Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with
infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P.
Gallegos.—Eastern Tierra del Fuego; leaves of extinct
beech-trees.—Summary on the Patagonian tertiary formations.—Tertiary
formations of the Western Coast.—Chonos and Chiloe groups, volcanic
rocks of.—Concepcion.—Navidad.—Coquimbo.—Summary.—Age of the tertiary
formations.—Lines of elevation.—Silicified wood.—Comparative ranges of
the extinct and living mollusca on the West Coast of S.
America.—Climate of the tertiary period.—On the causes of the absence
of recent conchiferous deposits on the coast of S. America.—On the
contemporaneous deposition and preservation of sedimentary formations.
_Rio Negro._—I can add little to the details given by M. d’Orbigny[1]
on the sandstone formation of this district. The cliffs to the south of
the
river are about two hundred feet in height, and are composed of
sandstone of various tints and degrees of hardness. One layer, which
thinned out at both ends, consisted of earthy matter, of a pale reddish
colour, with some gypsum, and very like (I speak after comparison of
the specimens brought home) Pampean mud: above this was a layer of
compact marly rock with dendritic manganese. Many blocks of a
conglomerate of pumice-pebbles embedded in hard sandstone were strewed
at the foot of the cliff, and had evidently fallen from above. A few
miles N.E. of the town, I found, low down in the sandstone, a bed, a
few inches in thickness, of a white, friable, harsh-feeling sediment,
which adheres to the tongue, is of easy fusibility, and of little
specific gravity; examined under the microscope, it is seen to be
pumiceous tuff, formed of broken transparent crystals. In the cliffs
south of the river, there is, also, a thin layer of nearly similar
nature, but finer grained, and not so white; it might easily have been
mistaken for a calcareous tuff, but it contains no lime: this substance
precisely resembles a most widely extended and thick formation in
Southern Patagonia, hereafter to be described, and which is remarkable
for being partially formed of infusoria. These beds, conjointly with
the conglomerate of pumice, are interesting, as showing the nature of
the volcanic action in the Cordillera during this old tertiary period.
[1] “Voyage,” Part. Géolog., pp. 57-65.
In a bed at the base of the southern cliffs, M. d’Orbigny found two
extinct fresh-water shells, namely, a Unio and Chilina. This bed rested
on one with bones of an extinct rodent, namely, the _ Megamys
Patagoniensis_; and this again on another with extinct marine shells.
The species found by M. d’Orbigny in different parts of this formation
consist of:—
Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and whole
coast of Patagonia).
Ostrea Ferrarisi, d’Orbigny, “Voyage, Pal.”
Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and S.
Josef).
Pecten Patagoniensis, d’Orbigny, “Voyage, Pal.”
Venus Munsterii, d’Orbigny, “Voyage, Pal.” (also at St. Fé).
Arca Bonplandiana, d’Orbigny, “Voyage, Pal.” (also at St. Fé).
According to M. d’Orbigny, the sandstone extends westward along the
coast as far as Port S. Antonio, and up the R. Negro far into the
interior: northward I traced it to the southern side of the Rio
Colorado, where it forms a low denuded plain. This formation, though
contemporaneous with that of the rest of Patagonia, is quite different
in mineralogical composition, being connected with it only by the one
thin white layer: this difference may be reasonably attributed to the
sediment brought down in ancient times by the Rio Negro; by which
agency, also, we can understand the presence of the fresh-water shells,
and of the bones of land animals. Judging from the identity of four of
the above shells, this formation is contemporaneous (as remarked by M.
d’Orbigny) with that under the Pampean deposit in Entre Rios and in
Banda Oriental. The gravel capping the sandstone plain, with its
calcareous cement and nodules of gypsum, is probably, from the reasons
given in the First Chapter, contemporaneous with the uppermost beds of
the Pampean formation on the upper plain north of the Colorado.
_San Josef._—My examination here was very short: the cliffs are about a
hundred feet high; the lower third consists of yellowish-brown, soft,
slightly calcareous, muddy sandstone, parts of which when struck emit a
fetid smell. In this bed the great Ostræa Patagonica, often marked with
dendritic manganese and small coral-lines, were extraordinarily
numerous. I found here the following shells:—
Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé and whole
coast of Patagonia).
Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fé and R.
Negro).
Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. Fé, S.
Julian, and Port Desire).
Pecten Darwinianus, d’Orbigny, “Voyage, Pal.” (also at St. Fé).
