionian Islands. (I exclude from this
measurement, the small volcanic islands of Culpepper and Wenman, lying
seventy miles northward of the group. Craters were visible on all the
islands of the group, except on Towers Island, which is one of the
lowest; this island is, however, formed of volcanic rocks.) They are
all volcanic: on two, craters have been seen in eruption, and on
several of the other islands, streams of lava have a recent appearance.
The larger islands are chiefly composed of solid rock, and they rise
with a tame outline to a height of between one and four thousand feet.
They are sometimes, but not generally, surmounted by one principal
orifice. The craters vary in size from mere spiracles to huge caldrons
several miles in circumference; they are extraordinarily numerous, so
that I should think, if enumerated, they would be found to exceed two
thousand; they are formed either of scoriae and lava, or of a
brown-coloured tuff; and these latter craters are in several respects
remarkable. The whole group was surveyed by the officers of the
“Beagle.” I visited myself four of the principal islands, and received
specimens from all the others. Under the head of the different islands
I will describe only that which appears to me deserving of attention.
CHATHAM ISLAND. CRATERS COMPOSED OF A SINGULAR KIND OF TUFF.
Towards the eastern end of this island there occur two craters composed
of two kinds of tuff; one kind being friable, like slightly
consolidated ashes; and the other compact, and of a different nature
from anything which I have met with described. This latter substance,
where it is best characterised, is of a yellowish-brown colour,
translucent, and with a lustre somewhat resembling resin; it is
brittle, with an angular, rough, and very irregular fracture,
sometimes, however, being slightly granular, and even obscurely
crystalline: it can readily be scratched with a knife, yet some points
are hard enough just to mark common glass; it fuses with ease into a
blackish-green glass. The mass contains numerous broken crystals of
olivine and augite, and small particles of black and brown scoriae; it
is often traversed by thin seams of calcareous matter. It generally
affects a nodular or concretionary structure. In a hand specimen, this
substance would certainly be mistaken for a pale and peculiar variety
of pitchstone; but when seen in mass its stratification, and the
numerous layers of fragments of basalt, both angular and rounded, at
once render its subaqueous origin evident. An examination of a series
of specimens shows that this resin-like substance results from a
chemical change on small particles of pale and dark-coloured
scoriaceous rocks; and this change could be distinctly traced in
different stages round the edges of even the same particle. The
position near the coast of all the craters composed of this kind of
tuff or peperino, and their breached condition, renders it probable
that they were all formed when standing immersed in the sea;
considering this circumstance, together with the remarkable absence of
large beds of ashes in the whole archipelago, I think it highly
probable that much the greater part of the tuff has originated from the
trituration of fragments of the grey, basaltic lavas in the mouths of
craters standing in the sea. It may be asked whether the heated water
within these craters has produced this singular change in the small
scoriaceous particles and given to them their translucent, resin-like
fracture. Or has the associated lime played any part in this change? I
ask these questions from having found at St. Jago, in the Cape de Verde
Islands, that where a great stream of molten lava has flowed over a
calcareous bottom into the sea, the outermost film, which in other
parts resembles pitchstone, is changed, apparently by its contact with
the carbonate of lime, into a resin-like substance, precisely like the
best characterised specimens of the tuff from this archipelago. (The
concretions containing lime, which I have described at Ascension, as
formed in a bed of ashes, present some degree of resemblance to this
substance, but they have not a resinous fracture. At St. Helena, also,
I found veins of a somewhat similar, compact, but non- resinous
substance, occurring in a bed of pumiceous ashes, apparently free from
calcareous matter: in neither of these cases could heat have acted.)
