|
Chapter V
ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.
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 on the sandstone
formation of this district. ("Voyage" Part Geolog. pages 57-65.) 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.
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:--
1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe, and whole
coast of Patagonia).
2. Ostrea Ferrarisi, d'Orbigny, "Voyage, Pal."
3. Ostrea Alvarezii, d'Orbigny, "Voyage, Pal." (also at St. Fe, and S.
Josef).
4. Pecten Patagoniensis, d'Orbigny, "Voyage, Pal."
5. Venus Munsterii, d'Orbigny, "Voyage, Pal." (also at St. Fe).
6. Arca Bonplandiana, d'Orbigny, "Voyage, Pal." (also at St. Fe).
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 Ostraea Patagonica, often marked with dendritic
manganese and small coral-lines, were extraordinarily numerous. I found
here the following shells:--
1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe and whole
coast of Patagonia).
2. Ostrea Alvarezii, d'Orbigny, "Voyage, Pal." (also at St. Fe and R.
Negro).
3. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (also at St. Fe, S. Julian,
and Port Desire).
4. Pecten Darwinianus, d'Orbigny, "Voyage, Pal." (also at St. Fe).
5. Pecten actinodes, G.B. Sowerby.
6. Terebratula Patagonica, G.B. Sowerby (also S. Julian).
7. Casts of a Turritella.
The four first of these species occur at St. Fe 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 laminae 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 Ostraea 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:--
1. Ostrea Patagonica, d'Orbigny, (also at St. Fe, and whole coast of
Patagonia).
2. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (also at St. Fe, S. Josef,
S. Julian).
3. Pecten centralis, G.B. Sowerby (also at S. Julian and S. Cruz).
4. Cucullaea alta, G.B. Sowerby (also at S. Cruz).
5. Nucula ornata, G.B. Sowerby.
6. 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 laminae 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 laminae) of gypsum. Under the microscope, according to Professor
Ehrenberg, it consists of minute, triturated, cellular, glassy fragments of
pumice, with some broken crystals. ("Monatsberichten de konig. Akad. zu
Berlin" vom April 1845.) 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.
PORT S. JULIAN.
(FIGURE 17. SECTION OF THE STRATA EXHIBITED IN THE CLIFFS OF THE NINETY
FEET PLAIN AT PORT S. JULIAN.
(Section through beds from top to bottom: A, B, C, D, E, F.))
On the south side of the harbour, Figure 17 gives the nature of the beds
seen in the cliffs of the ninety feet plain. Beginning at the top:--
1st, 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 Scutellae; and the lowest bed (F)
is an indurated, greenish, sandy clay, including large concretions of
calcareous sandstone, many shells of the great oyster, and in parts almost
made up of fragments of Balanidae. Out of these three beds, I procured the
following twelve species, of which the two first were exceedingly numerous
in individuals, as were the Terebratulae and Turritellae in certain
layers:--
1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe, and whole
coast of Patagonia).
2. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (St. Fe, S. Josef, Port
Desire).
3. Pecten centralis, G.B. Sowerby (also at Port Desire and S. Cruz).
4. Pecten geminatus, G.B. Sowerby.
5. Terebratula Patagonica, G.B. Sowerby (also S. Josef).
6. Struthiolaria ornata, G.B. Sowerby (also S. Cruz).
7. Fusus Patagonicus, G.B. Sowerby.
8. Fusus Noachinus, G.B. Sowerby.
9. Scalaria rugulosa, G.B. Sowerby.
10. Turritella ambulacrum, G.B. Sowerby (also S. Cruz).
11. Pyrula, cast of, like P. ventricosa of Sowerby, Tank Cat.
12. Balanus varians, G.B. Sowerby.
13. Scutella, differing from the species from Nuevo Gulf.
At the head of the inner harbour of Port S. Julian, the fossiliferous mass
is not displayed, and the sea-cliffs from the water's edge to a height of
between one and two hundred feet are formed of the white pumiceous
mudstone, which here includes innumerable, far-extended, sometimes
horizontal, sometimes inclined or vertical laminae of transparent gypsum,
often about an inch in thickness. Further inland, with the exception of the
superficial gravel, the whole thickness of the truncated hills, which
represent a formerly continuous plain 950 feet in height, appears to be
formed of this white mudstone: here and there, however, at various heights,
thin earthy layers, containing the great oyster, Pecten Paranensis and
Turritella ambulacrum, are interstratified; thus showing that the whole
mass belongs to the same epoch. I nowhere found even a fragment of a shell
actually in the white deposit, and only a single cast of a Turritella. Out
of the eighteen microscopic organisms discovered by Ehrenberg in the
specimens from this place, ten are common to the same deposit at Port
Desire. I may add that specimens of this white mudstone, with the same
identical characters were brought me from two points,--one twenty miles
north of S. Julian, where a wide gravel-capped plain, 350 feet in height,
is thus composed; and the other forty miles south of S. Julian, where, on
the old charts, the cliffs are marked as "Chalk Hills."
SANTA CRUZ.
The gravel-capped cliffs at the mouth of the river are 355 feet in height:
the lower part, to a thickness of fifty or sixty feet, consists of a more
or less hardened, darkish, muddy, or argillaceous sandstone (like the
lowest bed of Port Desire), containing very many shells, some silicified
and some converted into yellow calcareous spar. The great oyster is here
numerous in layers; the Trigonocelia and Turritella are also very numerous:
it is remarkable that the Pecten Paranensis, so common in all other parts
of the coast, is here absent: the shells consist of:--
1. Ostrea Patagonica, d'Orbigny; "Voyage Pal." (also at St. Fe and whole
coast of Patagonia).
