into two varieties (C and D), both closely resembling the lower, fine-
grained, more unctuous and compact kind at Port Desire; and, as at that
place, including much selenite.
Fourth, a fossiliferous mass, divided into three main beds, of which
the uppermost is thin, and consists of ferruginous sandstone, with many
shells of the great oyster and Pecten Paranensis; the middle bed (E) is
a yellowish earthy sandstone abounding with Scutellae; and the lowest
bed (F) is an indurated, greenish, sandy clay, including large
concretions of calcareous sandstone, many shells of the great oyster,
and in parts almost made up of fragments of Balanidae. Out of these
three beds, I procured the following twelve species, of which the two
first were exceedingly numerous in individuals, as were the
Terebratulae and Turritellae in certain layers:—
1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fe, and
whole coast of Patagonia). 2. Pecten Paranensis, d’Orbigny, “Voyage,
Pal.” (St. Fe, S. Josef, Port Desire). 3. Pecten centralis, G.B.
Sowerby (also at Port Desire and S. Cruz). 4. Pecten geminatus, G.B.
Sowerby. 5. Terebratula Patagonica, G.B. Sowerby (also S. Josef). 6.
Struthiolaria ornata, G.B. Sowerby (also S. Cruz). 7. Fusus
Patagonicus, G.B. Sowerby. 8. Fusus Noachinus, G.B. Sowerby. 9.
Scalaria rugulosa, G.B. Sowerby. 10. Turritella ambulacrum, G.B.
Sowerby (also S. Cruz). 11. Pyrula, cast of, like P. ventricosa of
Sowerby, Tank Cat. 12. Balanus varians, G.B. Sowerby. 13. Scutella,
differing from the species from Nuevo Gulf.
At the head of the inner harbour of Port S. Julian, the fossiliferous
mass is not displayed, and the sea-cliffs from the water’s edge to a
height of between one and two hundred feet are formed of the white
pumiceous mudstone, which here includes innumerable, far-extended,
sometimes horizontal, sometimes inclined or vertical laminae of
transparent gypsum, often about an inch in thickness. Further inland,
with the exception of the superficial gravel, the whole thickness of
the truncated hills, which represent a formerly continuous plain 950
feet in height, appears to be formed of this white mudstone: here and
there, however, at various heights, thin earthy layers, containing the
great oyster, Pecten Paranensis and Turritella ambulacrum, are
interstratified; thus showing that the whole mass belongs to the same
epoch. I nowhere found even a fragment of a shell actually in the white
deposit, and only a single cast of a Turritella. Out of the eighteen
microscopic organisms discovered by Ehrenberg in the specimens from
this place, ten are common to the same deposit at Port Desire. I may
add that specimens of this white mudstone, with the same identical
characters were brought me from two points,—one twenty miles north of
S. Julian, where a wide gravel-capped plain, 350 feet in height, is
thus composed; and the other forty miles south of S. Julian, where, on
the old charts, the cliffs are marked as “Chalk Hills.”
SANTA CRUZ.
The gravel-capped cliffs at the mouth of the river are 355 feet in
height: the lower part, to a thickness of fifty or sixty feet, consists
of a more or less hardened, darkish, muddy, or argillaceous sandstone
(like the lowest bed of Port Desire), containing very many shells, some
silicified and some converted into yellow calcareous spar. The great
oyster is here numerous in layers; the Trigonocelia and Turritella are
also very numerous: it is remarkable that the Pecten Paranensis, so
common in all other parts of the coast, is here absent: the shells
consist of:—
1. Ostrea Patagonica, d’Orbigny; “Voyage Pal.” (also at St. Fe and
whole coast of Patagonia).
2. Pecten centralis, G.B. Sowerby (also P. Desire and S. Julian).
3. Venus meridionalis of G.B. Sowerby.
4. Crassatella Lyellii, G.B. Sowerby.
5. Cardium puelchum, G.B. Sowerby.
6. Cardita Patagonica, G.B. Sowerby.
7. Mactra rugata, G.B. Sowerby.
8. Mactra Darwinii, G.B. Sowerby.
9. Cucullaea alta, G.B. Sowerby (also P. Desire).
10. Trigonocelia insolita, G.B. Sowerby.
11. Nucula (?) glabra, G.B. Sowerby.
12. Crepidula gregaria, G.B. Sowerby.
13. Voluta alta, G.B. Sowerby.
14. Trochus collaris, G.B. Sowerby.
15. Natica solida (?), G.B. Sowerby
16. Struthiolaria ornata, G.B. Sowerby (also P. Desire).
17. Turritella ambulacrum, G.B. Sowerby (also P. S. Julian).
Imperfect fragments of the genera Byssoarca, Artemis, and Fusus.
The upper part of the cliff is generally divided into three great
strata, differing slightly in composition, but essentially resembling
the pumiceous mudstone of the places farther north; the deposit,
however, here is more arenaceous, of greater specific gravity, and not
so white: it is interlaced with numerous thin veins, partially or quite
filled with transverse fibres of gypsum; these fibres were too short to
reach across the vein, have their extremities curved or bent: in the
same veins with the gypsum, and likewise in separate veins as well as
in little nests, there is much powdery sulphate of magnesia (as
ascertained by Mr. Reeks) in an uncompressed form: I believe that this
salt has not heretofore been found in veins. Of the three beds, the
central one is the most compact, and more like ordinary sandstone: it
includes numerous flattened spherical concretions, often united like a
necklace, composed of hard calcareous sandstone, containing a few
shells: some of these concretions were four feet in diameter, and in a
horizontal line nine feet apart, showing that the calcareous matter
must have been drawn to the centres of attraction, from a distance of
four feet and a half on both sides. In the upper and lower
finer-grained strata, there were other concretions of a grey colour,
containing calcareous matter, and so fine-grained and compact, as
almost to resemble porcelain- rock: I have seen exactly similar
concretions in a volcanic tufaceous bed in Chiloe. Although in this
upper fine-grained strata, organic remains were very rare, yet I
noticed a few of the great oyster; and in one included soft ferruginous
layer, there were some specimens of the Cucullaea alta (found at Port
Desire in the lower fossiliferous mass) and of the Mactra rugata, which
latter shell has been partially converted into gypsum.
(FIGURE 18. SECTION OF THE PLAINS OF PATAGONIA, ON THE BANKS OF THE S.
CRUZ.
(Section through strata (from top to bottom)): Surface of plain with
erratic boulders; 1,146 feet above the sea. a. Gravel and boulders, 212
feet thick. b. Basaltic lava, 322 feet thick. c, d and e. Sedimentary
layers, bed of small pebbles and talus respectively, total 592 feet
thick. River of S. Cruz; here 280 feet above sea.)
In ascending the valley of the S. Cruz, the upper strata of the coast-
cliffs are prolonged, with nearly the same characters, for fifty miles:
at about this point, they begin in the most gradual and scarcely
perceptible manner, to be banded with white lines; and after ascending
ten miles farther, we meet with distinct thin layers of whitish,
greenish, and yellowish fine-grained, fusible sediments. At eighty
miles from the coast, in a cliff thus composed, there were a few layers
of ferruginous sandstone, and of an argillaceous sandstone with
concretions of marl like those in the Pampas. (At this spot, for a
space of three-quarters of a mile along the north side of the river,
and for a width of half a mile, there has been a great slip, which has
formed hills between sixty and seventy feet in height, and has tilted
the strata into highly inclined and even vertical positions. The strata
generally dipped at an angle of 45 degrees towards the cliff from which
they had slided. I have observed in slips, both on a small and large
scale, that this inward dip is very general. Is it due to the
hydrostatic pressure of water percolating with difficulty through the
strata acting with greater force at the base of the mass than against
the upper part?) At one hundred miles from the coast, that is at a
central point between the Atlantic and the Cordillera, we have the
section in Figure 18.
The upper half of the sedimentary mass, under the basaltic lava,
consists of innumerable zones of perfectly white bright green,
yellowish and brownish, fine-grained, sometimes incoherent, sedimentary
matter. The white, pumiceous, trachytic tuff-like varieties are of
rather greater specific gravity than the pumiceous mudstone on the
coast to the north; some of the layers, especially the browner ones,
are coarser, so that the broken crystals are distinguishable with a
weak lens. The layers vary in character in short distances. With the
exception of a few of the Ostrea Patagonica, which appeared to have
rolled down from the cliff above, no organic remains were found. The
chief difference between these layers taken as a whole, and the upper
beds both at the mouth of the river and on the coast northward, seems
to lie in the occasional presence of more colouring matter, and in the
supply having been intermittent; these characters, as we have seen,
very gradually disappear in descending the valley, and this fact may
perhaps be accounted for by the currents of a more open sea having
blended together the sediment from a distant and intermittent source.
The coloured layers in the foregoing section rest on a mass, apparently
of great thickness (but much hidden by the talus), of soft sandstone,
almost composed of minute pebbles, from one-tenth to two-tenths of an
inch in diameter, of the rocks (with the entire exception of the
basaltic lava) composing the great boulders on the surface of the
plain, and probably composing the neighbouring Cordillera. Five miles
higher up the valley, and again thirty miles higher up (that is twenty
miles from the nearest range of the Cordillera), the lower plain
included within the upper escarpments, is formed, as seen on the banks
of the river, of a nearly similar but finer-grained, more earthy,
laminated sandstone, alternating with argillaceous beds, and containing
numerous moderately sized pebbles of the same rocks, and some shells of
the great Ostrea Patagonica. (I found at both places, but not in situ,
quantities of coniferous and ordinary dicotyledonous silicified wood,
which was examined for me by Mr. R. Brown.) As most of these shells had
been rolled before being here embedded, their presence does not prove
that the sandstone belongs to the great Patagonian tertiary formation,
for they might have been redeposited in it, when the valley existed as
a sea-strait; but as amongst the pebbles there were none of basalt,
although the cliffs on both sides of the valley are composed of this
rock, I believe that the sandstone does belong to this formation. At
the highest point to which we ascended, twenty miles distant from the
nearest slope of the Cordillera, I could see the horizontally zoned
white beds, stretching under the black basaltic lava, close up to the
mountains; so that the valley of the S. Cruz gives a fair idea of the
constitution of the whole width of Patagonia.
BASALTIC LAVA OF THE S. CRUZ.
This formation is first met with sixty-seven miles from the mouth of
the river; thence it extends uninterruptedly, generally but not
exclusively on the northern side of the valley, close up to the
Cordillera. The basalt is generally black and fine-grained, but
sometimes grey and laminated; it contains some olivine, and high up the
valley much glassy feldspar, where, also, it is often amygdaloidal; it
is never highly vesicular, except on the sides of rents and on the
upper and lower, spherically laminated surfaces. It is often columnar;
and in one place I saw magnificent columns, each face twelve feet in
width, with their interstices filled up with calcareous tuff. The
streams rest conformably on the white sedimentary beds, but I nowhere
saw the actual junction; nor did I anywhere see the white beds actually
superimposed on the lava; but some way up the valley at the foot of the
uppermost escarpments, they must be thus superimposed. Moreover, at the
lowest point down the valley, where the streams thin out and terminate
in irregular projections, the spaces or intervals between these
projections are filled up to the level of the now denuded and
gravel-capped surfaces of the plains, with the white-zoned sedimentary
beds; proving that this matter continued to be deposited after the
streams had flowed. Hence we may conclude that the basalt is
contemporaneous with the upper parts of the great tertiary formation.
The lava where first met with is 130 feet in thickness: it there
consists of two, three, or perhaps more streams, divided from each
other by vesicular spheroids like those on the surface. From the
streams having, as it appears, extended to different distances, the
terminal points are of unequal heights. Generally the surface of the
basalt is smooth them in one part high up the valley, it was so uneven
and hummocky, that until I afterwards saw the streams extending
continuously on both sides of the valley up to a height of about three
thousand feet close to the Cordillera, I thought that the craters of
eruption were probably close at hand. This hummocky surface I believe
to have been caused by the crossing and heaping up of different
streams. In one place, there were several rounded ridges about twenty
feet in height, some of them as broad as high, and some broader, which
certainly had been formed whilst the lava was fluid, for in transverse
sections each ridge was seen to be concentrically laminated, and to be
composed of imperfect columns radiating from common centres, like the
spokes of wheels.
The basaltic mass where first met with is, as I have said, 130 feet in
thickness, and, thirty-five miles higher up the valley, it increases to
322 feet. In the first fourteen and a half miles of this distance, the
upper surface of the lava, judging from three measurements taken above
the level of the river (of which the apparently very uniform
inclination has been calculated from its total height at a point 135
miles from the mouth), slopes towards the Atlantic at an angle of only
0 degrees 7 minutes twenty seconds: this must be considered only as an
approximate measurement, but it cannot be far wrong. Taking the whole
thirty-five miles, the upper surface slopes at an angle of 0 degrees 10
minutes 53 seconds; but this result is of no value in showing the
inclination of any one stream, for halfway between the two points of
measurement, the surface suddenly rises between one hundred and two
hundred feet, apparently caused by some of the uppermost streams having
extended thus far and no farther. From the measurement made at these
two points, thirty-five miles apart, the mean inclination of the
sedimentary beds, over which the lava has flowed, is NOW (after
elevation from under the sea) only 0 degrees 7 minutes 52 seconds: for
the sake of comparison, it may be mentioned that the bottom of the
present sea in a line from the mouth of the S. Cruz to the Falkland
Islands, from a depth of seventeen fathoms to a depth of eighty-five
fathoms, declines at an angle of 0 degrees 1 minute 22 seconds; between
the beach and the depth of seventeen fathoms, the slope is greater.