Pecten actinodes, G. B. Sowerby.
Terebratula Patagonica, G. B. Sowerby (also S. Julian).
Casts of a Turritella.
The four first of these species occur at St. Fé in Entre Rios, and the
two first in the sandstone of the Rio Negro. Above this fossiliferous
mass, there is a stratum of very fine-grained, pale brown mudstone,
including numerous laminæ of selenite. All the strata appear
horizontal, but when followed by the eye for a long distance, they are
seen to have a small easterly dip. On the surface we have the
porphyritic gravel, and on it sand with recent shells.
_Nuevo Gulf._—From specimens and notes given me by Lieutenant Stokes,
it appears that the lower bed consists of soft muddy sandstone, like
that of S. Josef, with many imperfect shells, including the _Pecten
Paranensis_, d’Orbigny, casts of a Turritella and Scutella. On this
there are two strata of the pale brown mudstone, also like that of _S.
Josef_, separated by a darker-coloured, more argillaceous variety,
including the _Ostrea Patagonica._ Professor Ehrenberg has examined
this mudstone for me: he finds in it three already known microscopic
organisms, enveloped in a fine-grained pumiceous tuff, which I shall
have immediately to describe in detail. Specimens brought to me from
the uppermost bed, north of the Rio Chupat, consist of this same
substance, but of a whiter colour.
Tertiary strata, such as here described, appear to extend along the
whole coast between Rio Chupat and Port Desire, except where
interrupted by the underlying claystone porphyry, and by some
metamorphic rocks; these hard rocks, I may add, are found at intervals
over a space of about five degrees of latitude, from Point Union to a
point between Port S. Julian and S. Cruz, and will be described in the
ensuing chapter. Many gigantic specimens of the _Ostrea Patagonica_
were collected in the Gulf of St. George.
_Port Desire._—A good section of the lowest fossiliferous mass, about
forty feet in thickness, resting on claystone porphyry, is exhibited a
few miles south of the harbour. The shells sufficiently perfect to be
recognised consist of:—
Ostrea Patagonica, d’Orbigny, (also at St. Fé, and whole coast of
Patagonia).
Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. Fé, S. Josef,
S. Julian).
Pecten centralis, G. B. Sowerby (also at S. Julian and S. Cruz).
Cucullæa alta, G. B. Sowerby (also at S. Cruz).
Nucula ornata, G. B. Sowerby.
Turritella Patagonica, G. B. Sowerby.
The fossiliferous strata, when not denuded, are conformably covered by
a considerable thickness of the fine-grained pumiceous mudstone,
divided into two masses: the lower half is very fine-grained, slightly
unctuous, and so compact as to break with a semi-conchoidal fracture,
though yielding to the nail; it includes laminæ of selenite: the upper
half precisely resembles the one layer at the Rio Negro, and with the
exception of being whiter, the upper beds at San Josef and Nuevo Gulf.
In neither mass is there any trace to the naked eye of organic forms.
Taking the entire deposit, it is generally quite white, or yellowish,
or feebly tinted with green; it is either almost friable under the
finger, or as hard as chalk; it is of easy fusibility, of little
specific gravity, is not harsh to the touch, adheres to the tongue, and
when breathed on exhales a strong aluminous odour; it sometimes
contains a very little calcareous matter, and traces (besides the
included laminæ) of gypsum. Under the microscope, according to
Professor Ehrenberg,[2] it consists of minute, triturated, cellular,
glassy fragments of pumice, with some broken crystals. In the minute
glassy fragments, Professor Ehrenberg recognises organic structures,
which have been affected by volcanic heat: in the specimens from this
place, and from Port S. Julian, he finds sixteen Polygastrica and
twelve Phytolitharia. Of these organisms, seven are new forms, the
others being previously known: all are of marine, and chiefly of
oceanic, origin. This deposit to the naked eye resembles the crust
which often appears on weathered surfaces of feldspathic rocks; it
likewise resembles those beds of earthy feldspathic matter, sometimes
interstratified with porphyritic rocks, as is the case in this very
district with the underlying purple claystone porphyry. From examining
specimens under a common microscope, and comparing them with other
specimens undoubtedly of volcanic origin, I had come to the same
conclusion with Professor Ehrenberg, namely, that this great deposit,
in its first origin, is of volcanic nature.
[2] “Monatsberichten de könig. Akad. zu Berlin,” vom April 1845.