To return to the two craters: one of them stands at the distance of a
league from the coast, the intervening tract consisting of a calcareous
tuff, apparently of submarine origin. This crater consists of a circle
of hills some of which stand quite detached, but all have a very
regular, qua- qua versal dip, at an inclination of between thirty and
forty degrees. The lower beds, to the thickness of several hundred
feet, consist of the resin- like stone, with embedded fragments of
lava. The upper beds, which are between thirty and forty feet in
thickness, are composed of a thinly stratified, fine-grained, harsh,
friable, brown-coloured tuff, or peperino. (Those geologists who
restrict the term of “tuff” to ashes of a white colour, resulting from
the attrition of feldspathic lavas, would call these brown-coloured
strata “peperino.”) A central mass without any stratification, which
must formerly have occupied the hollow of the crater, but is now
attached only to a few of the circumferential hills, consists of a
tuff, intermediate in character between that with a resin-like, and
that with an earthy fracture. This mass contains white calcareous
matter in small patches. The second crater (520 feet in height) must
have existed until the eruption of a recent, great stream of lava, as a
separate islet; a fine section, worn by the sea, shows a grand
funnel-shaped mass of basalt, surrounded by steep, sloping flanks of
tuff, having in parts an earthy, and in others a semi-resinous
fracture. The tuff is traversed by several broad, vertical dikes, with
smooth and parallel sides, which I did not doubt were formed of basalt,
until I actually broke off fragments. These dikes, however, consist of
tuff like that of the surrounding strata, but more compact, and with a
smoother fracture; hence we must conclude, that fissures were formed
and filled up with the finer mud or tuff from the crater, before its
interior was occupied, as it now is, by a solidified pool of basalt.
Other fissures have been subsequently formed, parallel to these
singular dikes, and are merely filled with loose rubbish. The change
from ordinary scoriaceous particles to the substance with a
semi-resinous fracture, could be clearly followed in portions of the
compact tuff of these dikes.
(FIGURE 12. THE KICKER ROCK, 400 FEET HIGH.)
At the distance of a few miles from these two craters, stands the
Kicker Rock, or islet, remarkable from its singular form. It is
unstratified, and is composed of compact tuff, in parts having the
resin-like fracture. It is probable that this amorphous mass, like that
similar mass in the case first described, once filled up the central
hollow of a crater, and that its flanks, or sloping walls, have since
been worn quite away by the sea, in which it stands exposed.
SMALL BASALTIC CRATERS.
A bare, undulating tract, at the eastern end of Chatham Island, is
remarkable from the number, proximity, and form of the small basaltic
craters with which it is studded. They consist, either of a mere
conical pile, or, but less commonly, of a circle, of black and red,
glossy scoriae, partially cemented together. They vary in diameter from
thirty to one hundred and fifty yards, and rise from about fifty to one
hundred feet above the level of the surrounding plain. From one small
eminence, I counted sixty of these craters, all of which were within a
third of a mile from each other, and many were much closer. I measured
the distance between two very small craters, and found that it was only
thirty yards from the summit-rim of one to the rim of the other. Small
streams of black, basaltic lava, containing olivine and much glassy
feldspar, have flowed from many, but not from all of these craters. The
surfaces of the more recent streams were exceedingly rugged, and were
crossed by great fissures; the older streams were only a little less
rugged; and they were all blended and mingled together in complete
confusion. The different growth, however, of the trees on the streams,
often plainly marked their different ages. Had it not been for this
latter character, the streams could in few cases have been
distinguished; and, consequently, this wide undulatory tract might have
(as probably many tracts have) been erroneously considered as formed by
one great deluge of lava, instead of by a multitude of small streams,
erupted from many small orifices.
In several parts of this tract, and especially at the base of the small
craters, there are circular pits, with perpendicular sides, from twenty
to forty feet deep. At the foot of one small crater, there were three
of these pits. They have probably been formed, by the falling in of the
roofs of small caverns. (M. Elie de Beaumont has described (“Mem. pour
servir” etc. tome 4 page 113) many “petits cirques d’eboulement” on
Etna, of some of which the origin is historically known.) In other
parts, there are mammiform hillocks, which resemble great bubbles of
lava, with their summits fissured by irregular cracks, which appeared,
upon entering them, to be very deep; lava has not flowed from these
hillocks. There are, also, other very regular, mammiform hillocks,
composed of stratified lava, and surmounted by circular, steep-sided
hollows, which, I suppose have been formed by a body of gas, first,
arching the strata into one of the bubble- like hillocks, and then,
blowing off its summit. These several kinds of hillocks and pits, as
well as the numerous, small, scoriaceous craters, all show that this
tract has been penetrated, almost like a sieve, by the passage of
heated vapours. The more regular hillocks could only have been heaved
up, whilst the lava was in a softened state. (Sir G. Mackenzie “Travels
in Iceland” pages 389 to 392, has described a plain of lava at the foot
of Hecla, everywhere heaved up into great bubbles or blisters. Sir
George states that this cavernous lava composes the uppermost stratum;
and the same fact is affirmed by Von Buch “Descript. des Isles
Canaries” page 159, with respect to the basaltic stream near Rialejo,
in Teneriffe. It appears singular that it should be the upper streams
that are chiefly cavernous, for one sees no reason why the upper and
lower should not have been equally affected at different times;—have
the inferior streams flowed beneath the pressure of the sea, and thus
been flattened, after the passage through them, of bodies of gas?)