From a point about half-way up the valley, the basaltic mass rises more
abruptly towards the foot of the Cordillera, namely, from a height of
1,204 feet, to about 3,000 feet above the sea.
This great deluge of lava is worthy, in its dimensions, of the great
continent to which it belongs. The aggregate streams have flowed from
the Cordillera to a distance (unparalleled, I believe, in any case yet
known) of about one hundred geographical miles. Near their furthest
extremity their total thickness is 130 feet, which increase thirty-five
miles farther inland, as we have just seen, to 322 feet. The least
inclination given by M. E. de Beaumont of the upper surface of a
lava-stream, namely 0 degrees 30 minutes, is that of the great
subaerial eruption in 1783 from Skaptar Jukul in Iceland; and M. E. de
Beaumont shows that it must have flowed down a mean inclination of less
than 0 degrees 20 minutes. (“Memoires pour servir” etc. pages 178 and
217.) But we now see that under the pressure of the sea, successive
streams have flowed over a smooth bottom with a mean inclination of not
more than 0 degrees 7 minutes 52 seconds; and that the upper surface of
the terminal portion (over a space of fourteen and a half miles) has an
inclination of not more than 0 degrees 7 minutes 20 seconds. If the
elevation of Patagonia has been greater nearer the Cordillera than near
the Atlantic (as is probable), then these angles are now all too large.
I must repeat, that although the foregoing measurements, which were all
carefully taken with the barometer, may not be absolutely correct, they
cannot be widely erroneous.
Southward of the S. Cruz, the cliffs of the 840 feet plain extend to
Coy Inlet, and owing to the naked patches of the white sediment, they
are said on the charts to be “like the coast of Kent.” At Coy Inlet the
high plain trends inland, leaving flat-topped outliers. At Port
Gallegos (latitude 51 degrees 35 minutes, and ninety miles south of S.
Cruz), I am informed by Captain Sulivan, R.N., that there is a
gravel-capped plain from two to three hundred feet in height, formed of
numerous strata, some fine-grained and pale-coloured, like the upper
beds at the mouth of the S. Cruz, others rather dark and coarser, so as
to resemble gritstones or tuffs; these latter include rather large
fragments of apparently decomposed volcanic rocks; there are, also,
included layers of gravel. This formation is highly remarkable, from
abounding with mammiferous remains, which have not as yet been examined
by Professor Owen, but which include some large, but mostly small,
species of Pachydermata, Edentata, and Rodentia. From the appearance of
the pale-coloured, fine-grained beds, I was inclined to believe that
they corresponded with the upper beds of the S. Cruz; but Professor
Ehrenberg, who has examined some of the specimens, informs me that the
included microscopical organisms are wholly different, being fresh and
brackish-water forms. Hence the two to three hundred feet plain at Port
Gallegos is of unknown age, but probably of subsequent origin to the
great Patagonian tertiary formation.
EASTERN TIERRA DEL FUEGO.
Judging from the height, the general appearance, and the white colour
of the patches visible on the hill sides, the uppermost plain, both on
the north and western side of the Strait of Magellan, and along the
eastern coast of Tierra del Fuego as far south as near Port St.
Polycarp, probably belongs to the great Patagonian tertiary formation,
These higher table- ranges are fringed by low, irregular, extensive
plains, belonging to the boulder formation (Described in the
“Geological Transactions” volume 6 page 415.), and composed of coarse
unstratified masses, sometimes associated (as north of C. Virgin’s)
with fine, laminated, muddy sandstones. The cliffs in Sebastian Bay are
200 feet in height, and are composed of fine sandstones, often in
curvilinear layers, including hard concretions of calcareous sandstone,
and layers of gravel. In these beds there are fragments of wood, legs
of crabs, barnacles encrusted with corallines still partially retaining
their colour, imperfect fragments of a Pholas distinct from any known
species, and of a Venus, approaching very closely to, but slightly
different in form from, the V. lenticularis, a species living on the
coast of Chile. Leaves of trees are numerous between the laminae of the
muddy sandstone; they belong, as I am informed by Dr. J.D. Hooker, to
three species of deciduous beech, different from the two species which
compose the great proportion of trees in this forest-clad land.
(“Botany of the Antarctic Voyage” page 212.) From these facts it is
difficult to conjecture, whether we here see the basal part of the
great Patagonian formation, or some later deposit.
A SUMMARY ON THE PATAGONIAN TERTIARY FORMATION.
Four out of the seven fossil shells, from St. Fe in Entre Rios, were
found by M. d’Orbigny in the sandstone of the Rio Negro, and by me at
San Josef. Three out of the six from San Josef are identical with those
from Port Desire and S. Julian, which two places have together fifteen
species, out of which three are common to both. Santa Cruz has
seventeen species, out of which five are common to Port Desire and S.
Julian. Considering the difference in latitude between these several
places, and the small number of species altogether collected, namely
thirty-six, I conceive the above proportional number of species in
common, is sufficient to show that the lower fossiliferous mass belongs
nearly, I do not say absolutely, to the same epoch. What this epoch may
be, compared with the European tertiary stages, M. d’Orbigny will not
pretend to determine. The thirty-six species (including those collected
by myself and by M. d’Orbigny) are all extinct, or at least unknown;
but it should be borne in mind, that the present coast consists of
shingle, and that no one, I believe, has dredged here for shells; hence
it is not improbable that some of the species may hereafter be found
living. Some few of the species are closely related with existing ones;
this is especially the case, according to M. d’Orbigny and Mr. Sowerby,
with the Fusus Patagonicus; and, according to Mr. Sowerby, with the
Pyrula, the Venus meridionalis, the Crepidula gregaria, and the
Turritella ambulacrum, and T. Patagonica. At least three of the genera,
namely, Cucullaea, Crassatella, and (as determined by Mr. Sowerby)
Struthiolaria, are not found in this quarter of the world; and
Trigonocelia is extinct. The evidence taken altogether indicates that
this great tertiary formation is of considerable antiquity; but when
treating of the Chilean beds, I shall have to refer again to this
subject.
The white pumiceous mudstone, with its abundant gypsum, belongs to the
same general epoch with the underlying fossiliferous mass, as may be
inferred from the shells included in the intercalated layers at Nuevo
Gulf, S. Julian, and S. Cruz. Out of the twenty-seven marine
microscopic structures found by Professor Ehrenberg in the specimens
from S. Julian and Port Desire, ten are common to these two places: the
three found at Nuevo Gulf are distinct. I have minutely described this
deposit, from its remarkable characters and its wide extension. From
Coy Inlet to Port Desire, a distance of 230 miles, it is certainly
continuous; and I have reason to believe that it likewise extends to
the Rio Chupat, Nuevo Gulf, and San Josef, a distance of 570 miles: we
have, also, seen that a single layer occurs at the Rio Negro. At Port
S. Julian it is from eight to nine hundred feet in thickness; and at S.
Cruz it extends, with a slightly altered character, up to the
Cordillera. From its microscopic structure, and from its analogy with
other formations in volcanic districts, it must be considered as
originally of volcanic origin: it may have been formed by the
long-continued attrition of vast quantities of pumice, or judging from
the manner in which the mass becomes, in ascending the valley of S.
Cruz, divided into variously coloured layers, from the long-continued
eruption of clouds of fine ashes. In either case, we must conclude,
that the southern volcanic orifices of the Cordillera, now in a dormant
state, were at about this period over a wide space, and for a great
length of time, in action. We have evidence of this fact, in the
latitude of the Rio Negro, in the sandstone-conglomerate with pumice,
and demonstrative proof of it, at S. Cruz, in the vast deluges of
basaltic lava: at this same tertiary period, also, there is distinct
evidence of volcanic action in Western Banda Oriental.
The Patagonian tertiary formation extends continuously, judging from
fossils alone, from S. Cruz to near the Rio Colorado, a distance of
above six hundred miles, and reappears over a wide area in Entre Rios
and Banda Oriental, making a total distance of 1,100 miles; but this
formation undoubtedly extends (though no fossils were collected) far
south of the S. Cruz, and, according to M. d’Orbigny, 120 miles north
of St. Fe. At S. Cruz we have seen that it extends across the
continent; being on the coast about eight hundred feet in thickness
(and rather more at S. Julian), and rising with the contemporaneous
lava-streams to a height of about three thousand feet at the base of
the Cordillera. It rests, wherever any underlying formation can be
seen, on plutonic and metamorphic rocks. Including the newer Pampean
deposit, and those strata in Eastern Tierra del Fuego of doubtful age,
as well as the boulder formation, we have a line of more than
twenty-seven degrees of latitude, equal to that from the Straits of
Gibraltar to the south of Iceland, continuously composed of tertiary
formations. Throughout this great space the land has been upraised,
without the strata having been in a single instance, as far as my means
of observation went, unequally tilted or dislocated by a fault.
TERTIARY FORMATIONS ON THE WEST COAST.
CHONOS ARCHIPELAGO.
The numerous islands of this group, with the exception of Lemus, Ypun,
consist of metamorphic schists; these two islands are formed of softish
grey and brown, fusible, often laminated sandstones, containing a few
pebbles, fragments of black lignite, and numerous mammillated
concretions of hard calcareous sandstone. Out of these concretions at
Ypun (latitude 40 degrees 30 minutes S.), I extracted the four
following extinct species of shells:—
1. Turritella suturalis, G.B. Sowerby (also Navidad). 2. Sigaretus
subglobosus, G.B. Sowerby (also Navidad). 3. Cytheraea (?) sulculosa
(?), G.B. Sowerby (also Chiloe and Huafo?). 4. Voluta, fragments of.
In the northern parts of this group there are some cliffs of gravel and
of the boulder formation. In the southern part (at P. Andres in Tres
Montes), there is a volcanic formation, probably of tertiary origin.
The lavas attain a thickness of from two to three hundred feet; they
are extremely variable in colour and nature, being compact, or
brecciated, or cellular, or amygdaloidal with zeolite, agate and bole,
or porphyritic with glassy albitic feldspar. There is also much
imperfect rubbly pitchstone, with the interstices charged with powdery
carbonate of lime apparently of contemporaneous origin. These lavas are
conformably associated with strata of breccia and of brown tuff
containing lignite. The whole mass has been broken up and tilted at an
angle of 45 degrees, by a series of great volcanic dikes, one of which
was thirty yards in breadth. This volcanic formation resembles one,
presently to be described, in Chiloe.
HUAFO.
This island lies between the Chonos and Chiloe groups: it is about
eight hundred feet high, and perhaps has a nucleus of metamorphic
rocks. The strata which I examined consisted of fine-grained muddy
sandstones, with fragments of lignite and concretions of calcareous
sandstone. I collected the following extinct shells, of which the
Turritella was in great numbers:—
1. Bulla cosmophila, G.B. Sowerby. 2. Pleurotoma subaequalis, G.B.
Sowerby. 3. Fusus cleryanus, d’Orbigny, “Voyage Pal.” (also at
Coquimbo). 4. Triton leucostomoides, G.B. Sowerby. 5. Turritella
Chilensis, G.B. Sowerby (also Mocha). 6. Venus, probably a distinct
species, but very imperfect. 7. Cytheraea (?) sulculosa (?), probably a
distinct species, but very imperfect. 8. Dentalium majus, G.B. Sowerby.
CHILOE.
This fine island is about one hundred miles in length. The entire
southern part, and the whole western coast, consists of mica-schist,
which likewise is seen in the ravines of the interior. The central
mountains rise to a height of 3,000 feet, and are said to be partly
formed of granite and greenstone: there are two small volcanic
districts. The eastern coast, and large parts of the northern extremity
of the island are composed of gravel, the boulder formation, and
underlying horizontal strata. The latter are well displayed for twenty
miles north and south of Castro; they vary in character from common
sandstone to fine-grained, laminated mudstones: all the specimens which
I examined are easily fusible, and some of the beds might be called
volcanic grit-stones. These latter strata are perhaps related to a mass
of columnar trachyte which occurs behind Castro. The sandstone
occasionally includes pebbles, and many fragments and layers of
lignite; of the latter, some are apparently formed of wood and others
of leaves: one layer on the N.W. side of Lemuy is nearly two feet in
thickness. There is also much silicified wood, both common
dicotyledonous and coniferous: a section of one specimen in the
direction of the medullary rays has, as I am informed by Mr. R. Brown,
the discs in a double row placed alternately, and not opposite as in
the true Araucaria. I found marine remains only in one spot, in some
concretions of hard calcareous sandstone: in several other districts I
have observed that organic remains were exclusively confined to such
concretions; are we to account for this fact, by the supposition that
the shells lived only at these points, or is it not more probable that
their remains were preserved only where concretions were formed? The
shells here are in a bad state, they consist of:—
1. Tellinides (?) oblonga, G.B. Sowerby (a solenella in M. d’Orbigny’s
opinion). 2. Natica striolata, G.B. Sowerby. 3. Natica (?) pumila, G.B.