2. Pecten centralis, G.B. Sowerby (also P. Desire and S. Julian).
3. Venus meridionalis of G.B. Sowerby.
4. Crassatella Lyellii, G.B. Sowerby.
5. Cardium puelchum, G.B. Sowerby.
6. Cardita Patagonica, G.B. Sowerby.
7. Mactra rugata, G.B. Sowerby.
8. Mactra Darwinii, G.B. Sowerby.
9. Cucullaea alta, G.B. Sowerby (also P. Desire).
10. Trigonocelia insolita, G.B. Sowerby.
11. Nucula (?) glabra, G.B. Sowerby.
12. Crepidula gregaria, G.B. Sowerby.
13. Voluta alta, G.B. Sowerby.
14. Trochus collaris, G.B. Sowerby.
15. Natica solida (?), G.B. Sowerby
16. Struthiolaria ornata, G.B. Sowerby (also P. Desire).
17. Turritella ambulacrum, G.B. Sowerby (also P. S. Julian).
Imperfect fragments of the genera Byssoarca, Artemis, and Fusus.
The upper part of the cliff is generally divided into three great strata,
differing slightly in composition, but essentially resembling the pumiceous
mudstone of the places farther north; the deposit, however, here is more
arenaceous, of greater specific gravity, and not so white: it is interlaced
with numerous thin veins, partially or quite filled with transverse fibres
of gypsum; these fibres were too short to reach across the vein, have their
extremities curved or bent: in the same veins with the gypsum, and likewise
in separate veins as well as in little nests, there is much powdery
sulphate of magnesia (as ascertained by Mr. Reeks) in an uncompressed form:
I believe that this salt has not heretofore been found in veins. Of the
three beds, the central one is the most compact, and more like ordinary
sandstone: it includes numerous flattened spherical concretions, often
united like a necklace, composed of hard calcareous sandstone, containing a
few shells: some of these concretions were four feet in diameter, and in a
horizontal line nine feet apart, showing that the calcareous matter must
have been drawn to the centres of attraction, from a distance of four feet
and a half on both sides. In the upper and lower finer-grained strata,
there were other concretions of a grey colour, containing calcareous
matter, and so fine-grained and compact, as almost to resemble porcelain-
rock: I have seen exactly similar concretions in a volcanic tufaceous bed
in Chiloe. Although in this upper fine-grained strata, organic remains were
very rare, yet I noticed a few of the great oyster; and in one included
soft ferruginous layer, there were some specimens of the Cucullaea alta
(found at Port Desire in the lower fossiliferous mass) and of the Mactra
rugata, which latter shell has been partially converted into gypsum.
(FIGURE 18. SECTION OF THE PLAINS OF PATAGONIA, ON THE BANKS OF THE S.
CRUZ.
(Section through strata (from top to bottom)):
Surface of plain with erratic boulders; 1,146 feet above the sea.
a. Gravel and boulders, 212 feet thick.
b. Basaltic lava, 322 feet thick.
c, d and e. Sedimentary layers, bed of small pebbles and talus
respectively, total 592 feet thick.
River of S. Cruz; here 280 feet above sea.)
In ascending the valley of the S. Cruz, the upper strata of the coast-
cliffs are prolonged, with nearly the same characters, for fifty miles: at
about this point, they begin in the most gradual and scarcely perceptible
manner, to be banded with white lines; and after ascending ten miles
farther, we meet with distinct thin layers of whitish, greenish, and
yellowish fine-grained, fusible sediments. At eighty miles from the coast,
in a cliff thus composed, there were a few layers of ferruginous sandstone,
and of an argillaceous sandstone with concretions of marl like those in the
Pampas. (At this spot, for a space of three-quarters of a mile along the
north side of the river, and for a width of half a mile, there has been a
great slip, which has formed hills between sixty and seventy feet in
height, and has tilted the strata into highly inclined and even vertical
positions. The strata generally dipped at an angle of 45 degrees towards
the cliff from which they had slided. I have observed in slips, both on a
small and large scale, that this inward dip is very general. Is it due to
the hydrostatic pressure of water percolating with difficulty through the
strata acting with greater force at the base of the mass than against the
upper part?) At one hundred miles from the coast, that is at a central
point between the Atlantic and the Cordillera, we have the section in
Figure 18.
The upper half of the sedimentary mass, under the basaltic lava, consists
of innumerable zones of perfectly white bright green, yellowish and
brownish, fine-grained, sometimes incoherent, sedimentary matter. The
white, pumiceous, trachytic tuff-like varieties are of rather greater
specific gravity than the pumiceous mudstone on the coast to the north;
some of the layers, especially the browner ones, are coarser, so that the
broken crystals are distinguishable with a weak lens. The layers vary in
character in short distances. With the exception of a few of the Ostrea
Patagonica, which appeared to have rolled down from the cliff above, no
organic remains were found. The chief difference between these layers taken
as a whole, and the upper beds both at the mouth of the river and on the
coast northward, seems to lie in the occasional presence of more colouring
matter, and in the supply having been intermittent; these characters, as we
have seen, very gradually disappear in descending the valley, and this fact
may perhaps be accounted for by the currents of a more open sea having
blended together the sediment from a distant and intermittent source.
The coloured layers in the foregoing section rest on a mass, apparently of
great thickness (but much hidden by the talus), of soft sandstone, almost
composed of minute pebbles, from one-tenth to two-tenths of an inch in
diameter, of the rocks (with the entire exception of the basaltic lava)
composing the great boulders on the surface of the plain, and probably
composing the neighbouring Cordillera. Five miles higher up the valley, and
again thirty miles higher up (that is twenty miles from the nearest range
of the Cordillera), the lower plain included within the upper escarpments,
is formed, as seen on the banks of the river, of a nearly similar but
finer-grained, more earthy, laminated sandstone, alternating with
argillaceous beds, and containing numerous moderately sized pebbles of the
same rocks, and some shells of the great Ostrea Patagonica. (I found at
both places, but not in situ, quantities of coniferous and ordinary
dicotyledonous silicified wood, which was examined for me by Mr. R. Brown.)
As most of these shells had been rolled before being here embedded, their
presence does not prove that the sandstone belongs to the great Patagonian
tertiary formation, for they might have been redeposited in it, when the
valley existed as a sea-strait; but as amongst the pebbles there were none
of basalt, although the cliffs on both sides of the valley are composed of
this rock, I believe that the sandstone does belong to this formation. At
the highest point to which we ascended, twenty miles distant from the
nearest slope of the Cordillera, I could see the horizontally zoned white
beds, stretching under the black basaltic lava, close up to the mountains;
so that the valley of the S. Cruz gives a fair idea of the constitution of
the whole width of Patagonia.