ALBEMARLE ISLAND.
This island consists of five, great, flat-topped craters, which,
together with the one on the adjoining island of Narborough, singularly
resemble each other, in form and height. The southern one is 4,700 feet
high, two others are 3,720 feet, a third only 50 feet higher, and the
remaining ones apparently of nearly the same height. Three of these are
situated on one line, and their craters appear elongated in nearly the
same direction. The northern crater, which is not the largest, was
found by the triangulation to measure, externally, no less than three
miles and one-eighth of a mile in diameter. Over the lips of these
great, broad caldrons, and from little orifices near their summits,
deluges of black lava have flowed down their naked sides.
FLUIDITY OF DIFFERENT LAVAS.
Near Tagus or Banks’ Cove, I examined one of these great streams of
lava, which is remarkable from the evidence of its former high degree
of fluidity, especially when its composition is considered. Near the
sea-coast this stream is several miles in width. It consists of a
black, compact base, easily fusible into a black bead, with angular and
not very numerous air-cells, and thickly studded with large, fractured
crystals of glassy albite, varying from the tenth of an inch to half an
inch in diameter. (In the Cordillera of Chile, I have seen lava very
closely resembling this variety at the Galapagos Archipelago. It
contained, however, besides the albite, well-formed crystals of augite,
and the base (perhaps in consequence of the aggregation of the augitic
particles) was a shade lighter in colour. I may here remark, that in
all these cases, I call the feldspathic crystals, “albite,” from their
cleavage-planes (as measured by the reflecting goniometer)
corresponding with those of that mineral. As, however, other species of
this genus have lately been discovered to cleave in nearly the same
planes with albite, this determination must be considered as only
provisional. I examined the crystals in the lavas of many different
parts of the Galapagos group, and I found that none of them, with the
exception of some crystals from one part of James Island, cleaved in
the direction of orthite or potash-feldspar.) This lava, although at
first sight appearing eminently porphyritic, cannot properly be
considered so, for the crystals have evidently been enveloped, rounded,
and penetrated by the lava, like fragments of foreign rock in a
trap-dike. This was very clear in some specimens of a similar lava,
from Abingdon Island, in which the only difference was, that the
vesicles were spherical and more numerous. The albite in these lavas is
in a similar condition with the leucite of Vesuvius, and with the
olivine, described by Von Buch, as projecting in great balls from the
basalt of Lanzarote. (“Description des Isles Canaries” page 295.)
Besides the albite, this lava contains scattered grains of a green
mineral, with no distinct cleavage, and closely resembling olivine
(Humboldt mentions that he mistook a green augitic mineral, occurring
in the volcanic rocks of the Cordillera of Quito, for olivine.); but as
it fuses easily into a green glass, it belongs probably to the augitic
family: at James Island, however, a similar lava contained true
olivine. I obtained specimens from the actual surface, and from a depth
of four feet, but they differed in no respect. The high degree of
fluidity of this lava-stream was at once evident, from its smooth and
gently sloping surface, from the manner in which the main stream was
divided by small inequalities into little rills, and especially from
the manner in which its edges, far below its source, and where it must
have been in some degree cooled, thinned out to almost nothing; the
actual margin consisting of loose fragments, few of which were larger
than a man’s head. The contrast between this margin, and the steep
walls, above twenty feet high, bounding many of the basaltic streams at
Ascension, is very remarkable. It has generally been supposed that
lavas abounding with large crystals, and including angular vesicles,
have possessed little fluidity; but we see that the case has been very
different at Albemarle Island. (The irregular and angular form of the
vesicles is probably caused by the unequal yielding of a mass composed,
in almost equal proportion, of solid crystals and of a viscid base. It
certainly seems a general circumstance, as might have been expected,
that in lava, which has possessed a high degree of fluidity, AS WELL AS
AN EVEN-SIZED GRAIN, the vesicles are internally smooth and spherical.)