Sowerby. 4. Cytheraea (?) sulculosa, G.B. Sowerby (also Ypun and
Huafo?).
At the northern extremity of the island, near S. Carlos, there is a
large volcanic formation, between five and seven hundred feet in
thickness. The commonest lava is blackish-grey or brown, either
vesicular, or amygdaloidal with calcareous spar and bole: most even of
the darkest varieties fuse into a pale-coloured glass. The next
commonest variety is a rubbly, rarely well characterised pitchstone
(fusing into a white glass) which passes in the most irregular manner
into stony grey lavas. This pitchstone, as well as some purple
claystone porphyry, certainly flowed in the form of streams. These
various lavas often pass, at a considerable depth from the surface, in
the most abrupt and singular manner into wacke. Great masses of the
solid rock are brecciated, and it was generally impossible to discover
whether the recementing process had been an igneous or aqueous action.
(In a cliff of the hardest fragmentary mass, I found several tortuous,
vertical veins, varying in thickness from a few tenths of an inch to
one inch and a half, of a substance which I have not seen described. It
is glossy, and of a brown colour; it is thinly laminated, with the
laminae transparent and elastic; it is a little harder than calcareous
spar; it is infusible under the blowpipe, sometimes decrepitates, gives
out water, curls up, blackens, and becomes magnetic. Borax easily
dissolves a considerable quantity of it, and gives a glass tinged with
green. I have no idea what its true nature is. On first seeing it, I
mistook it for lignite!) The beds are obscurely separated from each
other; they are sometimes parted by seams of tuff and layers of
pebbles. In one place they rested on, and in another place were capped
by, tuffs and girt-stones, apparently of submarine origin.
The neighbouring peninsula of Lacuy is almost wholly formed of
tufaceous deposits, connected probably in their origin with the
volcanic hills just described. The tuffs are pale-coloured, alternating
with laminated mudstones and sandstones (all easily fusible), and
passing sometimes into fine-grained white beds strikingly resembling
the great upper infusorial deposit of Patagonia, and sometimes into
brecciolas with pieces of pumice in the last stage of decay; these
again pass into ordinary coarse breccias and conglomerates of hard
rocks. Within very short distances, some of the finer tuffs often
passed into each other in a peculiar manner, namely, by irregular
polygonal concretions of one variety increasing so much and so suddenly
in size, that the second variety, instead of any longer forming the
entire mass, was left merely in thin veins between the concretions. In
a straight line of cliffs, at Point Tenuy, I examined the following
remarkable section (Figure 19):—
(FIGURE 19.)
On the left hand, the lower part (AA) consists of regular, alternating
strata of brown tuffs and greenish laminated mudstone, gently inclined
to the right, and conformably covered by a mass (B left) of a white,
tufaceous and brecciolated deposit. On the right hand, the whole cliff
(BB right) consists of the same white tufaceous matter, which on this
side presents scarcely a trace of stratification, but to the left
becomes very gradually and rather indistinctly divided into strata
quite conformable with the underlying beds (AA): moreover, a few
hundred yards further to the left, where the surface has been less
denuded, the tufaceous strata (B left) are conformably covered by
another set of strata, like the underlying ones (AA) of this section.
In the middle of the diagram, the beds (AA) are seen to be abruptly cut
off, and to abut against the tufaceous non-stratified mass; but the
line of junction has been accidentally not represented steep enough,
for I particularly noticed that before the beds had been tilted to the
right, this line must have been nearly vertical. It appears that a
current of water cut for itself a deep and steep submarine channel, and
at the same time or afterwards filled it up with the tufaceous and
brecciolated matter, and spread the same over the surrounding submarine
beds; the matter becoming stratified in these more distant and less
troubled parts, and being moreover subsequently covered up by other
strata (like AA) not shown in the diagram. It is singular that three of
the beds (of AA) are prolonged in their proper direction, as
represented, beyond the line of junction into the white tufaceous
matter: the prolonged portions of two of the beds are rounded; in the
third, the terminal fragment has been pushed upwards: how these beds
could have been left thus prolonged, I will not pretend to explain. In
another section on the opposite side of a promontory, there was at the
foot of this same line of junction, that is at the bottom of the old
submarine channel, a pile of fragments of the strata (AA), with their
interstices filled up with white tufaceous matter: this is exactly what
might have been anticipated under such circumstances.
(FIGURE 20. GROUND PLAN SHOWING THE RELATION BETWEEN VEINS AND
CONCRETIONARY ZONES IN A MASS OF TUFF.)
The various tufaceous and other beds at this northern end of Chiloe
probably belong to about the same age with those near Castro, and they
contain, as there, many fragments of black lignite and of silicified
and pyritous wood, often embedded close together. They also contain
many and singular concretions: some are of hard calcareous sandstone,
in which it would appear that broken volcanic crystals and scales of
mica have been better preserved (as in the case of the organic remains
near Castro) than in the surrounding mass. Other concretions in the
white brecciola are of a hard, ferruginous, yet fusible, nature; they
are as round as cannon-balls, and vary from two or three inches to two
feet in diameter; their insides generally consist either of fine,
scarcely coherent volcanic sand (The frequent tendency in iron to form
hollow concretions or shell containing incoherent matter is singular;
D’Aubuisson (“Traite de Geogn.” tome 1 page 318) remarks on this
circumstance.), or of an argillaceous tuff; in this latter case, the
external crust was quite thin and hard. Some of these spherical balls
were encircled in the line of their equators, by a necklace-like row of
smaller concretions. Again there were other concretions, irregularly
formed, and composed of a hard, compact, ash- coloured stone, with an
almost porcelainous fracture, adhesive to the tongue, and without any
calcareous matter. These beds are, also, interlaced by many veins,
containing gypsum, ferruginous matter, calcareous spar, and agate. It
was here seen with remarkable distinctness, how intimately
concretionary action and the production of fissures and veins are
related together. Figure 20 is an accurate representation of a
horizontal space of tuff, about four feet long by two and a half in
width: the double lines represent the fissures partially filled with
oxide of iron and agate: the curvilinear lines show the course of the
innumerable, concentric, concretionary zones of different shades of
colour and of coarseness in the particles of tuff. The symmetry and
complexity of the arrangement gave the surface an elegant appearance.
It may be seen how obviously the fissures determine (or have been
determined by) the shape, sometimes of the whole concretion, and
sometimes only of its central parts. The fissures also determine the
curvatures of the long undulating zones of concretionary action. From
the varying composition of the veins and concretions, the amount of
chemical action which the mass has undergone is surprisingly great; and
it would likewise appear from the difference in size in the particles
of the concretionary zones, that the mass, also, has been subjected to
internal mechanical movements.
In the peninsula of Lacuy, the strata over a width of four miles have
been upheaved by three distinct, and some other indistinct, lines of
elevation, ranging within a point of north and south. One line, about
two hundred feet in height, is regularly anticlinal, with the strata
dipping away on both sides, at an angle of 15 degrees, from a central
“valley of elevation,” about three hundred yards in width. A second
narrow steep ridge, only sixty feet high, is uniclinal, the strata
throughout dipping westward; those on both flanks being inclined at an
angle of from ten to fifteen degrees; whilst those on the ridge dip in
the same direction at an angle of between thirty and forty degrees.
This ridge, traced northwards, dies away; and the beds at its terminal
point, instead of dipping westward, are inclined 12 degrees to the
north. This case interested me, as being the first in which I found in
South America, formations perhaps of tertiary origin, broken by lines
of elevation.
VALDIVIA: ISLAND OF MOCHA.
The formations of Chiloe seem to extend with nearly the same character
to Valdivia, and for some leagues northward of it: the underlying rocks
are micaceous schists, and are covered up with sandstone and other
sedimentary beds, including, as I was assured, in many places layers of
lignite. I did not land on Mocha (latitude 38 degrees 20 minutes), but
Mr. Stokes brought me specimens of the grey, fine-grained, slightly
calcareous sandstone, precisely like that of Huafo, containing lignite
and numerous Turritellae. The island is flat topped, 1,240 feet in
height, and appears like an outlier of the sedimentary beds on the
mainland. The few shells collected consist of:—
1. Turritella Chilensis, G.B. Sowerby (also at Huafo). 2. Fusus, very
imperfect, somewhat resembling F. subreflexus of Navidad, but probably
different. 3. Venus, fragments of.
CONCEPCION.
Sailing northward from Valdivia, the coast-cliffs are seen, first to
assume near the R. Tolten, and thence for 150 miles northward, to be
continued with the same mineralogical characters, immediately to be
described at Concepcion. I heard in many places of beds of lignite,
some of it fine and glossy, and likewise of silicified wood; near the
Tolten the cliffs are low, but they soon rise in height; and the
horizontal strata are prolonged, with a nearly level surface, until
coming to a more lofty tract between points Rumena and Lavapie. Here
the beds have been broken up by at least eight or nine parallel lines
of elevation, ranging E. or E.N.E. and W. or W.S.W. These lines can be
followed with the eye many miles into the interior; they are all
uniclinal, the strata in each dipping to a point between S. and S.S.E.
with an inclination in the central lines of about forty degrees, and in
the outer ones of under twenty degrees. This band of symmetrically
troubled country is about eight miles in width.
The island of Quiriquina, in the Bay of Concepcion, is formed of
various soft and often ferruginous sandstones, with bands of pebbles,
and with the lower strata sometimes passing into a conglomerate resting
on the underlying metamorphic schists. These beds include subordinate
layers of greenish impure clay, soft micaceous and calcareous
sandstones, and reddish friable earthy matter with white specks like
decomposed crystals of feldspar; they include, also, hard concretions,
fragments of shells, lignite, and silicified wood. In the upper part
they pass into white, soft sediments and brecciolas, very like those
described at Chiloe; as indeed is the whole formation. At Lirguen and
other places on the eastern side of the bay, there are good sections of
the lower sandstones, which are generally ferruginous, but which vary
in character, and even pass into an argillaceous nature; they contain
hard concretions, fragments of lignite, silicified wood, and pebbles
(of the same rocks with the pebbles in the sandstones of Quiriquina),
and they alternate with numerous, often very thin layers of imperfect
coal, generally of little specific gravity. The main bed here is three
feet thick; and only the coal of this one bed has a glossy fracture.
Another irregular, curvilinear bed of brown, compact lignite, is
remarkable for being included in a mass of coarse gravel. These
imperfect coals, when placed in a heap, ignite spontaneously. The
cliffs on this side of the bay, as well as on the island of Quiriquina,
are capped with red friable earth, which, as stated in the Second
Chapter, is of recent formation. The stratification in this
neighbourhood is generally horizontal; but near Lirguen the beds dip
N.W. at an angle of 23 degrees; near Concepcion they are also inclined:
at the northern end of Quiriquina they have been tilted at an angle of
30 degrees, and at the southern end at angles varying from 15 degrees
to 40 degrees: these dislocations must have taken place under the sea.
A collection of shells, from the island of Quiriquina, has been
described by M. d’Orbigny: they are all extinct, and from their generic
character, M. d’Orbigny inferred that they were of tertiary origin:
they consist of:—
1. Scalaria Chilensis, d’Orbigny, “Voyage, Part Pal.” 2. Natica
Araucana, d’Orbigny, “Voyage, Part Pal.” 3. Natica australis,
d’Orbigny, “Voyage, Part Pal.” 4. Fusus difficilis, d’Orbigny, “Voyage,
Part Pal.” 5. Pyrula longirostra, d’Orbigny, “Voyage, Part Pal.” 6.
Pleurotoma Araucana, d’Orbigny, “Voyage, Part Pal.” 7. Cardium auca,
d’Orbigny, “Voyage, Part Pal.” 8. Cardium acuticostatum, d’Orbigny,
“Voyage, Part Pal.” 9. Venus auca, d’Orbigny, “Voyage, Part Pal.” 10.
Mactra cecileana, d’Orbigny, “Voyage, Part Pal.” 11. Mactra Araucana,
d’Orbigny, “Voyage, Part Pal.” 12. Arca Araucana, d’Orbigny, “Voyage,
Part Pal.” 13. Nucula Largillierti, d’Orbigny, “Voyage, Part Pal.” 14.
Trigonia Hanetiana, d’Orbigny, “Voyage, Part Pal.”