_Port S. Julian._—On the south side of the harbour, the following
section (figure No. 17) gives the nature of the beds seen in the cliffs
of the ninety feet plain. Beginning at the top:—first, the earthy mass
(AA), including the remains of the Macrauchenia, with recent shells on
the surface; second, the porphyritic shingle (B), which in its lower
part is interstratified (owing, I believe, to redisposition during
denudation) with the white pumiceous mudstone; third, this white
mudstone, about twenty feet in thickness, and divided 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 Scutellæ;
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 Balanidæ. Out of
these three beds, I procured the following twelve species, of which the
two first were exceedingly numerous in individuals, as were the
Terebratulæ and Turritellæ in certain layers:—
Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and whole
coast of Patagonia).
Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (St. Fé, S. Josef, Port
Desire).
Pecten centralis, G. B. Sowerby (also at Port Desire and S. Cruz).
Pecten geminatus, G. B. Sowerby.
Terebratula Patagonica, G. B. Sowerby (also S. Josef).
Struthiolaria ornata, G. B. Sowerby (also S. Cruz).
Fusus Patagonicus, G. B. Sowerby.
Fusus Noachinus, G. B. Sowerby.
Scalaria rugulosa, G. B. Sowerby.
Turritella ambulacrum, G. B. Sowerby (also S. Cruz).
Pyrula, cast of, like P. ventricosa of Sowerby, Tank Cat.
Balanus varians, G. B. Sowerby.
Scutella, differing from the species from Nuevo Gulf.
No. 17
Section of the strata exhibited in the cliffs of the ninety feet plain
at Port S. Julian.
[Illustration: Section of the strata exhibited in the cliffs of the
ninety feet plain at Port S. Julian.]
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 laminæ 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:—
Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé and whole
coast of Patagonia).
Pecten centralis, G. B. Sowerby (also P. Desire and S. Julian).
Venus meridionalis of G. B. Sowerby.
Crassatella Lyellii, G. B. Sowerby.
Cardium puelchum, G. B. Sowerby.
Cardita Patagonica, G. B. Sowerby.
Mactra rugata, G. B. Sowerby.
Mactra Darwinii, G. B. Sowerby.
Cucullæa alta, G. B. Sowerby (also P. Desire).
Trigonocelia insolita, G. B. Sowerby.
Nucula (?) glabra, G. B. Sowerby.
Crepidula gregaria, G. B. Sowerby.
Voluta alta, G. B. Sowerby.
Trochus collaris, G. B. Sowerby.
Natica solida (?), G. B. Sowerby.
Struthiolaria ornata, G. B. Sowerby (also P. Desire).
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 _
Cucullæa alta_ (found at Port Desire in the lower fossiliferous mass)
and of the _Mactra rugata_, which latter shell has been partially
converted into gypsum.
No. 18
Section of the plain at Patagonia, on the banks of the S. Cruz.
[Illustration: Section of the plain at Patagonia, on the banks of the
S. Cruz.]
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,[3] 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 one hundred miles from the coast, that is
at a central point between the Atlantic and the Cordillera, we have the
section in figure No. 18.
[3] 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° 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?
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[4] (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._ 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.
[4] 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.
_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° 7′ 20″: 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° 10′ 53″; 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° 7′ 52″: 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° 1′ 22″; 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° 30′, is that of the great subaerial eruption in
1783 from Skaptar Jukul in Iceland; and M. E. de Beaumont shows[5] that
it must have flowed down a mean inclination of less than 0° 20′. 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°
7′ 52″; 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° 7′ 20″. 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.
[5] “Mémoires pour servir,” etc., pp. 178 and 217.
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 (lat. 51° 35′, 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,[6] 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 laminæ of the muddy sandstone;
they belong, as I am informed by Dr. J. D. Hooker,[7] to three species
of deciduous beech, different from the two species which compose the
great proportion of trees in this forest-clad land. From these facts it
is difficult to conjecture, whether we here see the basal part of the
great Patagonian formation, or some later deposit.
[6] Described in the “Geological Transactions,” vol. vi, p. 415.
[7] “Botany of the Antarctic Voyage,” p. 212.
_Summary on the Patagonian tertiary formation._—Four out of the seven
fossil shells, from St. Fé 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, Cucullæa, 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. Fé. 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 (lat. 40° 30′ S.), I extracted the four
following extinct species of shells:—
Turritella suturalis, G. B. Sowerby (also Navidad).
Sigaretus subglobosus, G. B. Sowerby (also Navidad).