BASALTIC LAVA OF THE S. CRUZ.
This formation is first met with sixty-seven miles from the mouth of the
river; thence it extends uninterruptedly, generally but not exclusively on
the northern side of the valley, close up to the Cordillera. The basalt is
generally black and fine-grained, but sometimes grey and laminated; it
contains some olivine, and high up the valley much glassy feldspar, where,
also, it is often amygdaloidal; it is never highly vesicular, except on the
sides of rents and on the upper and lower, spherically laminated surfaces.
It is often columnar; and in one place I saw magnificent columns, each face
twelve feet in width, with their interstices filled up with calcareous
tuff. The streams rest conformably on the white sedimentary beds, but I
nowhere saw the actual junction; nor did I anywhere see the white beds
actually superimposed on the lava; but some way up the valley at the foot
of the uppermost escarpments, they must be thus superimposed. Moreover, at
the lowest point down the valley, where the streams thin out and terminate
in irregular projections, the spaces or intervals between these projections
are filled up to the level of the now denuded and gravel-capped surfaces of
the plains, with the white-zoned sedimentary beds; proving that this matter
continued to be deposited after the streams had flowed. Hence we may
conclude that the basalt is contemporaneous with the upper parts of the
great tertiary formation.
The lava where first met with is 130 feet in thickness: it there consists
of two, three, or perhaps more streams, divided from each other by
vesicular spheroids like those on the surface. From the streams having, as
it appears, extended to different distances, the terminal points are of
unequal heights. Generally the surface of the basalt is smooth them in one
part high up the valley, it was so uneven and hummocky, that until I
afterwards saw the streams extending continuously on both sides of the
valley up to a height of about three thousand feet close to the Cordillera,
I thought that the craters of eruption were probably close at hand. This
hummocky surface I believe to have been caused by the crossing and heaping
up of different streams. In one place, there were several rounded ridges
about twenty feet in height, some of them as broad as high, and some
broader, which certainly had been formed whilst the lava was fluid, for in
transverse sections each ridge was seen to be concentrically laminated, and
to be composed of imperfect columns radiating from common centres, like the
spokes of wheels.
The basaltic mass where first met with is, as I have said, 130 feet in
thickness, and, thirty-five miles higher up the valley, it increases to 322
feet. In the first fourteen and a half miles of this distance, the upper
surface of the lava, judging from three measurements taken above the level
of the river (of which the apparently very uniform inclination has been
calculated from its total height at a point 135 miles from the mouth),
slopes towards the Atlantic at an angle of only 0 degrees 7 minutes twenty
seconds: this must be considered only as an approximate measurement, but it
cannot be far wrong. Taking the whole thirty-five miles, the upper surface
slopes at an angle of 0 degrees 10 minutes 53 seconds; but this result is
of no value in showing the inclination of any one stream, for halfway
between the two points of measurement, the surface suddenly rises between
one hundred and two hundred feet, apparently caused by some of the
uppermost streams having extended thus far and no farther. From the
measurement made at these two points, thirty-five miles apart, the mean
inclination of the sedimentary beds, over which the lava has flowed, is NOW
(after elevation from under the sea) only 0 degrees 7 minutes 52 seconds:
for the sake of comparison, it may be mentioned that the bottom of the
present sea in a line from the mouth of the S. Cruz to the Falkland
Islands, from a depth of seventeen fathoms to a depth of eighty-five
fathoms, declines at an angle of 0 degrees 1 minute 22 seconds; between the
beach and the depth of seventeen fathoms, the slope is greater. From a
point about half-way up the valley, the basaltic mass rises more abruptly
towards the foot of the Cordillera, namely, from a height of 1,204 feet, to
about 3,000 feet above the sea.
This great deluge of lava is worthy, in its dimensions, of the great
continent to which it belongs. The aggregate streams have flowed from the
Cordillera to a distance (unparalleled, I believe, in any case yet known)
of about one hundred geographical miles. Near their furthest extremity
their total thickness is 130 feet, which increase thirty-five miles farther
inland, as we have just seen, to 322 feet. The least inclination given by
M. E. de Beaumont of the upper surface of a lava-stream, namely 0 degrees
30 minutes, is that of the great subaerial eruption in 1783 from Skaptar
Jukul in Iceland; and M. E. de Beaumont shows that it must have flowed down
a mean inclination of less than 0 degrees 20 minutes. ("Memoires pour
servir" etc. pages 178 and 217.) But we now see that under the pressure of
the sea, successive streams have flowed over a smooth bottom with a mean
inclination of not more than 0 degrees 7 minutes 52 seconds; and that the
upper surface of the terminal portion (over a space of fourteen and a half
miles) has an inclination of not more than 0 degrees 7 minutes 20 seconds.
If the elevation of Patagonia has been greater nearer the Cordillera than
near the Atlantic (as is probable), then these angles are now all too
large. I must repeat, that although the foregoing measurements, which were
all carefully taken with the barometer, may not be absolutely correct, they
cannot be widely erroneous.
Southward of the S. Cruz, the cliffs of the 840 feet plain extend to Coy
Inlet, and owing to the naked patches of the white sediment, they are said
on the charts to be "like the coast of Kent." At Coy Inlet the high plain
trends inland, leaving flat-topped outliers. At Port Gallegos (latitude 51
degrees 35 minutes, and ninety miles south of S. Cruz), I am informed by
Captain Sulivan, R.N., that there is a gravel-capped plain from two to
three hundred feet in height, formed of numerous strata, some fine-grained
and pale-coloured, like the upper beds at the mouth of the S. Cruz, others
rather dark and coarser, so as to resemble gritstones or tuffs; these
latter include rather large fragments of apparently decomposed volcanic
rocks; there are, also, included layers of gravel. This formation is highly
remarkable, from abounding with mammiferous remains, which have not as yet
been examined by Professor Owen, but which include some large, but mostly
small, species of Pachydermata, Edentata, and Rodentia. From the appearance
of the pale-coloured, fine-grained beds, I was inclined to believe that
they corresponded with the upper beds of the S. Cruz; but Professor
Ehrenberg, who has examined some of the specimens, informs me that the
included microscopical organisms are wholly different, being fresh and
brackish-water forms. Hence the two to three hundred feet plain at Port
Gallegos is of unknown age, but probably of subsequent origin to the great
Patagonian tertiary formation.