During a second visit of the “Beagle” to Concepcion, Mr. Kent collected
for me some silicified wood and shells out of the concretions in the
sandstone from Tome, situated a short distance north of Lirguen. They
consist of:—
1. Natica australis, d’Orbigny, “Voyage, Part Pal.” 2. Mactra Araucana,
d’Orbigny, “Voyage, Part Pal.” 3. Trigonia Hanetiana, d’Orbigny,
“Voyage, Part Pal.” 4. Pecten, fragments of, probably two species, but
too imperfect for description. 5. Baculites vagina, E. Forbes. 6.
Nautilus d’Orbignyanus, E. Forbes.
Besides these shells, Captain Belcher found here an Ammonite, nearly
three feet in diameter, and so heavy that he could not bring it away;
fragments are deposited at Haslar Hospital: he also found the
silicified vertebrae of some very large animal. (“Zoology of Captain
Beechey’s Voyage” page 163.) From the identity in mineralogical nature
of the rocks, and from Captain Belcher’s minute description of the
coast between Lirguen and Tome, the fossiliferous concretions at this
latter place certainly belong to the same formation with the beds
examined by myself at Lirguen; and these again are undoubtedly the same
with the strata of Quiriquina; moreover; the three first of the shells
from Tome, though associated in the same concretions with the Baculite,
are identical with the species from Quiriquina. Hence all the sandstone
and lignitiferous beds in this neighbourhood certainly belong to the
same formation. Although the generic character of the Quiriquina
fossils naturally led M. d’Orbigny to conceive that they were of
tertiary origin, yet as we now find them associated with the Baculites
vagina and with an Ammonite, we must, in the opinion of M. d’Orbigny,
and if we are guided by the analogy of the northern hemisphere, rank
them in the Cretaceous system. Moreover, the Baculites vagina, which is
in a tolerable state of preservation, appears to Professor E. Forbes
certainly to be identical with a species, so named by him, from
Pondicherry in India; where it is associated with numerous decidedly
cretaceous species, which approach most nearly to Lower Greensand or
Neocomian forms: this fact, considering the vast distance between Chile
and India, is truly surprising. Again, the Nautilus d’Orbignyanus, as
far as its imperfect state allows of comparison, resembles, as I am
informed by Professor Forbes, both in its general form and in that of
its chambers, two species from the Upper Greensand. It may be added
that every one of the above-named genera from Quiriquina, which have an
apparently tertiary character, are found in the Pondicherry strata.
There are, however, some difficulties on this view of the formations at
Concepcion being cretaceous, which I shall afterwards allude to; and I
will here only state that the Cardium auca is found also at Coquimbo,
the beds at which place, there can be no doubt, are tertiary.
NAVIDAD. (I was guided to this locality by the Report on M. Gay’s
“Geological Researches” in the “Annales des Scienc. Nat.” 1st series
tome 28.)
The Concepcion formation extends some distance northward, but how far I
know not; for the next point at which I landed was at Navidad, 160
miles north of Concepcion, and 60 miles south of Valparaiso. The cliffs
here are about eight hundred feet in height: they consist, wherever I
could examine them, of fine-grained, yellowish, earthy sandstones, with
ferruginous veins, and with concretions of hard calcareous sandstone.
In one part, there were many pebbles of the common metamorphic
porphyries of the Cordillera: and near the base of the cliff, I
observed a single rounded boulder of greenstone, nearly a yard in
diameter. I traced this sandstone formation beneath the superficial
covering of gravel, for some distance inland: the strata are slightly
inclined from the sea towards the Cordillera, which apparently has been
caused by their having been accumulated against or round outlying
masses of granite, of which some points project near the coast. The
sandstone contains fragments of wood, either in the state of lignite or
partially silicified, sharks’ teeth, and shells in great abundance,
both high up and low down the sea-cliffs. Pectunculus and Oliva were
most numerous in individuals, and next to them Turritella and Fusus. I
collected in a short time, though suffering from illness, the following
thirty-one species, all of which are extinct, and several of the genera
do not now range (as we shall hereafter show) nearly so far south:—
1. Gastridium cepa, G.B. Sowerby. 2. Monoceros, fragments of,
considered by M. d’Orbigny as a new species. 3. Voluta alta, G.B.
Sowerby (considered by M. d’Orbigny as distinct from the V. alta of
Santa Cruz). 4. Voluta triplicata, G.B. Sowerby. 5. Oliva dimidiata,
G.B. Sowerby. 6. Pleurotoma discors, G.B. Sowerby. 7. Pleurotoma
turbinelloides, G.B. Sowerby. 8. Fusus subreflexus, G.B. Sowerby. 9.
Fusus pyruliformis, G.B. Sowerby. 10. Fusus, allied to F. regularis
(considered by M. d’Orbigny as a distinct species). 11. Turritella
suturalis, G.B. Sowerby. 12. Turritella Patagonica, G.B. Sowerby
(fragments of). 13. Trochus laevis, G.B. Sowerby. 14. Trochus collaris,
G.B. Sowerby (considered by M. d’Orbigny as the young of the T.
laevis). 15. Cassis monilifer, G.B. Sowerby. 16. Pyrula distans, G.B.
Sowerby. 17. Triton verruculosus, G.B. Sowerby. 18. Sigaretus
subglobosus, G.B. Sowerby. 19. Natica solida, G.B. Sowerby. (It is
doubtful whether the Natica solida of S. Cruz is the same species with
this.) 20. Terebra undulifera, G.B. Sowerby. 21. Terebra costellata,
G.B. Sowerby. 22. Bulla (fragments of). 23. Dentalium giganteum, do.
24. Dentalium sulcosum, do. 25. Corbis (?) laevigata, do. 26. Cardium
multiradiatum, do. 27. Venus meridionalis, do. 28. Pectunculus dispar,
(?) Desh. (considered by M. d’Orbigny as a distinct species). 29, 30.
Cytheraea and Mactra, fragments of (considered by M. d’Orbigny as new
species). 31. Pecten, fragments of.
COQUIMBO.
(FIGURE 21. SECTION OF THE TERTIARY FORMATION AT COQUIMBO.
From Level of Sea to Surface of plain, 252 feet above sea, through
levels F, E, D and C:
F.—Lower sandstone, with concretions and silicified bones, with fossil
shells, all, or nearly all, extinct.
E.—Upper ferruginous sandstone, with numerous Balani, with fossil
shells, all, or nearly all, extinct.
C and D.—Calcareous beds with recent shells.
A.—Stratified sand in a ravine, also with recent shells.)
For more than two hundred miles northward of Navidad, the coast
consists of plutonic and metamorphic rocks, with the exception of some
quite insignificant superficial beds of recent origin. At Tonguay,
twenty-five miles south of Coquimbo, tertiary beds recommence. I have
already minutely described in the Second Chapter, the step-formed
plains of Coquimbo, and the upper calcareous beds (from twenty to
thirty feet in thickness) containing shells of recent species, but in
different proportions from those on the beach. There remains to be
described only the underlying ancient tertiary beds, represented in
Figure 21 by the letters F and E:—
I obtained good sections of bed F only in Herradura Bay: it consists of
soft whitish sandstone, with ferruginous veins, some pebbles of
granite, and concretionary layers of hard calcareous sandstone. These
concretions are remarkable from the great number of large silicified
bones, apparently of cetaceous animals, which they contain; and
likewise of a shark’s teeth, closely resembling those of the Carcharias
megalodon. Shells of the following species, of which the gigantic
Oyster and Perna are the most conspicuous, are numerously embedded in
the concretions:—
1. Bulla ambigua, d’Orbigny “Voyage” Pal. 2. Monoceros Blainvillii,
d’Orbigny “Voyage” Pal. 3. Cardium auca, d’Orbigny “Voyage” Pal. 4.
Panopaea Coquimbensis, d’Orbigny “Voyage” Pal. 5. Perna Gaudichaudi,
d’Orbigny “Voyage” Pal. 6. Artemis ponderosa; Mr. Sowerby can find no
distinguishing character between this fossil and the recent A.
ponderosa; it is certainly an Artemis, as shown by the pallial
impression. 7. Ostrea Patagonica (?); Mr. Sowerby can point out no
distinguishing character between this species and that so eminently
characteristic of the great Patagonian formation; but he will not
pretend to affirm that they are identical. 8. Fragments of a Venus and
Natica.
The cliffs on one side of Herradura Bay are capped by a mass of
stratified shingle, containing a little calcareous matter, and I did
not doubt that it belonged to the same recent formation with the gravel
on the surrounding plains, also cemented by calcareous matter, until to
my surprise, I found in the midst of it, a single thin layer almost
entirely composed of the above gigantic oyster.
At a little distance inland, I obtained several sections of the bed E,
which, though different in appearance from the lower bed F, belongs to
the same formation: it consists of a highly ferruginous sandy mass,
almost composed, like the lowest bed at Port S. Julian, of fragments of
Balanidae; it includes some pebbles, and layers of yellowish-brown
mudstone. The embedded shells consist of:—
1. Monoceros Blainvillii, d’Orbigny “Voyage” Pal. 2. Monoceros
ambiguus, G.B. Sowerby. 3. Anomia alternans, G.B. Sowerby. 4. Pecten
rudis, G.B. Sowerby. 5. Perna Gaudichaudi, d’Orbigny “Voyage” Pal. 6.
Ostrea Patagonica (?), d’Orbigny “Voyage” Pal. 7. Ostrea, small
species, in imperfect state; it appeared to me like a small kind now
living in, but very rare in the bay. 8. Mytilus Chiloensis; Mr. Sowerby
can find no distinguishing character between this fossil, as far as its
not very perfect condition allows of comparison, and the recent
species. 9. Balanus Coquimbensis, G.B. Sowerby. 10. Balanus psittacus?
King. This appears to Mr. Sowerby and myself identical with a very
large and common species now living on the coast.
The uppermost layers of this ferrugino-sandy mass are conformably
covered by, and impregnated to the depth of several inches with, the
calcareous matter of the bed D called losa: hence I at one time
imagined that there was a gradual passage between them; but as all the
species are recent in the bed D, whilst the most characteristic shells
of the uppermost layers of E are the extinct Perna, Pecten, and
Monoceros, I agree with M. d’Orbigny, that this view is erroneous, and
that there is only a mineralogical passage between them, and no gradual
transition in the nature of their organic remains. Besides the fourteen
species enumerated from these two lower beds, M. d’Orbigny has
described ten other species given to him from this locality; namely:—
1. Fusus Cleryanus, d’Orbigny “Voyage” Pal. 2. Fusus petitianus,
d’Orbigny “Voyage” Pal. 3. Venus hanetiana, d’Orbigny “Voyage” Pal. 4.
Venus incerta (?) d’Orbigny “Voyage” Pal. 5. Venus Cleryana, d’Orbigny
“Voyage” Pal. 6. Venus petitiana, d’Orbigny “Voyage” Pal. 7. Venus
Chilensis, d’Orbigny “Voyage” Pal. 8. Solecurtus hanetianus, d’Orbigny
“Voyage” Pal. 9. Mactra auca, d’Orbigny “Voyage” Pal. 10. Oliva serena,
d’Orbigny “Voyage” Pal.
Of these twenty-four shells, all are extinct, except, according to Mr.
Sowerby, the Artemis ponderosa, Mytilus Chiloensis, and probably the
great Balanus.
COQUIMBO TO COPIAPO.
A few miles north of Coquimbo, I met with the ferruginous, balaniferous
mass E with many silicified bones; I was informed that these silicified
bones occur also at Tonguay, south of Coquimbo: their number is
certainly remarkable, and they seem to take the place of the silicified
wood, so common on the coast-formations of Southern Chile. In the
valley of Chaneral, I again saw this same formation, capped with the
recent calcareous beds. I here left the coast, and did not see any more
of the tertiary formations, until descending to the sea at Copiapo:
here in one place I found variously coloured layers of sand and soft
sandstone, with seams of gypsum, and in another place, a comminuted
shelly mass, with layers of rotten-stone and seams of gypsum, including
many of the extinct gigantic oyster: beds with these oysters are said
to occur at English Harbour, a few miles north of Copiapo.
COAST OF PERU.
With the exception of deposits containing recent shells and of quite
insignificant dimensions, no tertiary formations have been observed on
this coast, for a space of twenty-two degrees of latitude north of
Copiapo, until coming to Payta, where there is said to be a
considerable calcareous deposit: a few fossils have been described by
M. d’Orbigny from this place, namely:—
1. Rostellaria Gaudichaudi, d’Orbigny “Voyage” Pal. 2. Pectunculus
Paytensis, d’Orbigny “Voyage” Pal. 3. Venus petitiana, d’Orbigny
“Voyage” Pal. 4. Ostrea Patagonica? This great oyster (of which
specimens have been given me) cannot be distinguished by Mr. Sowerby
from some of the varieties from Patagonia; though it would be hazardous
to assert it is the same with that species, or with that from Coquimbo.
CONCLUDING REMARKS.