Cytheræa (?) sulculosa (?), G. B. Sowerby (also Chiloe and Huafo?).
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°, 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:—
Bulla cosmophila, G. B. Sowerby.
Pleurotoma subæqualis, G. B. Sowerby.
Fusus cleryanus, d’Orbigny, “Voyage Pal.” (also at Coquimbo).
Triton leucostomoides, G. B. Sowerby.
Turritella Chilensis, G. B. Sowerby (also Mocha).
Venus, probably a distinct species, but very imperfect.
Cytheræa (?) sulculosa (?), probably a distinct species, but very
imperfect.
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:—
Tellinides (?) oblonga, G. B. Sowerby (a solenella in M. d’Orbigny’s
opinion).
Natica striolata, G. B. Sowerby.
Natica (?) pumila, G. B. Sowerby.
Cytheræa (?) 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.[8] 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.
[8] 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 laminæ 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 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 No. 19):—
No. 19
[Illustration: Section at Point Tenuy]
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.
No. 20
Ground plan showing the relation between veins and concretionary zones
in a mass of tuff.
[Illustration: 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,[9] 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.
[9] The frequent tendency in iron to form hollow concretions or shell
containing incoherent matter is singular; D’Aubuisson (“Traité de
Géogn.” tome i, p. 318) remarks on this circumstance.
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°, 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° 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
(lat. 38° 20′), but Mr. Stokes brought me specimens of the grey,
fine-grained, slightly calcareous sandstone, precisely like that of
Huafo, containing lignite and numerous Turritellæ. 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:—
Turritella Chilensis, G. B. Sowerby (also at Huafo).
Fusus, very imperfect, somewhat resembling F. subreflexus of Navidad,
but probably different.
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°; near Concepcion they are also inclined: at the
northern end of Quiriquina they have been tilted at an angle of 30°,
and at the southern end at angles varying from 15° to 40°: 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:—
Scalaria Chilensis, d’Orbigny, “Voyage, Part Pal.”
Natica Araucana, d’Orbigny, “Voyage, Part Pal.”
Natica australis, d’Orbigny, “Voyage, Part Pal.”
Fusus difficilis, d’Orbigny, “Voyage, Part Pal.”
Pyrula longirostra, d’Orbigny, “Voyage, Part Pal.”
Pleurotoma Araucana, d’Orbigny, “Voyage, Part Pal.”
Cardium auca, d’Orbigny, “Voyage, Part Pal.”
Cardium acuticostatum, d’Orbigny, “Voyage, Part Pal.”
Venus auca, d’Orbigny, “Voyage, Part Pal.”
Mactra cecileana, d’Orbigny, “Voyage, Part Pal.”
Mactra Araucana, d’Orbigny, “Voyage, Part Pal.”
Arca Araucana, d’Orbigny, “Voyage, Part Pal.”
Nucula Largillierti, d’Orbigny, “Voyage, Part Pal.”
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:—
Natica australis, d’Orbigny, “Voyage, Part Pal.”
Mactra Araucana, d’Orbigny, “Voyage, Part Pal.”
Trigonia Hanetiana, d’Orbigny, “Voyage, Part Pal.”
Pecten, fragments of, probably two species, but too imperfect for
description.
Baculites vagina, E. Forbes.
Nautilus d’Orbignyanus, E. Forbes.
Besides these shells, Captain Belcher[10] 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 vertebræ of some very large animal. 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.
[10] “Zoology of Captain Beechey’s Voyage,” p. 163.
_Navidad._[11]—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:—
Gastridium cepa, G. B. Sowerby.
Monoceros, fragments of, considered by M. d’Orbigny as a new species.
Voluta alta, G. B. Sowerby (considered by M. d’Orbigny as distinct from
the V. alta of Santa Cruz).
Voluta triplicata, G. B. Sowerby.
Oliva dimidiata, G. B. Sowerby.
Pleurotoma discors, G. B. Sowerby.
Pleurotoma turbinelloides, G. B. Sowerby.
Fusus subreflexus, G. B. Sowerby.
Fusus pyruliformis, G. B. Sowerby.
Fusus, allied to F. regularis (considered by M. d’Orbigny as a distinct
species).
Turritella suturalis, G. B. Sowerby.
Turritella Patagonica, G. B. Sowerby (fragments of).
Trochus lævis, G. B. Sowerby.
Trochus collaris, G. B. Sowerby (considered by M. d’Orbigny as the
young of the T. lævis).