EASTERN TIERRA DEL FUEGO.
Judging from the height, the general appearance, and the white colour of
the patches visible on the hill sides, the uppermost plain, both on the
north and western side of the Strait of Magellan, and along the eastern
coast of Tierra del Fuego as far south as near Port St. Polycarp, probably
belongs to the great Patagonian tertiary formation, These higher table-
ranges are fringed by low, irregular, extensive plains, belonging to the
boulder formation (Described in the "Geological Transactions" volume 6 page
415.), and composed of coarse unstratified masses, sometimes associated (as
north of C. Virgin's) with fine, laminated, muddy sandstones. The cliffs in
Sebastian Bay are 200 feet in height, and are composed of fine sandstones,
often in curvilinear layers, including hard concretions of calcareous
sandstone, and layers of gravel. In these beds there are fragments of wood,
legs of crabs, barnacles encrusted with corallines still partially
retaining their colour, imperfect fragments of a Pholas distinct from any
known species, and of a Venus, approaching very closely to, but slightly
different in form from, the V. lenticularis, a species living on the coast
of Chile. Leaves of trees are numerous between the laminae of the muddy
sandstone; they belong, as I am informed by Dr. J.D. Hooker, to three
species of deciduous beech, different from the two species which compose
the great proportion of trees in this forest-clad land. ("Botany of the
Antarctic Voyage" page 212.) From these facts it is difficult to
conjecture, whether we here see the basal part of the great Patagonian
formation, or some later deposit.
SUMMARY ON THE PATAGONIAN TERTIARY FORMATION.
Four out of the seven fossil shells, from St. Fe in Entre Rios, were found
by M. d'Orbigny in the sandstone of the Rio Negro, and by me at San Josef.
Three out of the six from San Josef are identical with those from Port
Desire and S. Julian, which two places have together fifteen species, out
of which three are common to both. Santa Cruz has seventeen species, out of
which five are common to Port Desire and S. Julian. Considering the
difference in latitude between these several places, and the small number
of species altogether collected, namely thirty-six, I conceive the above
proportional number of species in common, is sufficient to show that the
lower fossiliferous mass belongs nearly, I do not say absolutely, to the
same epoch. What this epoch may be, compared with the European tertiary
stages, M. d'Orbigny will not pretend to determine. The thirty-six species
(including those collected by myself and by M. d'Orbigny) are all extinct,
or at least unknown; but it should be borne in mind, that the present coast
consists of shingle, and that no one, I believe, has dredged here for
shells; hence it is not improbable that some of the species may hereafter
be found living. Some few of the species are closely related with existing
ones; this is especially the case, according to M. d'Orbigny and Mr.
Sowerby, with the Fusus Patagonicus; and, according to Mr. Sowerby, with
the Pyrula, the Venus meridionalis, the Crepidula gregaria, and the
Turritella ambulacrum, and T. Patagonica. At least three of the genera,
namely, Cucullaea, Crassatella, and (as determined by Mr. Sowerby)
Struthiolaria, are not found in this quarter of the world; and Trigonocelia
is extinct. The evidence taken altogether indicates that this great
tertiary formation is of considerable antiquity; but when treating of the
Chilean beds, I shall have to refer again to this subject.
The white pumiceous mudstone, with its abundant gypsum, belongs to the same
general epoch with the underlying fossiliferous mass, as may be inferred
from the shells included in the intercalated layers at Nuevo Gulf, S.
Julian, and S. Cruz. Out of the twenty-seven marine microscopic structures
found by Professor Ehrenberg in the specimens from S. Julian and Port
Desire, ten are common to these two places: the three found at Nuevo Gulf
are distinct. I have minutely described this deposit, from its remarkable
characters and its wide extension. From Coy Inlet to Port Desire, a
distance of 230 miles, it is certainly continuous; and I have reason to
believe that it likewise extends to the Rio Chupat, Nuevo Gulf, and San
Josef, a distance of 570 miles: we have, also, seen that a single layer
occurs at the Rio Negro. At Port S. Julian it is from eight to nine hundred
feet in thickness; and at S. Cruz it extends, with a slightly altered
character, up to the Cordillera. From its microscopic structure, and from
its analogy with other formations in volcanic districts, it must be
considered as originally of volcanic origin: it may have been formed by the
long-continued attrition of vast quantities of pumice, or judging from the
manner in which the mass becomes, in ascending the valley of S. Cruz,
divided into variously coloured layers, from the long-continued eruption of
clouds of fine ashes. In either case, we must conclude, that the southern
volcanic orifices of the Cordillera, now in a dormant state, were at about
this period over a wide space, and for a great length of time, in action.
We have evidence of this fact, in the latitude of the Rio Negro, in the
sandstone-conglomerate with pumice, and demonstrative proof of it, at S.
Cruz, in the vast deluges of basaltic lava: at this same tertiary period,
also, there is distinct evidence of volcanic action in Western Banda
Oriental.
The Patagonian tertiary formation extends continuously, judging from
fossils alone, from S. Cruz to near the Rio Colorado, a distance of above
six hundred miles, and reappears over a wide area in Entre Rios and Banda
Oriental, making a total distance of 1,100 miles; but this formation
undoubtedly extends (though no fossils were collected) far south of the S.
Cruz, and, according to M. d'Orbigny, 120 miles north of St. Fe. At S. Cruz
we have seen that it extends across the continent; being on the coast about
eight hundred feet in thickness (and rather more at S. Julian), and rising
with the contemporaneous lava-streams to a height of about three thousand
feet at the base of the Cordillera. It rests, wherever any underlying
formation can be seen, on plutonic and metamorphic rocks. Including the
newer Pampean deposit, and those strata in Eastern Tierra del Fuego of
doubtful age, as well as the boulder formation, we have a line of more than
twenty-seven degrees of latitude, equal to that from the Straits of
Gibraltar to the south of Iceland, continuously composed of tertiary
formations. Throughout this great space the land has been upraised, without
the strata having been in a single instance, as far as my means of
observation went, unequally tilted or dislocated by a fault.