The formations described in this chapter, have, in the case of Chiloe
and probably in that of Concepcion and Navidad, apparently been
accumulated in troughs formed by submarine ridges extending parallel to
the ancient shores of the continent; in the case of the islands of
Mocha and Huafo it is highly probable, and in that of Ypun and Lemus
almost certain, that they were accumulated round isolated rocky centres
or nuclei, in the same manner as mud and sand are now collecting round
the outlying islets and reefs in the West Indian Archipelago. Hence, I
may remark, it does not follow that the outlying tertiary masses of
Mocha and Huafo were ever continuously united at the same level with
the formations on the mainland, though they may have been of
contemporaneous origin, and been subsequently upraised to the same
height. In the more northern parts of Chile, the tertiary strata seem
to have been separately accumulated in bays, now forming the mouths of
valleys.
The relation between these several deposits on the shores of the
Pacific, is not nearly so clear as in the case of the tertiary
formations on the Atlantic. Judging from the form and height of the
land (evidence which I feel sure is here much more trustworthy than it
can ever be in such broken continents as that of Europe), from the
identity of mineralogical composition, from the presence of fragments
of lignite and of silicified wood, and from the intercalated layers of
imperfect coal, I must believe that the coast-formations from Central
Chiloe to Concepcion, a distance of 400 miles, are of the same age:
from nearly similar reasons, I suspect that the beds of Mocha, Huafo,
and Ypun, belong also to the same period. The commonest shell in Mocha
and Huafo is the same species of Turritella; and I believe the same
Cytheraea is found on the islands of Huafo, Chiloe, and Ypun; but with
these trifling exceptions, the few organic remains found at these
places are distinct. The numerous shells from Navidad, with the
exception of two, namely, the Sigaretus and Turritella found at Ypun,
are likewise distinct from those found in any other part of this coast.
Coquimbo has Cardium auca in common with Concepcion, and Fusus
Cleryanus with Huafo; I may add, that Coquimbo has Venus petitiana, and
a gigantic oyster (said by M. d’Orbigny also to be found a little south
of Concepcion) in common with Payta, though this latter place is
situated twenty-two degrees northward of latitude 27 degrees, to which
point the Coquimbo formation extends.
From these facts, and from the generic resemblance of the fossils from
the different localities, I cannot avoid the suspicion that they all
belong to nearly the same epoch, which epoch, as we shall immediately
see, must be a very ancient tertiary one. But as the Baculite,
especially considering its apparent identity with the Cretaceous
Pondicherry species, and the presence of an Ammonite, and the
resemblance of the Nautilus to two upper greensand species, together
afford very strong evidence that the formation of Concepcion is a
Secondary one; I will, in my remarks on the fossils from the other
localities, put on one side those from Concepcion and from Eastern
Chiloe, which, whatever their age may be, appear to me to belong to one
group. I must, however, again call attention to the fact that the
Cardium auca is found both at Concepcion and in the undoubtedly
tertiary strata of Coquimbo: nor should the possibility be overlooked,
that as Trigonia, though known in the northern hemisphere only as a
Secondary genus, has living representatives in the Australian seas, so
a Baculite, Ammonite, and Trigonia may have survived in this remote
part of the southern ocean to a somewhat later period than to the north
of the equator.
Before passing in review the fossils from the other localities, there
are two points, with respect to the formations between Concepcion and
Chiloe, which deserve some notice. First, that though the strata are
generally horizontal, they have been upheaved in Chiloe in a set of
parallel anticlinal and uniclinal lines ranging north and south,—in the
district near P. Rumena by eight or nine far-extended, most
symmetrical, uniclinal lines ranging nearly east and west,—and in the
neighbourhood of Concepcion by less regular single lines, directed both
N.E. and S.W., and N.W. and S.E. This fact is of some interest, as
showing that within a period which cannot be considered as very ancient
in relation to the history of the continent, the strata between the
Cordillera and the Pacific have been broken up in the same variously
directed manner as have the old plutonic and metamorphic rocks in this
same district. The second point is, that the sandstone between
Concepcion and Southern Chiloe is everywhere lignitiferous, and
includes much silicified wood; whereas the formations in Northern Chile
do not include beds of lignite or coal, and in place of the fragments
of silicified wood there are silicified bones. Now, at the present day,
from Cape Horn to near Concepcion, the land is entirely concealed by
forests, which thin out at Concepcion, and in Central and Northern
Chile entirely disappear. This coincidence in the distribution of the
fossil wood and the living forests may be quite accidental; but I
incline to take a different view of it; for, as the difference in
climate, on which the presence of forests depends, is here obviously in
chief part due to the form of the land, and as the Cordillera
undoubtedly existed when the lignitiferous beds were accumulating, I
conceive it is not improbable that the climate, during the
lignitiferous period, varied on different parts of the coast in a
somewhat similar manner as it now does. Looking to an earlier epoch,
when the strata of the Cordillera were depositing, there were islands
which even in the latitude of Northern Chile, where now all is
irreclaimably desert, supported large coniferous forests.
TABLE 4.
Column 1. Genera, with living and tertiary species on the west coast of
South America. (M. d’Orbigny states that the genus Natica is not found
on the coast of Chile; but Mr. Cuming found it at Valparaiso. Scalaria
was found at Valparaiso; Arca, at Iquique, in latitude 20, by Mr.
Cuming; Arca, also, was found by Captain King, at Juan Fernandez, in
latitude 33 degrees 30′S.)
Column 2. Latitudes, in which found fossil on the coasts of Chile and
Peru. (In degrees and minutes.)
Column 3. Southernmost latitude, in which found living on the west
coast of South America. (In degrees and minutes.)
Bulla : 30 to 43 30 : 12 near Lima.
Cassis : 34 : 1 37.
Pyrula : 34 (and 36 30 at Concepcion) : 5 Payta.
Fusus : 30 and 43 30 : 23 Mexillones; reappears at the St. of Magellan.
Pleurotoma : 34 to 43 30 : 2 18 St. Elena.
Terebra : 34 : 5 Payta.
Sigaretus : 34 to 44 30 : 12 Lima.
Anomia : 30 : 7 48.
Perna : 30 : 1 23 Xixappa.
Cardium : 30 to 34 (and 36 30 at Concepcion) : 5 Payta.
Artemis : 30 : 5 Payta.
Voluta : 34 to 44 30 : Mr. Cuming does not know of any species living
on the west coast, between the equator and latitude 43 south; from this
latitude a species is found as far south as Tierra del Fuego.
Seventy-nine species of fossil shells, in a tolerably recognisable
condition, from the coast of Chile and Peru, are described in this
volume, and in the Palaeontological part of M. d’Orbigny’s “Voyage”: if
we put on one side the twenty species exclusively found at Concepcion
and Chiloe, fifty-nine species from Navidad and the other specified
localities remain. Of these fifty-nine species only an Artemis, a
Mytilus and Balanus, all from Coquimbo, are (in the opinion of Mr.
Sowerby, but not in that of M. d’Orbigny) identical with living shells;
and it would certainly require a better series of specimens to render
this conclusion certain. Only the Turritella Chilensis from Huafo and
Mocha, the T. Patagonica and Venus meridionalis from Navidad, come very
near to recent South American shells, namely, the two Turritellas to T.
cingulata, and the Venus to V. exalbida: some few other species come
rather less near; and some few resemble forms in the older European
tertiary deposits: none of the species resemble secondary forms. Hence
I conceive there can be no doubt that these formations are tertiary,—a
point necessary to consider, after the case of Concepcion. The
fifty-nine species belong to thirty-two genera; of these, Gastridium is
extinct, and three or four of the genera (viz. Panopaea, Rostellaria,
Corbis (?), and I believe Solecurtus) are not now found on the west
coast of South America. Fifteen of the genera have on this coast living
representatives in about the same latitudes with the fossil species;
but twelve genera now range very differently to what they formerly did.
The idea of Table 4, in which the difference between the extension in
latitude of the fossil and existing species is shown, is taken from M.
d’Orbigny’s work; but the range of the living shells is given on the
authority of Mr. Cuming, whose long-continued researches on the
conchology of South America are well-known.
When we consider that very few, if any, of the fifty-nine fossil shells
are identical with, or make any close approach to, living species; when
we consider that some of the genera do not now exist on the west coast
of South America, and that no less than twelve genera out of the
thirty-two formerly ranged very differently from the existing species
of the same genera, we must admit that these deposits are of
considerable antiquity, and that they probably verge on the
commencement of the tertiary era. May we not venture to believe, that
they are of nearly contemporaneous origin with the Eocene formations of
the northern hemisphere?
Comparing the fossil remains from the coast of Chile (leaving out, as
before, Concepcion and Chiloe) with those from Patagonia, we may
conclude, from their generic resemblance, and from the small number of
the species which from either coast approach closely to living forms,
that the formations of both belong to nearly the same epoch; and this
is the opinion of M. D’Orbigny. Had not a single fossil shell been
common to the two coasts, it could not have been argued that the
formations belonged to different ages; for Messrs. Cuming and Hinds
have found, on the comparison of nearly two thousand living species
from the opposite sides of South America, only one in common, namely,
the Purpura lapillus from both sides of the Isthmus of Panama: even the
shells collected by myself amongst the Chonos Islands and on the coast
of Patagonia, are dissimilar, and we must descend to the apex of the
continent, to Tierra del Fuego, to find these two great conchological
provinces united into one. Hence it is remarkable that four or five of
the fossil shells from Navidad, namely, Voluta alta, Turritella
Patagonica, Trochus collaris, Venus meridionalis, perhaps Natica
solida, and perhaps the large oyster from Coquimbo, are considered by
Mr. Sowerby as identical with species from Santa Cruz and P. Desire. M.
d’Orbigny, however, admits the perfect identity only of the Trochus.
ON THE TEMPERATURE OF THE TERTIARY PERIOD.
As the number of the fossil species and genera from the western and
eastern coasts is considerable, it will be interesting to consider the
probable nature of the climate under which they lived. We will first
take the case of Navidad, in latitude 34 degrees, where thirty-one
species were collected, and which, as we shall presently see, must have
inhabited shallow water, and therefore will necessarily well exhibit
the effects of temperature. Referring to Table 4 we find that the
existing species of the genera Cassis, Pyrula, Pleurotoma, Terebra, and
Sigaretus, which are generally (though by no means invariably)
characteristic of warmer latitudes, do not at the present day range
nearly so far south on this line of coast as the fossil species
formerly did. Including Coquimbo, we have Perna in the same
predicament. The first impression from this fact is, that the climate
must formerly have been warmer than it now is; but we must be very
cautious in admitting this, for Cardium, Bulla, and Fusus (and, if we
include Coquimbo, Anomia and Artemis) likewise formerly ranged farther
south than they now do; and as these genera are far from being
characteristic of hot climates, their former greater southern range may
well have been owing to causes quite distinct from climate: Voluta,
again, though generally so tropical a genus, is at present confined on
the west coast to colder or more southern latitudes than it was during
the tertiary period. The Trochus collaris, moreover, and, as we have
just seen according to Mr. Sowerby, two or three other species,
formerly ranged from Navidad as far south as Santa Cruz in latitude 50
degrees. If, instead of comparing the fossils of Navidad, as we have
hitherto done, with the shells now living on the west coast of South
America, we compare them with those found in other parts of the world,
under nearly similar latitudes; for instance, in the southern parts of
the Mediterranean or of Australia, there is no evidence that the sea
off Navidad was formerly hotter than what might have been expected from
its latitude, even if it was somewhat warmer than it now is when cooled
by the great southern polar current. Several of the most tropical
genera have no representative fossils at Navidad; and there are only
single species of Cassis, Pyrula, and Sigaretus, two of Pleurotoma and
two of Terebra, but none of these species are of conspicuous size. In
Patagonia, there is even still less evidence in the character of the
fossils, of the climate having been formerly warmer. (It may be worth
while to mention that the shells living at the present day on this
eastern side of South America, in latitude 40 degrees, have perhaps a
more tropical character than those in corresponding latitudes on the
shores of Europe: for at Bahia Blanca and S. Blas, there are two fine
species of Voluta and four of Oliva.) As from the various reasons
already assigned, there can be little doubt that the formations of
Patagonia and at least of Navidad and Coquimbo in Chile, are the
equivalents of an ancient stage in the tertiary formations of the
northern hemisphere, the conclusion that the climate of the southern
seas at this period was not hotter than what might have been expected
from the latitude of each place, appears to me highly important; for we
must believe, in accordance with the views of Mr. Lyell, that the
causes which gave to the older tertiary productions of the quite
temperate zones of Europe a tropical character, WERE OF A LOCAL
CHARACTER AND DID NOT AFFECT THE ENTIRE GLOBE. On the other hand, I
have endeavoured to show, in the “Geological Transactions,” that, at a
much later period, Europe and North and South America were nearly
contemporaneously subjected to ice- action, and consequently to a
colder, or at least more equable, climate than that now characteristic
of the same latitudes.
ON THE ABSENCE OF EXTENSIVE MODERN CONCHIFEROUS DEPOSITS IN SOUTH
AMERICA; AND ON THE CONTEMPORANEOUSNESS OF THE OLDER TERTIARY DEPOSITS
AT DISTANT POINTS BEING DUE TO CONTEMPORANEOUS MOVEMENTS OF SUBSIDENCE.