Cassis monilifer, G. B. Sowerby.
Pyrula distans, G. B. Sowerby.
Triton verruculosus, G. B. Sowerby.
Sigaretus subglobosus, G. B. Sowerby.
Natica solida, G. B. Sowerby. (It is doubtful whether the Natica solida
of S. Cruz is the same species with this.)
Terebra undulifera, G. B. Sowerby.
Terebra costellata, G. B. Sowerby.
Bulla (fragments of).
Dentalium giganteum, do.
Dentalium sulcosum, do.
Corbis (?) lævigata, do.
Cardium multiradiatum, do.
Venus meridionalis, do.
Pectunculus dispar, (?) Desh. (considered by M. d’Orbigny as a distinct
species).
and 30. Cytheræa and Mactra, fragments of (considered by M. d’Orbigny
as new species).
Pecten, fragments of.
[11] 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.
_Coquimbo._—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:—
No. 21
Section of the tertiary formation at Coquimbo.
[Illustration: Section of the tertiary formation at Coquimbo.]
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:—
Bulla ambigua, d’Orbigny, “Voyage,” Pal.
Monoceros Blainvillii, d’Orbigny, “Voyage,” Pal.
Cardium auca, d’Orbigny, “Voyage,” Pal.
Panopæa Coquimbensis, d’Orbigny, “Voyage,” Pal.
Perna Gaudichaudi, d’Orbigny, “Voyage,” Pal.
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.
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.
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 Balanidæ; it includes some pebbles, and layers of
yellowish-brown mudstone. The embedded shells consist of:—
Monoceros Blainvillii, d’Orbigny, “Voyage” Pal.
Monoceros ambiguus, G. B. Sowerby.
Anomia alternans, G. B. Sowerby.
Pecten rudis, G. B. Sowerby.
Perna Gaudichaudi, d’Orbigny, “Voyage” Pal.
Ostrea Patagonica (?), d’Orbigny, “Voyage” Pal.
Ostrea, small species, in imperfect state; it appeared to me like a
small kind now living in, but very rare in the bay.
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.
Balanus Coquimbensis, G. B. Sowerby.
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:—
Fusus Cleryanus, d’Orbigny, “Voyage” Pal.
Fusus petitianus, d’Orbigny, “Voyage” Pal.
Venus hanetiana, d’Orbigny, “Voyage” Pal.
Venus incerta (?) d’Orbigny, “Voyage” Pal.
Venus Cleryana, d’Orbigny, “Voyage” Pal.
Venus petitiana, d’Orbigny, “Voyage” Pal.
Venus Chilensis, d’Orbigny, “Voyage” Pal.
Solecurtus hanetianus, d’Orbigny, “Voyage” Pal.
Mactra auca, d’Orbigny, “Voyage” Pal.
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 Chañeral, 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:—
Rostellaria Gaudichaudi, d’Orbigny, “Voyage” Pal.
Pectunculus Paytensis, d’Orbigny, “Voyage” Pal.
Venus petitiana, d’Orbigny, “Voyage” Pal.
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 Cytheræa 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 lat. 27°,
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.
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 Palæontological 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. Panopæa,
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 the table on the following page, 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?
Genera, with living and tertiary species on the west coast of S.
America.[12] Latitudes, in which found fossil on the coasts of Chile
and Peru. Southernmost latitude, in which found living on the west
coast of
S. America. Bulla 30° to 43° 30′ 12° near Lima.
Cassis 34° 1° 37′ Pyrula 34° (and 36° 30′ at Concepcion) 5°
Payta Fusus 30° to 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 lat. 43°
south; from this latitude a species is found as far south as Tierra del
Fuego.
[12] 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 lat. 20°, by Mr. Cuming;
Arca, also, was found by Captain King, at Juan Fernandez, in lat. 33°
30′.
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 lat. 34°, 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 the table given in the previous page, 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.[13]
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.
[13] It may be worth while to mention that the shells living at the
present day on this eastern side of South America, in lat. 40°, 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.
_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 S. 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 S. 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 S. 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.[14] 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.
[14] Professor H. D. Rogers, in his excellent address to the
Association of American Geologists (_Silliman’s Journal,_ vol. xlvii,
p. 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.)
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 S. 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.[15]
[15] Professor Forbes has some admirable remarks on this subject, in
his “Report on the Shells of the Ægean Sea.” In a letter to Mr.
Maclaren (_Edinburgh New Phil. 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.