TERTIARY FORMATIONS ON THE WEST COAST.
CHONOS ARCHIPELAGO.
The numerous islands of this group, with the exception of Lemus, Ypun,
consist of metamorphic schists; these two islands are formed of softish
grey and brown, fusible, often laminated sandstones, containing a few
pebbles, fragments of black lignite, and numerous mammillated concretions
of hard calcareous sandstone. Out of these concretions at Ypun (latitude 40
degrees 30 minutes S.), I extracted the four following extinct species of
shells:--
1. Turritella suturalis, G.B. Sowerby (also Navidad).
2. Sigaretus subglobosus, G.B. Sowerby (also Navidad).
3. Cytheraea (?) sulculosa (?), G.B. Sowerby (also Chiloe and Huafo?).
4. Voluta, fragments of.
In the northern parts of this group there are some cliffs of gravel and of
the boulder formation. In the southern part (at P. Andres in Tres Montes),
there is a volcanic formation, probably of tertiary origin. The lavas
attain a thickness of from two to three hundred feet; they are extremely
variable in colour and nature, being compact, or brecciated, or cellular,
or amygdaloidal with zeolite, agate and bole, or porphyritic with glassy
albitic feldspar. There is also much imperfect rubbly pitchstone, with the
interstices charged with powdery carbonate of lime apparently of
contemporaneous origin. These lavas are conformably associated with strata
of breccia and of brown tuff containing lignite. The whole mass has been
broken up and tilted at an angle of 45 degrees, by a series of great
volcanic dikes, one of which was thirty yards in breadth. This volcanic
formation resembles one, presently to be described, in Chiloe.
HUAFO.
This island lies between the Chonos and Chiloe groups: it is about eight
hundred feet high, and perhaps has a nucleus of metamorphic rocks. The
strata which I examined consisted of fine-grained muddy sandstones, with
fragments of lignite and concretions of calcareous sandstone. I collected
the following extinct shells, of which the Turritella was in great
numbers:--
1. Bulla cosmophila, G.B. Sowerby.
2. Pleurotoma subaequalis, G.B. Sowerby.
3. Fusus cleryanus, d'Orbigny, "Voyage Pal." (also at Coquimbo).
4. Triton leucostomoides, G.B. Sowerby.
5. Turritella Chilensis, G.B. Sowerby (also Mocha).
6. Venus, probably a distinct species, but very imperfect.
7. Cytheraea (?) sulculosa (?), probably a distinct species, but very
imperfect.
8. Dentalium majus, G.B. Sowerby.
CHILOE.
This fine island is about one hundred miles in length. The entire southern
part, and the whole western coast, consists of mica-schist, which likewise
is seen in the ravines of the interior. The central mountains rise to a
height of 3,000 feet, and are said to be partly formed of granite and
greenstone: there are two small volcanic districts. The eastern coast, and
large parts of the northern extremity of the island are composed of gravel,
the boulder formation, and underlying horizontal strata. The latter are
well displayed for twenty miles north and south of Castro; they vary in
character from common sandstone to fine-grained, laminated mudstones: all
the specimens which I examined are easily fusible, and some of the beds
might be called volcanic grit-stones. These latter strata are perhaps
related to a mass of columnar trachyte which occurs behind Castro. The
sandstone occasionally includes pebbles, and many fragments and layers of
lignite; of the latter, some are apparently formed of wood and others of
leaves: one layer on the N.W. side of Lemuy is nearly two feet in
thickness. There is also much silicified wood, both common dicotyledonous
and coniferous: a section of one specimen in the direction of the medullary
rays has, as I am informed by Mr. R. Brown, the discs in a double row
placed alternately, and not opposite as in the true Araucaria. I found
marine remains only in one spot, in some concretions of hard calcareous
sandstone: in several other districts I have observed that organic remains
were exclusively confined to such concretions; are we to account for this
fact, by the supposition that the shells lived only at these points, or is
it not more probable that their remains were preserved only where
concretions were formed? The shells here are in a bad state, they consist
of:--
1. Tellinides (?) oblonga, G.B. Sowerby (a solenella in M. d'Orbigny's
opinion).
2. Natica striolata, G.B. Sowerby.
3. Natica (?) pumila, G.B. Sowerby.
4. Cytheraea (?) sulculosa, G.B. Sowerby (also Ypun and Huafo?).
At the northern extremity of the island, near S. Carlos, there is a large
volcanic formation, between five and seven hundred feet in thickness. The
commonest lava is blackish-grey or brown, either vesicular, or amygdaloidal
with calcareous spar and bole: most even of the darkest varieties fuse into
a pale-coloured glass. The next commonest variety is a rubbly, rarely well
characterised pitchstone (fusing into a white glass) which passes in the
most irregular manner into stony grey lavas. This pitchstone, as well as
some purple claystone porphyry, certainly flowed in the form of streams.
These various lavas often pass, at a considerable depth from the surface,
in the most abrupt and singular manner into wacke. Great masses of the
solid rock are brecciated, and it was generally impossible to discover
whether the recementing process had been an igneous or aqueous action. (In
a cliff of the hardest fragmentary mass, I found several tortuous, vertical
veins, varying in thickness from a few tenths of an inch to one inch and a
half, of a substance which I have not seen described. It is glossy, and of
a brown colour; it is thinly laminated, with the laminae transparent and
elastic; it is a little harder than calcareous spar; it is infusible under
the blowpipe, sometimes decrepitates, gives out water, curls up, blackens,
and becomes magnetic. Borax easily dissolves a considerable quantity of it,
and gives a glass tinged with green. I have no idea what its true nature
is. On first seeing it, I mistook it for lignite!) The beds are obscurely
separated from each other; they are sometimes parted by seams of tuff and
layers of pebbles. In one place they rested on, and in another place were
capped by, tuffs and girt-stones, apparently of submarine origin.