Knowing from the researches of Professor E. Forbes, that molluscous
animals chiefly abound within a depth of 100 fathoms and under, and
bearing in mind how many thousand miles of both coasts of South America
have been upraised within the recent period by a slow, long-continued,
intermittent movement,—seeing the diversity in nature of the shores and
the number of shells now living on them,—seeing also that the sea off
Patagonia and off many parts of Chile, was during the tertiary period
highly favourable to the accumulation of sediment,—the absence of
extensive deposits including recent shells over these vast spaces of
coast is highly remarkable. The conchiferous calcareous beds at
Coquimbo, and at a few isolated points northward, offer the most marked
exception to this statement; for these beds are from twenty to thirty
feet in thickness, and they stretch for some miles along shore,
attaining, however, only a very trifling breadth. At Valdivia there is
some sandstone with imperfect casts of shells, which POSSIBLY may
belong to the recent period: parts of the boulder formation and the
shingle-beds on the lower plains of Patagonia probably belong to this
same period, but neither are fossiliferous: it also so happens that the
great Pampean formation does not include, with the exception of the
Azara, any mollusca. There cannot be the smallest doubt that the
upraised shells along the shores of the Atlantic and Pacific, whether
lying on the bare surface, or embedded in mould or in sand-hillocks,
will in the course of ages be destroyed by alluvial action: this
probably will be the case even with the calcareous beds of Coquimbo, so
liable to dissolution by rain-water. If we take into consideration the
probability of oscillations of level and the consequent action of the
tidal-waves at different heights, their destruction will appear almost
certain. Looking to an epoch as far distant in futurity as we now are
from the past Miocene period, there seems to me scarcely a chance,
under existing conditions, of the numerous shells now living in those
zones of depths most fertile in life, and found exclusively on the
western and south-eastern coasts of South America, being preserved to
this imaginary distant epoch. A whole conchological series will in time
be swept away, with no memorials of their existence preserved in the
earth’s crust.
Can any light be thrown on this remarkable absence of recent
conchiferous deposits on these coasts, on which, at an ancient tertiary
epoch, strata abounding with organic remains were extensively
accumulated? I think there can, namely, by considering the conditions
necessary for the preservation of a formation to a distant age. Looking
to the enormous amount of denudation which on all sides of us has been
effected,—as evidenced by the lofty cliffs cutting off on so many
coasts horizontal and once far-extended strata of no great antiquity
(as in the case of Patagonia),—as evidenced by the level surface of the
ground on both sides of great faults and dislocations,—by inland lines
of escarpments, by outliers, and numberless other facts, and by that
argument of high generality advanced by Mr. Lyell, namely, that every
SEDIMENTARY formation, whatever its thickness may be, and over however
many hundred square miles it may extend, is the result and the measure
of an equal amount of wear and tear of pre-existing formations;
considering these facts, we must conclude that, as an ordinary rule, a
formation to resist such vast destroying powers, and to last to a
distant epoch, must be of wide extent, and either in itself, or
together with superincumbent strata, be of great thickness. In this
discussion, we are considering only formations containing the remains
of marine animals, which, as before mentioned, live, with some
exceptions within (most of them much within) depths of 100 fathoms.
How, then, can a thick and widely extended formation be accumulated,
which shall include such organic remains? First, let us take the case
of the bed of the sea long remaining at a stationary level: under these
circumstances it is evident that CONCHIFEROUS strata can accumulate
only to the same thickness with the depth at which the shells can live;
on gently inclined coasts alone can they accumulate to any considerable
width; and from the want of superincumbent pressure, it is probable
that the sedimentary matter will seldom be much consolidated: such
formations have no very good chance, when in the course of time they
are upraised, of long resisting the powers of denudation. The chance
will be less if the submarine surface, instead of having remained
stationary, shall have gone on slowly rising during the deposition of
the strata, for in this case their total thickness must be less, and
each part, before being consolidated or thickly covered up by
superincumbent matter, will have had successively to pass through the
ordeal of the beach; and on most coasts, the waves on the beach tend to
wear down and disperse every object exposed to their action. Now, both
on the south-eastern and western shores of South America, we have had
clear proofs that the land has been slowly rising, and in the long
lines of lofty cliffs, we have seen that the tendency of the sea is
almost everywhere to eat into the land. Considering these facts, it
ceases, I think, to be surprising, that extensive recent conchiferous
deposits are entirely absent on the southern and western shores of
America.
Let us take the one remaining case, of the bed of the sea slowly
subsiding during a length of time, whilst sediment has gone on being
deposited. It is evident that strata might thus accumulate to any
thickness, each stratum being deposited in shallow water, and
consequently abounding with those shells which cannot live at great
depths: the pressure, also, I may observe, of each fresh bed would aid
in consolidating all the lower ones. Even on a rather steep coast,
though such must ever be unfavourable to widely extended deposits, the
formations would always tend to increase in breadth from the water
encroaching on the land. Hence we may admit that periods of slow
subsidence will commonly be most favourable to the accumulation of
CONCHIFEROUS deposits, of sufficient thickness, extension, and
hardness, to resist the average powers of denudation.
We have seen that at an ancient tertiary epoch, fossiliferous deposits
were extensively deposited on the coasts of South America; and it is a
very interesting fact, that there is evidence that these ancient
tertiary beds were deposited during a period of subsidence. Thus, at
Navidad, the strata are about eight hundred feet in thickness, and the
fossil shells are abundant both at the level of the sea and some way up
the cliffs; having sent a list of these fossils to Professor E. Forbes,
he thinks they must have lived in water between one and ten fathoms in
depth: hence the bottom of the sea on which these shells once lived
must have subsided at least 700 feet to allow of the superincumbent
matter being deposited. I must here remark, that, as all these and the
following fossil shells are extinct species, Professor Forbes
necessarily judges of the depths at which they lived only from their
generic character, and from the analogical distribution of shells in
the northern hemisphere; but there is no just cause from this to doubt
the general results. At Huafo the strata are about the same thickness,
namely, 800 feet, and Professor Forbes thinks the fossils found there
cannot have lived at a greater depth than fifty fathoms, or 300 feet.
These two points, namely, Navidad and Huafo, are 570 miles apart, but
nearly halfway between them lies Mocha, an island 1,200 feet in height,
apparently formed of tertiary strata up to its level summit, and with
many shells, including the same Turritella with that found at Huafo,
embedded close to the level of the sea. In Patagonia, shells are
numerous at Santa Cruz, at the foot of the 350 feet plain, which has
certainly been formed by the denudation of the 840 feet plain, and
therefore was originally covered by strata that number of feet in
thickness, and these shells, according to Professor Forbes, probably
lived at a depth of between seven and fifteen fathoms: at Port S.
Julian, sixty miles to the north, shells are numerous at the foot of
the ninety feet plain (formed by the denudation of the 950 feet plain),
and likewise occasionally at the height of several hundred feet in the
upper strata; these shells must have lived in water somewhere between
five and fifty fathoms in depth. Although in other parts of Patagonia I
have no direct evidence of shoal-water shells having been buried under
a great thickness of superincumbent submarine strata, yet it should be
borne in mind that the lower fossiliferous strata with several of the
same species of Mollusca, the upper tufaceous beds, and the high
summit-plain, stretch for a considerable distance southward, and for
hundreds of miles northward; seeing this uniformity of structure, I
conceive it may be fairly concluded that the subsidence by which the
shells at Santa Cruz and S. Julian were carried down and covered up,
was not confined to these two points, but was co-extensive with a
considerable portion of the Patagonian tertiary formation. In a
succeeding chapter it will be seen, that we are led to a similar
conclusion with respect to the secondary fossiliferous strata of the
Cordillera, namely, that they also were deposited during a long-
continued and great period of subsidence. From the foregoing reasoning,
and from the facts just given, I think we must admit the probability of
the following proposition: namely, that when the bed of the sea is
either stationary or rising, circumstances are far less favourable,
than when the level is sinking, to the accumulation of CONCHIFEROUS
deposits of sufficient thickness and extension to resist, when
upheaved, the average vast amount of denudation. This result appears to
me, in several respects, very interesting: every one is at first
inclined to believe that at innumerable points, wherever there is a
supply of sediment, fossiliferous strata are now forming, which at some
future distant epoch will be upheaved and preserved; but on the views
above given, we must conclude that this is far from being the case; on
the contrary, we require (1st), a long-continued supply of sediment;
(2nd), an extensive shallow area; and (3rd), that this area shall
slowly subside to a great depth, so as to admit the accumulation of a
widely extended thick mass of superincumbent strata. In how few parts
of the world, probably, do these conditions at the present day concur!
We can thus, also, understand the general want of that close sequence
in fossiliferous formations which we might theoretically have
anticipated; for, without we suppose a subsiding movement to go on at
the same spot during an enormous period, from one geological era to
another, and during the whole of this period sediment to accumulate at
the proper rate, so that the depth should not become too great for the
continued existence of molluscous animals, it is scarcely possible that
there should be a perfect sequence at the same spot in the fossil
shells of the two geological formations. (Professor H.D. Rogers, in his
excellent address to the Association of American Geologists
(“Silliman’s Journal” volume 47 page 277) makes the following remark:
“I question if we are at all aware how COMPLETELY the whole history of
all departed time lies indelibly recorded with the amplest minuteness
of detail in the successive sediments of the globe, how effectually, in
other words, every period of time HAS WRITTEN ITS OWN HISTORY,
carefully preserving every created form and every trace of action.” I
think the correctness of such remarks is more than doubtful, even if we
except (as I suppose he would) all those numerous organic forms which
contain no hard parts.) So far from a very long-continued subsidence
being probable, many facts lead to the belief that the earth’s surface
oscillates up and down; and we have seen that during the elevatory
movements there is but a small chance of DURABLE fossiliferous deposits
accumulating.
Lastly, these same considerations appear to throw some light on the
fact that certain periods appear to have been favourable to the
deposition, or at least to the preservation, of contemporaneous
formations at very distant points. We have seen that in South America
an enormous area has been rising within the recent period; and in other
quarters of the globe immense spaces appear to have risen
contemporaneously. From my examination of the coral- reefs of the great
oceans, I have been led to conclude that the bed of the sea has gone on
slowly sinking within the present era, over truly vast areas: this,
indeed, is in itself probable, from the simple fact of the rising areas
having been so large. In South America we have distinct evidence that
at nearly the same tertiary period, the bed of the sea off parts of the
coast of Chile and off Patagonia was sinking, though these regions are
very remote from each other. If, then, it holds good, as a general
rule, that in the same quarter of the globe the earth’s crust tends to
sink and rise contemporaneously over vast spaces, we can at once see,
that we have at distant points, at the same period, those very
conditions which appear to be requisite for the accumulation of
fossiliferous masses of sufficient extension, thickness, and hardness,
to resist denudation, and consequently to last unto an epoch distant in
futurity. (Professor Forbes has some admirable remarks on this subject,
in his “Report on the Shells of the Aegean Sea.” In a letter to Mr.
Maclaren (“Edinburgh New Philosophical Journal” January 1843), I
partially entered into this discussion, and endeavoured to show that it
was highly improbable, that upraised atolls or barrier-reefs, though of
great thickness, should, owing to their small extension or breadth, be
preserved to a distant future period.)
CHAPTER VI.
PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.
Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of
foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in,
decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S.
Ventana.—Claystone porphyry formation of Patagonia; singular
metamorphic rocks; pseudo-dikes.—Falkland Islands, Palaeozoic fossils
of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of;
cleavage and foliation; form of land.—Chonos Archipelago, mica-schists,
foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes,
successive formation of.—Central and Northern Chile.—Concluding remarks
on cleavage and foliation.—Their close analogy and similar
origin.—Stratification of metamorphic schists.—Foliation of intrusive
rocks.—Relation of cleavage and foliation to the lines of tension
during metamorphosis.
The metamorphic and plutonic formations of the several districts
visited by the “Beagle” will be here chiefly treated of, but only such
cases as appear to me new, or of some special interest, will be
described in detail; at the end of the chapter I will sum up all the
facts on cleavage and foliation,— to which I particularly attended.
BAHIA, BRAZIL: latitude 13 degrees south.
The prevailing rock is gneiss, often passing, by the disappearance of
the quartz and mica, and by the feldspar losing its red colour, into a
brilliantly grey primitive greenstone. Not unfrequently quartz and
hornblende are arranged in layers in almost amorphous feldspar. There
is some fine-grained syenitic granite, orbicularly marked by
ferruginous lines, and weathering into vertical, cylindrical holes,
almost touching each other. In the gneiss, concretions of granular
feldspar and others of garnets with mica occur. The gneiss is traversed
by numerous dikes composed of black, finely crystallised, hornblendic
rock, containing a little glassy feldspar and sometimes mica, and
varying in thickness from mere threads to ten feet: these threads,
which are often curvilinear, could sometimes be traced running into the
larger dikes. One of these dikes was remarkable from having been in two
or three places laterally disjointed, with unbroken gneiss interposed
between the broken ends, and in one part with a portion of the gneiss
driven, apparently whilst in a softened state, into its side or wall.