The degree of fluidity in different lavas, does not seem to correspond
with any APPARENT corresponding amount of difference in their
composition: at Chatham Island, some streams, containing much glassy
albite and some olivine, are so rugged, that they may be compared to a
sea frozen during a storm; whilst the great stream at Albemarle Island
is almost as smooth as a lake when ruffled by a breeze. At James
Island, black basaltic lava, abounding with small grains of olivine,
presents an intermediate degree of roughness; its surface being glossy,
and the detached fragments resembling, in a very singular manner, folds
of drapery, cables, and pieces of the bark of trees. (A specimen of
basaltic lava, with a few small broken crystals of albite, given me by
one of the officers, is perhaps worthy of description. It consists of
cylindrical ramifications, some of which are only the twentieth of an
inch in diameter, and are drawn out into the sharpest points. The mass
has not been formed like a stalactite, for the points terminate both
upwards and downwards. Globules, only the fortieth of an inch in
diameter, have dropped from some of the points, and adhere to the
adjoining branches. The lava is vesicular, but the vesicles never reach
the surface of the branches, which are smooth and glossy. As it is
generally supposed that vesicles are always elongated in the direction
of the movement of the fluid mass, I may observe, that in these
cylindrical branches, which vary from a quarter to only the twentieth
of an inch in diameter, every air-cell is spherical.)
CRATERS OF TUFF.
About a mile southward of Banks’ Cove, there is a fine elliptic crater,
about five hundred feet in depth, and three-quarters of a mile in
diameter. Its bottom is occupied by a lake of brine, out of which some
little crateriform hills of tuff rise. The lower beds are formed of
compact tuff, appearing like a subaqueous deposit; whilst the upper
beds, round the entire circumference, consist of a harsh, friable tuff,
of little specific gravity, but often containing fragments of rock in
layers. This upper tuff contains numerous pisolitic balls, about the
size of small bullets, which differ from the surrounding matter, only
in being slightly harder and finer grained. The beds dip away very
regularly on all sides, at angles varying, as I found by measurement,
from twenty-five to thirty degrees. The external surface of the crater
slopes at a nearly similar inclination, and is formed by slightly
convex ribs, like those on the shell of a pecten or scallop, which
become broader as they extend from the mouth of the crater to its base.
These ribs are generally from eight to twenty feet in breadth, but
sometimes they are as much as forty feet broad; and they resemble old,
plastered, much flattened vaults, with the plaster scaling off in
plates: they are separated from each other by gullies, deepened by
alluvial action. At their upper and narrow ends, near the mouth of the
crater, these ribs often consist of real hollow passages, like, but
rather smaller than, those often formed by the cooling of the crust of
a lava-stream, whilst the inner parts have flowed onward;—of which
structure I saw many examples at Chatham Island. There can be no doubt
but that these hollow ribs or vaults have been formed in a similar
manner, namely, by the setting or hardening of a superficial crust on
streams of mud, which have flowed down from the upper part of the
crater. In another part of this same crater, I saw open concave gutters
between one and two feet wide, which appear to have been formed by the
hardening of the lower surface of a mud stream, instead of, as in the
former case, of the upper surface. From these facts I think it is
certain that the tuff must have flowed as mud. (This conclusion is of
some interest, because M. Dufrenoy “Mem. pour servir” tome 4 page 274,
has argued from strata of tuff, apparently of similar composition with
that here described, being inclined at angles between 18 degrees and 20
degrees, that Monte Nuevo and some other craters of Southern Italy have
been formed by upheaval. From the facts given above, of the vaulted
character of the separate rills, and from the tuff not extending in
horizontal sheets round these crateriform hills, no one will suppose
that the strata have here been produced by elevation; and yet we see
that their inclination is above 20 degrees, and often as much as 30
degrees. The consolidated strata also, of the internal talus, as will
be immediately seen, dips at an angle of above 30 degrees.) This mud
may have been formed either within the crater, or from ashes deposited
on its upper parts, and afterwards washed down by torrents of rain. The
former method, in most of the cases, appears the more probable one; at
James Island, however, some beds of the friable kind of tuff extend so
continuously over an uneven surface, that probably they were formed by
the falling of showers of ashes.
Within this same crater, strata of coarse tuff, chiefly composed of
fragments of lava, abut, like a consolidated talus, against the inside
walls. They rise to a height of between one hundred and one hundred and
fifty feet above the surface of the internal brine-lake; they dip
inwards, and are inclined at an angle varying from thirty to thirty-six
degrees. They appear to have been formed beneath water, probably at a
period when the sea occupied the hollow of the crater. I was surprised
to observe that beds having this great inclination did not, as far as
they could be followed, thicken towards their lower extremities.
BANKS’ COVE.