The neighbouring peninsula of Lacuy is almost wholly formed of tufaceous
deposits, connected probably in their origin with the volcanic hills just
described. The tuffs are pale-coloured, alternating with laminated
mudstones and sandstones (all easily fusible), and passing sometimes into
fine-grained white beds strikingly resembling the great upper infusorial
deposit of Patagonia, and sometimes into brecciolas with pieces of pumice
in the last stage of decay; these again pass into ordinary coarse breccias
and conglomerates of hard rocks. Within very short distances, some of the
finer tuffs often passed into each other in a peculiar manner, namely, by
irregular polygonal concretions of one variety increasing so much and so
suddenly in size, that the second variety, instead of any longer forming
the entire mass, was left merely in thin veins between the concretions. In
a straight line of cliffs, at Point Tenuy, I examined the following
remarkable section (Figure 19):--
(FIGURE 19.)
On the left hand, the lower part (AA) consists of regular, alternating
strata of brown tuffs and greenish laminated mudstone, gently inclined to
the right, and conformably covered by a mass (B left) of a white, tufaceous
and brecciolated deposit. On the right hand, the whole cliff (BB right)
consists of the same white tufaceous matter, which on this side presents
scarcely a trace of stratification, but to the left becomes very gradually
and rather indistinctly divided into strata quite conformable with the
underlying beds (AA): moreover, a few hundred yards further to the left,
where the surface has been less denuded, the tufaceous strata (B left) are
conformably covered by another set of strata, like the underlying ones (AA)
of this section. In the middle of the diagram, the beds (AA) are seen to be
abruptly cut off, and to abut against the tufaceous non-stratified mass;
but the line of junction has been accidentally not represented steep
enough, for I particularly noticed that before the beds had been tilted to
the right, this line must have been nearly vertical. It appears that a
current of water cut for itself a deep and steep submarine channel, and at
the same time or afterwards filled it up with the tufaceous and
brecciolated matter, and spread the same over the surrounding submarine
beds; the matter becoming stratified in these more distant and less
troubled parts, and being moreover subsequently covered up by other strata
(like AA) not shown in the diagram. It is singular that three of the beds
(of AA) are prolonged in their proper direction, as represented, beyond the
line of junction into the white tufaceous matter: the prolonged portions of
two of the beds are rounded; in the third, the terminal fragment has been
pushed upwards: how these beds could have been left thus prolonged, I will
not pretend to explain. In another section on the opposite side of a
promontory, there was at the foot of this same line of junction, that is at
the bottom of the old submarine channel, a pile of fragments of the strata
(AA), with their interstices filled up with white tufaceous matter: this is
exactly what might have been anticipated under such circumstances.
(FIGURE 20. GROUND PLAN SHOWING THE RELATION BETWEEN VEINS AND
CONCRETIONARY ZONES IN A MASS OF TUFF.)
The various tufaceous and other beds at this northern end of Chiloe
probably belong to about the same age with those near Castro, and they
contain, as there, many fragments of black lignite and of silicified and
pyritous wood, often embedded close together. They also contain many and
singular concretions: some are of hard calcareous sandstone, in which it
would appear that broken volcanic crystals and scales of mica have been
better preserved (as in the case of the organic remains near Castro) than
in the surrounding mass. Other concretions in the white brecciola are of a
hard, ferruginous, yet fusible, nature; they are as round as cannon-balls,
and vary from two or three inches to two feet in diameter; their insides
generally consist either of fine, scarcely coherent volcanic sand (The
frequent tendency in iron to form hollow concretions or shell containing
incoherent matter is singular; D'Aubuisson ("Traite de Geogn." tome 1 page
318) remarks on this circumstance.), or of an argillaceous tuff; in this
latter case, the external crust was quite thin and hard. Some of these
spherical balls were encircled in the line of their equators, by a
necklace-like row of smaller concretions. Again there were other
concretions, irregularly formed, and composed of a hard, compact, ash-
coloured stone, with an almost porcelainous fracture, adhesive to the
tongue, and without any calcareous matter. These beds are, also, interlaced
by many veins, containing gypsum, ferruginous matter, calcareous spar, and
agate. It was here seen with remarkable distinctness, how intimately
concretionary action and the production of fissures and veins are related
together. Figure 20 is an accurate representation of a horizontal space of
tuff, about four feet long by two and a half in width: the double lines
represent the fissures partially filled with oxide of iron and agate: the
curvilinear lines show the course of the innumerable, concentric,
concretionary zones of different shades of colour and of coarseness in the
particles of tuff. The symmetry and complexity of the arrangement gave the
surface an elegant appearance. It may be seen how obviously the fissures
determine (or have been determined by) the shape, sometimes of the whole
concretion, and sometimes only of its central parts. The fissures also
determine the curvatures of the long undulating zones of concretionary
action. From the varying composition of the veins and concretions, the
amount of chemical action which the mass has undergone is surprisingly
great; and it would likewise appear from the difference in size in the
particles of the concretionary zones, that the mass, also, has been
subjected to internal mechanical movements.
In the peninsula of Lacuy, the strata over a width of four miles have been
upheaved by three distinct, and some other indistinct, lines of elevation,
ranging within a point of north and south. One line, about two hundred feet
in height, is regularly anticlinal, with the strata dipping away on both
sides, at an angle of 15 degrees, from a central "valley of elevation,"
about three hundred yards in width. A second narrow steep ridge, only sixty
feet high, is uniclinal, the strata throughout dipping westward; those on
both flanks being inclined at an angle of from ten to fifteen degrees;
whilst those on the ridge dip in the same direction at an angle of between
thirty and forty degrees. This ridge, traced northwards, dies away; and the
beds at its terminal point, instead of dipping westward, are inclined 12
degrees to the north. This case interested me, as being the first in which
I found in South America, formations perhaps of tertiary origin, broken by
lines of elevation.
VALDIVIA: ISLAND OF MOCHA.
The formations of Chiloe seem to extend with nearly the same character to
Valdivia, and for some leagues northward of it: the underlying rocks are
micaceous schists, and are covered up with sandstone and other sedimentary
beds, including, as I was assured, in many places layers of lignite. I did
not land on Mocha (latitude 38 degrees 20 minutes), but Mr. Stokes brought
me specimens of the grey, fine-grained, slightly calcareous sandstone,
precisely like that of Huafo, containing lignite and numerous Turritellae.