In several neighbouring places, the gneiss included angular, well-
defined, sometimes bent, masses of hornblende rock, quite like, except
in being more perfectly crystallised, that forming the dikes, and, at
least in one instance, containing (as determined by Professor Miller)
augite as well as hornblende. In one or two cases these angular masses,
though now quite separate from each other by the solid gneiss, had,
from their exact correspondence in size and shape, evidently once been
united; hence I cannot doubt that most or all of the fragments have
been derived from the breaking up of the dikes, of which we see the
first stage in the above- mentioned laterally disjointed one. The
gneiss close to the fragments generally contained many large crystals
of hornblende, which are entirely absent or rare in other parts: its
folia or laminae were gently bent round the fragments, in the same
manner as they sometimes are round concretions. Hence the gneiss has
certainly been softened, its composition modified, and its folia
arranged, subsequently to the breaking up of the dikes, these latter
also having been at the same time bent and softened. (Professor
Hitchcock “Geology of Massachusetts” volume 2 page 673, gives a closely
similar case of a greenstone dike in syenite.)
I must here take the opportunity of premising, that by the term
CLEAVAGE I imply those planes of division which render a rock,
appearing to the eye quite or nearly homogeneous, fissile. By the term
FOLIATION, I refer to the layers or plates of different mineralogical
nature of which most metamorphic schists are composed; there are, also,
often included in such masses, alternating, homogeneous, fissile layers
or folia, and in this case the rock is both foliated and has a
cleavage. By STRATIFICATION, as applied to these formations, I mean
those alternate, parallel, large masses of different composition, which
are themselves frequently either foliated or fissile,—such as the
alternating so-called strata of mica-slate, gneiss, glossy clay-slate,
and marble.
The folia of the gneiss within a few miles round Bahia generally strike
irregularly, and are often curvilinear, dipping in all directions at
various angles: but where best defined, they extended most frequently
in a N.E. by N. (or East 50 degrees N.) and S.W. by S. line,
corresponding nearly with the coast-line northwards of the bay. I may
add that Mr. Gardner found in several parts of the province of Ceara,
which lies between four and five hundred miles north of Bahia, gneiss
with the folia extending E. 45 degrees N.; and in Guyana according to
Sir R. Schomburgk, the same rock strikes E. 57 degrees N. Again,
Humboldt describes the gneiss-granite over an immense area in Venezuela
and even in Colombia, as striking E. 50 degrees N., and dipping to the
N.W. at an angle of fifty degrees. (Gardner “Geological Section of the
British Association” 1840. For Sir R. Schomburgk’s observations see
“Geographical Journal” 1842 page 190. See also Humboldt’s discussion on
Loxodrism in the “Personal Narrative.”) Hence all the observations
hitherto made tend to show that the gneissic rocks over the whole of
this part of the continent have their folia extending generally within
almost a point of the compass of the same direction. (I landed at only
one place north of Bahia, namely, at Pernambuco. I found there only
soft, horizontally stratified matter, formed from disintegrated
granitic rocks, and some yellowish impure limestone, probably of a
tertiary epoch. I have described a most singular natural bar of hard
sandstone, which protects the harbour, in the 19th volume 1841 page 258
of the “London and Edinburgh Philosophical Magazine.”
ABROLHOS ISLETS, Latitude 18 degrees S. off the coast of Brazil.
Although not strictly in place, I do not know where I can more
conveniently describe this little group of small islands. The lowest
bed is a sandstone with ferruginous veins; it weathers into an
extraordinary honeycombed mass; above it there is a dark-coloured
argillaceous shale; above this a coarser sandstone—making a total
thickness of about sixty feet; and lastly, above these sedimentary
beds, there is a fine conformable mass of greenstone, in some parts
having a columnar structure. All the strata, as well as the surface of
the land, dip at an angle of about 12 degrees to N. by W. Some of the
islets are composed entirely of the sedimentary, others of the trappean
rocks, generally, however, with the sandstone, cropping out on the
southern shores.)
RIO DE JANEIRO.
This whole district is almost exclusively formed of gneiss, abounding
with garnets, and porphyritic with large crystals, even three and four
inches in length, of orthoclase feldspar: in these crystals mica and
garnets are often enclosed. At the western base of the Corcovado, there
is some ferruginous carious quartz-rock; and in the Tijeuka range, much
fine- grained granite. I observed boulders of greenstone in several
places; and on the islet of Villegagnon, and likewise on the coast some
miles northward, two large trappean dikes. The porphyritic gneiss, or
gneiss- granite as it has been called by Humboldt, is only so far
foliated that the constituent minerals are arranged with a certain
degree of regularity, and may be said to have a “GRAIN,” but they are
not separated into distinct folia or laminae. There are, however,
several other varieties of gneiss regularly foliated, and alternating
with each other in so-called strata. The stratification and foliation
of the ordinary gneisses, and the foliation or “grain” of the
gneiss-granite, are parallel to each other, and generally strike within
a point of N.E. and S.W. dipping at a high angle (between 50 and 60
degrees) generally to S.E.: so that here again we meet with the strike
so prevalent over the more northern parts of this continent. The
mountains of gneiss-granite are to a remarkable degree abruptly
conical, which seems caused by the rock tending to exfoliate in thick,
conically concentric layers: these peaks resemble in shape those of
phonolite and other injected rocks on volcanic islands; nor is the
grain or foliation (as we shall afterwards see) any difficulty on the
idea of the gneiss-granite having been an intrusive rather than a
metamorphic formation. The lines of mountains, but not always each
separate hill, range nearly in the same direction with the foliation
and so-called stratification, but rather more easterly.
(FIGURE 22. FRAGMENT OF GNEISS EMBEDDED IN ANOTHER VARIETY OF THE SAME
ROCK.)
On a bare gently inclined surface of the porphyritic gneiss in Botofogo
Bay, I observed the appearance represented in Figure 22. A fragment
seven yards long and two in width, with angular and distinctly defined
edges, composed of a peculiar variety of gneiss with dark layers of
mica and garnets, is surrounded on all sides by the ordinary gneiss-
granite; both having been dislocated by a granitic vein. The folia in
the fragment and in the surrounding rock strike in the same N.N.E. and
S.S.W. line; but in the fragment they are vertical, whereas in the
gneiss-granite they dip at a small angle, as shown by the arrows, to
S.S.E. This fragment, considering its great size, its solitary
position, and its foliated structure parallel to that of the
surrounding rock, is, as far as I know, a unique case: and I will not
attempt any explanation of its origin.
The numerous travellers in this country, have all been greatly
surprised at the depth to which the gneiss and other granitic rocks, as
well as the talcose slates of the interior, have been decomposed. (Spix
and Martius have collected in an Appendix to their “Travels,” the
largest body of facts on this subject. See also some remarks by M. Lund
in his communications to the Academy at Copenhagen; and others by M.
Gaudichaud in Freycinet “Voyage.”) Near Rio, every mineral except the
quartz has been completely softened, in some places to a depth little
less than one hundred feet. (Dr. Benza describes granitic rock, “Madras
Journal of Literature” etc. October 183? page 246), in the
Neelgherries, decomposed to a depth of forty feet.) The minerals retain
their positions in folia ranging in the usual direction; and fractured
quartz veins may be traced from the solid rock, running for some
distance into the softened, mottled, highly coloured, argillaceous
mass. It is said that these decomposed rocks abound with gems of
various kinds, often in a fractured state, owing, as some have
supposed, to the collapse of geodes, and that they contain gold and
diamonds. At Rio, it appeared to me that the gneiss had been softened
before the excavation (no doubt by the sea) of the existing, broad,
flat-bottomed valleys; for the depth of decomposition did not appear at
all conformable with the present undulations of the surface. The
porphyritic gneiss, where now exposed to the air, seems to withstand
decomposition remarkably well; and I could see no signs of any tendency
to the production of argillaceous masses like those here described. I
was also struck with the fact, that where a bare surface of this rock
sloped into one of the quiet bays, there were no marks of erosion at
the level of the water, and the parts both beneath and above it
preserved a uniform curve. At Bahia, the gneiss rocks are similarly
decomposed, with the upper parts insensibly losing their foliation, and
passing, without any distinct line of separation, into a bright red
argillaceous earth, including partially rounded fragments of quartz and
granite. From this circumstance, and from the rocks appearing to have
suffered decomposition before the excavation of the valleys, I suspect
that here, as at Rio, the decomposition took place under the sea. The
subject appeared to me a curious one, and would probably well repay
careful examination by an able mineralogist.
THE NORTHERN PROVINCES OF LA PLATA.
According to some observations communicated to me by Mr. Fox, the coast
from Rio de Janeiro to the mouth of the Plata seems everywhere to be
granitic, with a few trappean dikes. At Port Alegre, near the boundary
of Brazil, there are porphyries and diorites. (M. Isabelle “Voyage a
Buenos Ayres” page 479.) At the mouth of the Plata, I examined the
country for twenty-five miles west, and for about seventy miles north
of Maldonado: near this town, there is some common gneiss, and much, in
all parts of the country, of a coarse-grained mixture of quartz and
reddish feldspar, often, however, assuming a little dark-green
imperfect hornblende, and then immediately becoming foliated. The
abrupt hillocks thus composed, as well as the highly inclined folia of
the common varieties of gneiss, strike N.N.E. or a little more
easterly, and S.S.W. Clay-slate is occasionally met with, and near the
L. del Potrero, there is white marble, rendered fissile from the
presence of hornblende, mica, and asbestus; the cleavage of these rocks
and their stratification, that is the alternating masses thus composed,
strike N.N.E. and S.S.W. like the foliated gneisses, and have an almost
vertical dip. The Sierra Larga, a low range five miles west of
Maldonado, consists of quartzite, often ferruginous, having an
arenaceous feel, and divided into excessively thin, almost vertical
laminae or folia by microscopically minute scales, apparently of mica,
and striking in the usual N.N.E. and S.S.W. direction. The range itself
is formed of one principal line with some subordinate ones; and it
extends with remarkable uniformity far northward (it is said even to
the confines of Brazil), in the same line with the vertically ribboned
quartz rock of which it is composed. The S. de Las Animas is the
highest range in the country; I estimated it at 1,000 feet; it runs
north and south, and is formed of feldspathic porphyry; near its base
there is a N.N.W. and S.S.E. ridge of a conglomerate in a highly
porphyritic basis.
Northward of Maldonado, and south of Las Minas, there is an E. and W.
hilly band of country, some miles in width, formed of siliceous
clay-slate, with some quartz, rock, and limestone, having a tortuous
irregular cleavage, generally ranging east and west. E. and S.E. of Las
Minas there is a confused district of imperfect gneiss and laminated
quartz, with the hills ranging in various directions, but with each
separate hill generally running in the same line with the folia of the
rocks of which it is composed: this confusion appears to have been
caused by the intersection of the [E. and W.] and [N.N.E. and S.S.W.]
strikes. Northward of Las Minas, the more regular northerly ranges
predominate: from this place to near Polanco, we meet with the
coarse-grained mixture of quartz and feldspar, often with the imperfect
hornblende, and then becoming foliated in a N. and S. line—with
imperfect clay-slate, including laminae of red crystallised
feldspar—with white or black marble, sometimes containing asbestus and
crystals of gypsum—with quartz-rock—with syenite—and lastly, with much
granite. The marble and granite alternate repeatedly in apparently
vertical masses: some miles northward of the Polanco, a wide district
is said to be entirely composed of marble. It is remarkable, how rare
mica is in the whole range of country north and westward of Maldonado.
Throughout this district, the cleavage of the clay-slate and marble—the
foliation of the gneiss and the quartz—the stratification or
alternating masses of these several rocks—and the range of the hills,
all coincide in direction; and although the country is only hilly, the
planes of division are almost everywhere very highly inclined or
vertical.
Some ancient submarine volcanic rocks are worth mentioning, from their
rarity on this eastern side of the continent. In the valley of the
Tapas (fifty or sixty miles N. of Maldonado) there is a tract three or
four miles in length, composed of various trappean rocks with glassy
feldspar—of apparently metamorphosed grit-stones—of purplish
amygdaloids with large kernels of carbonate of lime (Near the Pan de
Azucar there is some greenish porphyry, in one place amygdaloidal with
agate.)—and much of a harshish rock with glassy feldspar intermediate
in character between claystone porphyry and trachyte. This latter rock
was in one spot remarkable from being full of drusy cavities, lined
with quartz crystals, and arranged in planes, dipping at an angle of 50
degrees to the east, and striking parallel to the foliation of an
adjoining hill composed of the common mixture of quartz, feldspar, and
imperfect hornblende: this fact perhaps indicates that these volcanic
rocks have been metamorphosed, and their constituent parts rearranged,
at the same time and according to the same laws, with the granitic and
metamorphic formations of this whole region. In the valley of the
Marmaraya, a few miles south of the Tapas, a band of trappean and
amygdaloidal rock is interposed between a hill of granite and an
extensive surrounding formation of red conglomerate, which (like that
at the foot of the S. Animas) has its basis porphyritic with crystals
of feldspar, and which hence has certainly suffered metamorphosis.