(FIGURE 13. A SECTIONAL SKETCH OF THE HEADLANDS FORMING BANKS’ COVE,
showing the diverging crateriform strata, and the converging stratified
talus. The highest point of these hills is 817 feet above the sea.)
This harbour occupies part of the interior of a shattered crater of
tuff larger than that last described. All the tuff is compact, and
includes numerous fragments of lava; it appears like a subaqueous
deposit. The most remarkable feature in this crater is the great
development of strata converging inwards, as in the last case, at a
considerable inclination, and often deposited in irregular curved
layers. These interior converging beds, as well as the proper,
diverging crateriform strata, are represented in Figure 13, a rude,
sectional sketch of the headlands, forming this Cove. The internal and
external strata differ little in composition, and the former have
evidently resulted from the wear and tear, and redeposition of the
matter forming the external crateriform strata. From the great
development of these inner beds, a person walking round the rim of this
crater might fancy himself on a circular anticlinal ridge of stratified
sandstone and conglomerate. The sea is wearing away the inner and outer
strata, and especially the latter; so that the inwardly converging
strata will, perhaps, in some future age, be left standing alone—a case
which might at first perplex a geologist. (I believe that this case
actually occurs in the Azores, where Dr. Webster “Description” page
185, has described a basin-formed, little island, composed of STRATA OF
TUFF, dipping inwards and bounded externally by steep sea-worn cliffs.
Dr. Daubeny supposes “Volcanoes” page 266, that this cavity must have
been formed by a circular subsidence. It appears to me far more
probable, that we here have strata which were originally deposited
within the hollow of a crater, of which the exterior walls have since
been removed by the sea.)
JAMES ISLAND.
Two craters of tuff on this island are the only remaining ones which
require any notice. One of them lies a mile and a half inland from
Puerto Grande: it is circular, about the third of a mile in diameter,
and 400 feet in depth. It differs from all the other tuff-craters which
I examined, in having the lower part of its cavity, to the height of
between one hundred and one hundred and fifty feet, formed by a
precipitous wall of basalt, giving to the crater the appearance of
having burst through a solid sheet of rock. The upper part of this
crater consists of strata of the altered tuff, with a semi-resinous
fracture. Its bottom is occupied by a shallow lake of brine, covering
layers of salt, which rest on deep black mud. The other crater lies at
the distance of a few miles, and is only remarkable from its size and
perfect condition. Its summit is 1,200 feet above the level of the sea,
and the interior hollow is 600 feet deep. Its external sloping surface
presented a curious appearance from the smoothness of the wide layers
of tuff, which resembled a vast plastered floor. Brattle Island is, I
believe, the largest crater in the Archipelago composed of tuff; its
interior diameter is nearly a nautical mile. At present it is in a
ruined condition, consisting of little more than half a circle open to
the south; its great size is probably due, in part, to internal
degradation, from the action of the sea.
SEGMENT OF A BASALTIC CRATER.
(FIGURE 14. SEGMENT OF A VERY SMALL ORIFICE OF ERUPTION, on the beach
of Fresh-water Bay.)
One side of Fresh-water Bay, in James Island, is bounded by a
promontory, which forms the last wreck of a great crater. On the beach
of this promontory, a quadrant-shaped segment of a small subordinate
point of eruption stands exposed. It consists of nine separate little
streams of lava piled upon each other; and of an irregular pinnacle,
about fifteen feet high, of reddish-brown, vesicular basalt, abounding
with large crystals of glassy albite, and with fused augite. This
pinnacle, and some adjoining paps of rock on the beach, represent the
axis of the crater. The streams of lava can be followed up a little
ravine, at right angles to the coast, for between ten and fifteen
yards, where they are hidden by detritus: along the beach they are
visible for nearly eighty yards, and I do not believe that they extend
much further. The three lower streams are united to the pinnacle; and
at the point of junction (as shown in Figure 14, a rude sketch made on
the spot), they are slightly arched, as if in the act of flowing over
the lip of the crater. The six upper streams no doubt were originally
united to this same column before it was worn down by the sea. The lava
of these streams is of similar composition with that of the pinnacle,
excepting that the crystals of albite appear to be more comminuted, and
the grains of fused augite are absent. Each stream is separated from
the one above it by a few inches, or at most by one or two feet in
thickness, of loose fragmentary scoriae, apparently derived from the
abrasion of the streams in passing over each other. All these streams
are very remarkable from their thinness. I carefully measured several
of them; one was eight inches thick, but was firmly coated with three
inches above, and three inches below, of red scoriaceous rock (which is
the case with all the streams), making altogether a thickness of
fourteen inches: this thickness was preserved quite uniformly along the
entire length of the section. A second stream was only eight inches
thick, including both the upper and lower scoriaceous surfaces. Until
examining this section, I had not thought it possible that lava could
have flowed in such uniformly thin sheets over a surface far from
smooth. These little streams closely resemble in composition that great
deluge of lava at Albemarle Island, which likewise must have possessed
a high degree of fluidity.