The island is flat topped, 1,240 feet in height, and appears like an
outlier of the sedimentary beds on the mainland. The few shells collected
consist of:--
1. Turritella Chilensis, G.B. Sowerby (also at Huafo).
2. Fusus, very imperfect, somewhat resembling F. subreflexus of Navidad,
but probably different.
3. Venus, fragments of.
CONCEPCION.
Sailing northward from Valdivia, the coast-cliffs are seen, first to assume
near the R. Tolten, and thence for 150 miles northward, to be continued
with the same mineralogical characters, immediately to be described at
Concepcion. I heard in many places of beds of lignite, some of it fine and
glossy, and likewise of silicified wood; near the Tolten the cliffs are
low, but they soon rise in height; and the horizontal strata are prolonged,
with a nearly level surface, until coming to a more lofty tract between
points Rumena and Lavapie. Here the beds have been broken up by at least
eight or nine parallel lines of elevation, ranging E. or E.N.E. and W. or
W.S.W. These lines can be followed with the eye many miles into the
interior; they are all uniclinal, the strata in each dipping to a point
between S. and S.S.E. with an inclination in the central lines of about
forty degrees, and in the outer ones of under twenty degrees. This band of
symmetrically troubled country is about eight miles in width.
The island of Quiriquina, in the Bay of Concepcion, is formed of various
soft and often ferruginous sandstones, with bands of pebbles, and with the
lower strata sometimes passing into a conglomerate resting on the
underlying metamorphic schists. These beds include subordinate layers of
greenish impure clay, soft micaceous and calcareous sandstones, and reddish
friable earthy matter with white specks like decomposed crystals of
feldspar; they include, also, hard concretions, fragments of shells,
lignite, and silicified wood. In the upper part they pass into white, soft
sediments and brecciolas, very like those described at Chiloe; as indeed is
the whole formation. At Lirguen and other places on the eastern side of the
bay, there are good sections of the lower sandstones, which are generally
ferruginous, but which vary in character, and even pass into an
argillaceous nature; they contain hard concretions, fragments of lignite,
silicified wood, and pebbles (of the same rocks with the pebbles in the
sandstones of Quiriquina), and they alternate with numerous, often very
thin layers of imperfect coal, generally of little specific gravity. The
main bed here is three feet thick; and only the coal of this one bed has a
glossy fracture. Another irregular, curvilinear bed of brown, compact
lignite, is remarkable for being included in a mass of coarse gravel. These
imperfect coals, when placed in a heap, ignite spontaneously. The cliffs on
this side of the bay, as well as on the island of Quiriquina, are capped
with red friable earth, which, as stated in the Second Chapter, is of
recent formation. The stratification in this neighbourhood is generally
horizontal; but near Lirguen the beds dip N.W. at an angle of 23 degrees;
near Concepcion they are also inclined: at the northern end of Quiriquina
they have been tilted at an angle of 30 degrees, and at the southern end at
angles varying from 15 degrees to 40 degrees: these dislocations must have
taken place under the sea.
A collection of shells, from the island of Quiriquina, has been described
by M. d'Orbigny: they are all extinct, and from their generic character, M.
d'Orbigny inferred that they were of tertiary origin: they consist of:--
1. Scalaria Chilensis, d'Orbigny, "Voyage, Part Pal."
2. Natica Araucana, d'Orbigny, "Voyage, Part Pal."
3. Natica australis, d'Orbigny, "Voyage, Part Pal."
4. Fusus difficilis, d'Orbigny, "Voyage, Part Pal."
5. Pyrula longirostra, d'Orbigny, "Voyage, Part Pal."
6. Pleurotoma Araucana, d'Orbigny, "Voyage, Part Pal."
7. Cardium auca, d'Orbigny, "Voyage, Part Pal."
8. Cardium acuticostatum, d'Orbigny, "Voyage, Part Pal."
9. Venus auca, d'Orbigny, "Voyage, Part Pal."
10. Mactra cecileana, d'Orbigny, "Voyage, Part Pal."
11. Mactra Araucana, d'Orbigny, "Voyage, Part Pal."
12. Arca Araucana, d'Orbigny, "Voyage, Part Pal."
13. Nucula Largillierti, d'Orbigny, "Voyage, Part Pal."
14. Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal."
During a second visit of the "Beagle" to Concepcion, Mr. Kent collected for
me some silicified wood and shells out of the concretions in the sandstone
from Tome, situated a short distance north of Lirguen. They consist of:--
1. Natica australis, d'Orbigny, "Voyage, Part Pal."
2. Mactra Araucana, d'Orbigny, "Voyage, Part Pal."
3. Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal."
4. Pecten, fragments of, probably two species, but too imperfect for
description.
5. Baculites vagina, E. Forbes.
6. Nautilus d'Orbignyanus, E. Forbes.
Besides these shells, Captain Belcher found here an Ammonite, nearly three
feet in diameter, and so heavy that he could not bring it away; fragments
are deposited at Haslar Hospital: he also found the silicified vertebrae of
some very large animal. ("Zoology of Captain Beechey's Voyage" page 163.)
From the identity in mineralogical nature of the rocks, and from Captain
Belcher's minute description of the coast between Lirguen and Tome, the
fossiliferous concretions at this latter place certainly belong to the same
formation with the beds examined by myself at Lirguen; and these again are
undoubtedly the same with the strata of Quiriquina; moreover; the three
first of the shells from Tome, though associated in the same concretions
with the Baculite, are identical with the species from Quiriquina. Hence
all the sandstone and lignitiferous beds in this neighbourhood certainly
belong to the same formation. Although the generic character of the
Quiriquina fossils naturally led M. d'Orbigny to conceive that they were of
tertiary origin, yet as we now find them associated with the Baculites
vagina and with an Ammonite, we must, in the opinion of M. d'Orbigny, and
if we are guided by the analogy of the northern hemisphere, rank them in
the Cretaceous system. Moreover, the Baculites vagina, which is in a
tolerable state of preservation, appears to Professor E. Forbes certainly
to be identical with a species, so named by him, from Pondicherry in India;
where it is associated with numerous decidedly cretaceous species, which
approach most nearly to Lower Greensand or Neocomian forms: this fact,
considering the vast distance between Chile and India, is truly surprising.