MONTE VIDEO.
The rocks here consist of several varieties of gneiss, with the
feldspar often yellowish, granular and imperfectly crystallised,
alternating with, and passing insensibly into, beds, from a few yards
to nearly a mile in thickness, of fine or coarse grained, dark-green
hornblendic slate; this again often passing into chloritic schist.
These passages seem chiefly due to changes in the mica, and its
replacement by other minerals. At Rat Island I examined a mass of
chloritic schist, only a few yards square, irregularly surrounded on
all sides by the gneiss, and intricately penetrated by many curvilinear
veins of quartz, which gradually BLEND into the gneiss: the cleavage of
the chloritic schist and the foliation of the gneiss were exactly
parallel. Eastward of the city there is much fine- grained,
dark-coloured gneiss, almost assuming the character of hornblende-
slate, which alternates in thin laminae with laminae of quartz, the
whole mass being transversely intersected by numerous large veins of
quartz: I particularly observed that these veins were absolutely
continuous with the alternating laminae of quartz. In this case and at
Rat Island, the passage of the gneiss into imperfect hornblendic or
into chloritic slate, seemed to be connected with the segregation of
the veins of quartz. (Mr. Greenough page 78 “Critical Examination”
etc., observes that quartz in mica-slate sometimes appears in beds and
sometimes in veins. Von Buch also in his “Travels in Norway” page 236,
remarks on alternating laminae of quartz and hornblende-slate replacing
mica-schist.)
The Mount, a hill believed to be 450 feet in height, from which the
place takes its name, is much the highest land in this neighbourhood:
it consists of hornblendic slate, which (except on the eastern and
disturbed base) has an east and west nearly vertical cleavage; the
longer axis of the hill also ranges in this same line. Near the summit
the hornblende-slate gradually becomes more and more coarsely
crystallised, and less plainly laminated, until it passes into a heavy,
sonorous greenstone, with a slaty conchoidal fracture; the laminae on
the north and south sides near the summit dip inwards, as if this upper
part had expanded or bulged outwards. This greenstone must, I conceive,
be considered as metamorphosed hornblende- slate. The Cerrito, the next
highest, but much less elevated point, is almost similarly composed. In
the more western parts of the province, besides gneiss, there is
quartz-rock, syenite, and granite; and at Colla, I heard of marble.
Near M. Video, the space which I more accurately examined was about
fifteen miles in an east and west line, and here I found the foliation
of the gneiss and the cleavage of the slates generally well developed,
and extending parallel to the alternating strata composed of the
gneiss, hornblendic and chloritic schists. These planes of division all
range within one point of east and west, frequently east by south and
west by north; their dip is generally almost vertical, and scarcely
anywhere under 45 degrees: this fact, considering how slightly
undulatory the surface of the country is, deserves attention. Westward
of M. Video, towards the Uruguay, wherever the gneiss is exposed, the
highly inclined folia are seen striking in the same direction; I must
except one spot where the strike was N.W. by W. The little Sierra de S.
Juan, formed of gneiss and laminated quartz, must also be excepted, for
it ranges between [N. to N.E.] and [S. to S.W.] and seems to belong to
the same system with the hills in the Maldonado district. Finally, we
have seen that, for many miles northward of Maldonado and for
twenty-five miles westward of it, as far as the S. de las Animas, the
foliation, cleavage, so-called stratification and lines of hills, all
range N.N.E. and S.S.W., which is nearly coincident with the adjoining
coast of the Atlantic. Westward of the S. de las Animas, as far as even
the Uruguay, the foliation, cleavage, and stratification (but not lines
of hills, for there are no defined ones) all range about E. by S. and
W. by N., which is nearly coincident with the direction of the northern
shore of the Plata; in the confused country near Las Minas, where these
two great systems appear to intersect each other, the cleavage,
foliation, and stratification run in various directions, but generally
coincide with the line of each separate hill.
SOUTHERN LA PLATA.
The first ridge, south of the Plata, which projects through the Pampean
formation, is the Sierra Tapalguen and Vulcan, situated 200 miles
southward of the district just described. This ridge is only a few
hundred feet in height, and runs from C. Corrientes in a W.N.W. line
for at least 150 miles into the interior: at Tapalguen, it is composed
of unstratified granular quartz, remarkable from forming tabular masses
and small plains, surrounded by precipitous cliffs: other parts of the
range are said to consist of granite: and marble is found at the S.
Tinta. It appears from M. Parchappe’s observations, that at Tandil
there is a range of quartzose gneiss, very like the rocks of the S.
Larga near Maldonado, running in the same N.N.E. and S.S.W. direction;
so that the framework of the country here is very similar to that on
the northern shore of the Plata. (M. d’Orbigny’s “Voyage” Part. Geolog.
page 46. I have given a short account of the peculiar forms of the
quartz hills of Tapalguen, so unusual in a metamorphic formation, in my
“Journal of Researches” 2nd edition page 116.)
The Sierra Guitru-gueyu is situated sixty miles south of the S.
Tapalguen: it consists of numerous parallel, sometimes blended together
ridges, about twenty-three miles in width, and five hundred feet in
height above the plain, and extending in a N.W. and S.E. direction.
Skirting round the extreme S.E. termination, I ascended only a few
points, which were composed of a fine-grained gneiss, almost composed
of feldspar with a little mica, and passing in the upper parts of the
hills into a rather compact purplish clay-slate. The cleavage was
nearly vertical, striking in a N.W. by W. and S.E. by E. line, nearly,
though not quite, coincident with the direction of the parallel ridges.
The Sierra Ventana lies close south of that of Guitru-gueyu; it is
remarkable from attaining a height, very unusual on this side of the
continent, of 3,340 feet. It consists up to its summit, of quartz,
generally pure and white, but sometimes reddish, and divided into thick
laminae or strata: in one part there is a little glossy clay-slate with
a tortuous cleavage. The thick layers of quartz strike in a W. 30
degrees N. line, dipping southerly at an angle of 45 degrees and
upwards. The principal line of mountains, with some quite subordinate
parallel ridges, range about W. 45 degrees N.: but at their S.E.
termination, only W. 25 degrees N. This Sierra is said to extend
between twenty and thirty leagues into the interior.
PATAGONIA.
With the exception perhaps of the hill of S. Antonio (600 feet high) in
the Gulf of S. Matias, which has never been visited by a geologist,
crystalline rocks are not met with on the coast of Patagonia for a
space of 380 miles south of the S. Ventana. At this point (latitude 43
degrees 50 minutes), at Points Union and Tombo, plutonic rocks are said
to appear, and are found, at rather wide intervals, beneath the
Patagonian tertiary formation for a space of about three hundred miles
southward, to near Bird Island, in latitude 48 degrees 56 minutes.
Judging from specimens kindly collected for me by Mr. Stokes, the
prevailing rock at Ports St. Elena, Camerones, Malaspina, and as far
south as the Paps of Pineda, is a purplish-pink or brownish claystone
porphyry, sometimes laminated, sometimes slightly vesicular, with
crystals of opaque feldspar and with a few grains of quartz; hence
these porphyries resemble those immediately to be described at Port
Desire, and likewise a series which I have seen from P. Alegre on the
southern confines of Brazil. This porphyritic formation further
resembles in a singularly close manner the lowest stratified formation
of the Cordillera of Chile, which, as we shall hereafter see, has a
vast range, and attains a great thickness. At the bottom of the Gulf of
St. George, only tertiary deposits appear to be present. At Cape
Blanco, there is quartz rock, very like that of the Falkland Islands,
and some hard, blue siliceous clay-slate.
At Port Desire there is an extensive formation of the claystone
porphyry, stretching at least twenty-five miles into the interior: it
has been denuded and deeply worn into gullies before being covered up
by the tertiary deposits, through which it here and there projects in
hills; those north of the bay being 440 feet in height. The strata have
in several places been tilted at small angles, generally either to
N.N.W. or S.S.E. By gradual passages and alternations, the porphyries
change incessantly in nature. I will describe only some of the
principal mineralogical changes, which are highly instructive, and
which I carefully examined. The prevailing rock has a compact purplish
base, with crystals of earthy or opaque feldspar, and often with grains
of quartz. There are other varieties, with an almost truly trachytic
base, full of little angular vesicles and crystals of glassy feldspar;
and there are beds of black perfect pitchstone, as well as of a
concretionary imperfect variety. On a casual inspection, the whole
series would be thought to be of the same plutonic or volcanic nature
with the trachytic varieties and pitchstone; but this is far from being
the case, as much of the porphyry is certainly of metamorphic origin.
Besides the true porphyries, there are many beds of earthy, quite white
or yellowish, friable, easily fusible matter, resembling chalk, which
under the microscope is seen to consist of minute broken crystals, and
which, as remarked in a former chapter, singularly resembles the upper
tufaceous beds of the Patagonian tertiary formation. This earthy
substance often becomes coarser, and contains minute rounded fragments
of porphyries and rounded grains of quartz, and in one case so many of
the latter as to resemble a common sandstone. These beds are sometimes
marked with true lines of aqueous deposition, separating particles of
different degrees of coarseness; in other cases there are parallel
ferruginous lines not of true deposition, as shown by the arrangement
of the particles, though singularly resembling them. The more indurated
varieties often include many small and some larger angular cavities,
which appear due to the removal of earthy matter: some varieties
contain mica. All these earthy and generally white stones insensibly
pass into more indurated sonorous varieties, breaking with a conchoidal
fracture, yet of small specific gravity; many of these latter varieties
assume a pale purple tint, being singularly banded and veined with
different shades, and often become plainly porphyritic with crystals of
feldspar. The formation of these crystals could be most clearly traced
by minute angular and often partially hollow patches of earthy matter,
first assuming a FIBROUS STRUCTURE, then passing into opaque
imperfectly shaped crystals, and lastly, into perfect glassy crystals.
When these crystals have appeared, and when the basis has become
compact, the rock in many places could not be distinguished from a true
claystone porphyry without a trace of mechanical structure.
In some parts, these earthy or tufaceous beds pass into jaspery and
into beautifully mottled and banded porcelain rocks, which break into
splinters, translucent at their edges, hard enough to scratch glass,
and fusible into white transparent beads: grains of quartz included in
the porcelainous varieties can be seen melting into the surrounding
paste. In other parts, the earthy or tufaceous beds either insensibly
pass into, or alternate with, breccias composed of large and small
fragments of various purplish porphyries, with the matrix generally
porphyritic: these breccias, though their subaqueous origin is in many
places shown both by the arrangement of their smaller particles and by
an oblique or current lamination, also pass into porphyries, in which
every trace of mechanical origin and stratification has been
obliterated.
Some highly porphyritic though coarse-grained masses, evidently of
sedimentary origin, and divided into thin layers, differing from each
other chiefly in the number of embedded grains of quartz, interested me
much from the peculiar manner in which here and there some of the
layers terminated in abrupt points, quite unlike those produced by a
layer of sediment naturally thinning out, and apparently the result of
a subsequent process of metamorphic aggregation. In another common
variety of a finer texture, the aggregating process had gone further,
for the whole mass consisted of quite short, parallel, often slightly
curved layers or patches, of whitish or reddish finely
granulo-crystalline feldspathic matter, generally terminating at both
ends in blunt points; these layers or patches further tended to pass
into wedge or almond-shaped little masses, and these finally into true
crystals of feldspar, with their centres often slightly drusy. The
series was so perfect that I could not doubt that these large crystals,
which had their longer axes placed parallel to each other, had
primarily originated in the metamorphosis and aggregation of
alternating layers of tuff; and hence their parallel position must be
attributed (unexpected though the conclusion may be), not to laws of
chemical action, but to the original planes of deposition. I am tempted
briefly to describe three other singular allied varieties of rock; the
first without examination would have passed for a stratified
porphyritic breccia, but all the included angular fragments consisted
of a border of pinkish crystalline feldspathic matter, surrounding a
dark translucent siliceous centre, in which grains of quartz not quite
blended into the paste could be distinguished: this uniformity in the
nature of the fragments shows that they are not of mechanical, but of
concretionary origin, having resulted perhaps from the self-breaking up
and aggregation of layers of indurated tuff containing numerous grains
of quartz,—into which, indeed, the whole mass in one part passed. The
second variety is a reddish non-porphyritic claystone, quite full of
spherical cavities, about half an inch in diameter, each lined with a
collapsed crust formed of crystals of quartz. The third variety also
consists of a pale purple non-porphyritic claystone, almost wholly
formed of concretionary balls, obscurely arranged in layers, of a less
compact and paler coloured claystone; each ball being on one side
partly hollow and lined with crystals of quartz.
PSEUDO-DIKES.