PSEUDO-EXTRANEOUS, EJECTED FRAGMENTS.
In the lava and in the scoriae of this little crater, I found several
fragments, which, from their angular form, their granular structure,
their freedom from air-cells, their brittle and burnt condition,
closely resembled those fragments of primary rocks which are
occasionally ejected, as at Ascension, from volcanoes. These fragments
consist of glassy albite, much mackled, and with very imperfect
cleavages, mingled with semi-rounded grains, having tarnished, glossy
surfaces, of a steel-blue mineral. The crystals of albite are coated by
a red oxide of iron, appearing like a residual substance; and their
cleavage-planes also are sometimes separated by excessively fine layers
of this oxide, giving to the crystals the appearance of being ruled
like a glass micrometer. There was no quartz. The steel-blue mineral,
which is abundant in the pinnacle, but which disappears in the streams
derived from the pinnacle, has a fused appearance, and rarely presents
even a trace of cleavage; I obtained, however, one measurement, which
proved that it was augite; and in one other fragment, which differed
from the others, in being slightly cellular, and in gradually blending
into the surrounding matrix the small grains of this mineral were
tolerably well crystallised. Although there is so wide a difference in
appearance between the lava of the little streams, and especially of
their red scoriaceous crusts, and one of these angular ejected
fragments, which at first sight might readily be mistaken for syenite,
yet I believe that the lava has originated from the melting and
movement of a mass of rock of absolutely similar composition with the
fragments. Besides the specimen above alluded to, in which we see a
fragment becoming slightly cellular, and blending into the surrounding
matrix, some of the grains of the steel-blue augite also have their
surfaces becoming very finely vesicular, and passing into the nature of
the surrounding paste; other grains are throughout, in an intermediate
condition. The paste seems to consist of the augite more perfectly
fused, or, more probably, merely disturbed in its softened state by the
movement of the mass, and mingled with the oxide of iron and with
finely comminuted, glassy albite. Hence probably it is that the fused
albite, which is abundant in the pinnacle, disappears in the streams.
The albite is in exactly the same state, with the exception of most of
the crystals being smaller in the lava and in the embedded fragments;
but in the fragments they appear to be less abundant: this, however,
would naturally happen from the intumescence of the augitic base, and
its consequent apparent increase in bulk. It is interesting thus to
trace the steps by which a compact granular rock becomes converted into
a vesicular, pseudo-porphyritic lava, and finally into red scoriae. The
structure and composition of the embedded fragments show that they are
parts either of a mass of primary rock which has undergone considerable
change from volcanic action, or more probably of the crust of a body of
cooled and crystallised lava, which has afterwards been broken up and
re-liquified; the crust being less acted on by the renewed heat and
movement.
CONCLUDING REMARKS ON THE TUFF-CRATERS.
These craters, from the peculiarity of the resin-like substance which
enters largely into their composition, from their structure, their size
and number, present the most striking feature in the geology of this
Archipelago. The majority of them form either separate islets, or
promontories attached to the larger islands; and those which now stand
at some little distance from the coast are worn and breached, as if by
the action of the sea. From this general circumstance of their
position, and from the small quantity of ejected ashes in any part of
the Archipelago, I am led to conclude, that the tuff has been chiefly
produced, by the grinding together of fragments of lava within active
craters, communicating with the sea. In the origin and composition of
the tuff, and in the frequent presence of a central lake of brine and
of layers of salt, these craters resemble, though on a gigantic scale,
the “salses,” or hillocks of mud, which are common in some parts of
Italy and in other countries. (D’Aubuisson “Traite de Geognosie” tome 1
page 189. I may remark, that I saw at Terceira, in the Azores, a crater
of tuff or peperino, very similar to these of the Galapagos
Archipelago. From the description given in Freycinet “Voyage,” similar
ones occur at the Sandwich Islands; and 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.