Again, the Nautilus d'Orbignyanus, as far as its imperfect state allows of
comparison, resembles, as I am informed by Professor Forbes, both in its
general form and in that of its chambers, two species from the Upper
Greensand. It may be added that every one of the above-named genera from
Quiriquina, which have an apparently tertiary character, are found in the
Pondicherry strata. There are, however, some difficulties on this view of
the formations at Concepcion being cretaceous, which I shall afterwards
allude to; and I will here only state that the Cardium auca is found also
at Coquimbo, the beds at which place, there can be no doubt, are tertiary.
NAVIDAD. (I was guided to this locality by the Report on M. Gay's
"Geological Researches" in the "Annales des Scienc. Nat." 1st series tome
28.)
The Concepcion formation extends some distance northward, but how far I
know not; for the next point at which I landed was at Navidad, 160 miles
north of Concepcion, and 60 miles south of Valparaiso. The cliffs here are
about eight hundred feet in height: they consist, wherever I could examine
them, of fine-grained, yellowish, earthy sandstones, with ferruginous
veins, and with concretions of hard calcareous sandstone. In one part,
there were many pebbles of the common metamorphic porphyries of the
Cordillera: and near the base of the cliff, I observed a single rounded
boulder of greenstone, nearly a yard in diameter. I traced this sandstone
formation beneath the superficial covering of gravel, for some distance
inland: the strata are slightly inclined from the sea towards the
Cordillera, which apparently has been caused by their having been
accumulated against or round outlying masses of granite, of which some
points project near the coast. The sandstone contains fragments of wood,
either in the state of lignite or partially silicified, sharks' teeth, and
shells in great abundance, both high up and low down the sea-cliffs.
Pectunculus and Oliva were most numerous in individuals, and next to them
Turritella and Fusus. I collected in a short time, though suffering from
illness, the following thirty-one species, all of which are extinct, and
several of the genera do not now range (as we shall hereafter show) nearly
so far south:--
1. Gastridium cepa, G.B. Sowerby.
2. Monoceros, fragments of, considered by M. d'Orbigny as a new species.
3. Voluta alta, G.B. Sowerby (considered by M. d'Orbigny as distinct from
the V. alta of Santa Cruz).
4. Voluta triplicata, G.B. Sowerby.
5. Oliva dimidiata, G.B. Sowerby.
6. Pleurotoma discors, G.B. Sowerby.
7. Pleurotoma turbinelloides, G.B. Sowerby.
8. Fusus subreflexus, G.B. Sowerby.
9. Fusus pyruliformis, G.B. Sowerby.
10. Fusus, allied to F. regularis (considered by M. d'Orbigny as a distinct
species).
11. Turritella suturalis, G.B. Sowerby.
12. Turritella Patagonica, G.B. Sowerby (fragments of).
13. Trochus laevis, G.B. Sowerby.
14. Trochus collaris, G.B. Sowerby (considered by M. d'Orbigny as the young
of the T. laevis).
15. Cassis monilifer, G.B. Sowerby.
16. Pyrula distans, G.B. Sowerby.
17. Triton verruculosus, G.B. Sowerby.
18. Sigaretus subglobosus, G.B. Sowerby.
19. Natica solida, G.B. Sowerby. (It is doubtful whether the Natica solida
of S. Cruz is the same species with this.)
20. Terebra undulifera, G.B. Sowerby.
21. Terebra costellata, G.B. Sowerby.
22. Bulla (fragments of).
23. Dentalium giganteum, do.
24. Dentalium sulcosum, do.
25. Corbis (?) laevigata, do.
26. Cardium multiradiatum, do.
27. Venus meridionalis, do.
28. Pectunculus dispar, (?) Desh. (considered by M. d'Orbigny as a distinct
species).
29, 30. Cytheraea and Mactra, fragments of (considered by M. d'Orbigny as
new species).
31. Pecten, fragments of.
COQUIMBO.
(FIGURE 21. SECTION OF THE TERTIARY FORMATION AT COQUIMBO.
From Level of Sea to Surface of plain, 252 feet above sea, through levels
F, E, D and C:
F.--Lower sandstone, with concretions and silicified bones, with fossil
shells, all, or nearly all, extinct.
E.--Upper ferruginous sandstone, with numerous Balani, with fossil shells,
all, or nearly all, extinct.
C and D.--Calcareous beds with recent shells.
A.--Stratified sand in a ravine, also with recent shells.)
For more than two hundred miles northward of Navidad, the coast consists of
plutonic and metamorphic rocks, with the exception of some quite
insignificant superficial beds of recent origin. At Tonguay, twenty-five
miles south of Coquimbo, tertiary beds recommence. I have already minutely
described in the Second Chapter, the step-formed plains of Coquimbo, and
the upper calcareous beds (from twenty to thirty feet in thickness)
containing shells of recent species, but in different proportions from
those on the beach. There remains to be described only the underlying
ancient tertiary beds, represented in Figure 21 by the letters F and E:--
I obtained good sections of bed F only in Herradura Bay: it consists of
soft whitish sandstone, with ferruginous veins, some pebbles of granite,
and concretionary layers of hard calcareous sandstone. These concretions
are remarkable from the great number of large silicified bones, apparently
of cetaceous animals, which they contain; and likewise of a shark's teeth,
closely resembling those of the Carcharias megalodon. Shells of the
following species, of which the gigantic Oyster and Perna are the most
conspicuous, are numerously embedded in the concretions:--
1. Bulla ambigua, d'Orbigny "Voyage" Pal.
2. Monoceros Blainvillii, d'Orbigny "Voyage" Pal.
3. Cardium auca, d'Orbigny "Voyage" Pal.
4. Panopaea Coquimbensis, d'Orbigny "Voyage" Pal.
5. Perna Gaudichaudi, d'Orbigny "Voyage" Pal.
6. Artemis ponderosa; Mr. Sowerby can find no distinguishing charac
< BackForward >
|
