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Chapter IV
ON THE FORMATIONS OF THE PAMPAS.
Mineralogical constitution.
Microscopical structure.
Buenos Ayres, shells embedded in tosca-rock.
Buenos Ayres to the Colorado.
San Ventana.
Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta, shells, bones,
and infusoria of; co-existence of the recent shells and extinct mammifers.
Buenos Ayres to Santa Fe.
Skeletons of Mastodon.
Infusoria.
Inferior marine tertiary strata, their age.
Horse's tooth.
BANDA ORIENTAL.
Superficial Pampean formation.
Inferior tertiary strata, variation of, connected with volcanic action;
Macrauchenia Patachonica at San Julian in Patagonia, age of, subsequent to
living mollusca and to the erratic block period.
SUMMARY.
Area of Pampean formation.
Theories of origin.
Source of sediment.
Estuary origin.
Contemporaneous with existing mollusca.
Relations to underlying tertiary strata.
Ancient deposit of estuary origin.
Elevation and successive deposition of the Pampean formation.
Number and state of the remains of mammifers; their habitation, food,
extinction, and range.
Conclusion.
Localities in Pampas at which mammiferous remains have been found.
The Pampean formation is highly interesting from its vast extent, its
disputed origin, and from the number of extinct gigantic mammifers embedded
in it. It has upon the whole a very uniform character: consisting of a more
or less dull reddish, slightly indurated, argillaceous earth or mud, often,
but not always, including in horizontal lines concretions of marl, and
frequently passing into a compact marly rock. The mud, wherever I examined
it, even close to the concretions, did not contain any carbonate of lime.
The concretions are generally nodular, sometimes rough externally,
sometimes stalactiformed; they are of a compact structure, but often
penetrated (as well as the mud) by hair-like serpentine cavities, and
occasionally with irregular fissures in their centres, lined with minute
crystals of carbonate of lime; they are of white, brown, or pale pinkish
tints, often marked by black dendritic manganese or iron; they are either
darker or lighter tinted than the surrounding mass; they contain much
carbonate of lime, but exhale a strong aluminous odour, and leave, when
dissolved in acids, a large but varying residue, of which the greater part
consists of sand. These concretions often unite into irregular strata; and
over very large tracts of country, the entire mass consists of a hard, but
generally cavernous marly rock: some of the varieties might be called
calcareous tuffs.
Dr. Carpenter has kindly examined under the microscope, sliced and polished
specimens of these concretions, and of the solid marl-rock, collected in
various places between the Colorado and Santa Fe Bajada. In the greater
number, Dr. Carpenter finds that the whole substance presents a tolerably
uniform amorphous character, but with traces of incipient crystalline
metamorphosis; in other specimens he finds microscopically minute rounded
concretions of an amorphous substance (resembling in size those in oolitic
rocks, but not having a concentric structure), united by a cement which is
often crystalline. In some, Dr. Carpenter can perceive distinct traces of
shells, corals, Polythalamia, and rarely of spongoid bodies. For the sake
of comparison, I sent Dr. Carpenter specimens of the calcareous rock,
formed chiefly of fragments of recent shells, from Coquimbo in Chile: in
one of these specimens, Dr. Carpenter finds, besides the larger fragments,
microscopical particles of shells, and a varying quantity of opaque
amorphous matter; in another specimen from the same bed, he finds the whole
composed of the amorphous matter, with layers showing indications of an
incipient crystalline metamorphosis: hence these latter specimens, both in
external appearance and in microscopical structure, closely resemble those
of the Pampas. Dr. Carpenter informs me that it is well known that chemical
precipitation throws down carbonate of lime in the opaque amorphous state;
and he is inclined to believe that the long-continued attrition of a
calcareous body in a state of crystalline or semi-crystalline aggregation
(as, for instance, in the ordinary shells of Mollusca, which, when sliced,
are transparent) may yield the same result. From the intimate relations
between all the Coquimbo specimens, I can hardly doubt that the amorphous
carbonate of lime in them has resulted from the attrition and decay of the
larger fragments of shell: whether the amorphous matter in the marly rocks
of the Pampas has likewise thus originated, it would be hazardous to
conjecture.
For convenience' sake, I will call the marly rock by the name given to it
by the inhabitants, namely, Tosca-rock; and the reddish argillaceous earth,
Pampean mud. This latter substance, I may mention, has been examined for me
by Professor Ehrenberg, and the result of his examination will be given
under the proper localities.
I will commence my descriptions at a central spot, namely, at Buenos Ayres,
and thence proceed first southward to the extreme limit of the deposit, and
afterwards northward. The plain on which Buenos Ayres stands is from thirty
to forty feet in height. The Pampean mud is here of a rather pale colour,
and includes small nearly white nodules, and other irregular strata of an
unusually arenaceous variety of tosca-rock. In a well at the depth of
seventy feet, according to Ignatio Nunez, much tosca-rock was met with, and
at several points, at one hundred feet deep, beds of sand have been found.
I have already given a list of the recent marine and estuary shells found
in many parts on the surface near Buenos Ayres, as far as three or four
leagues from the Plata. Specimens from near Ensenada, given me by Sir W.
Parish, where the rock is quarried just beneath the surface of the plain,
consist of broken bivalves, cemented by and converted into white
crystalline carbonate of lime. I have already alluded, in the first
chapter, to a specimen (also given me by Sir W. Parish) from the A. del
Tristan, in which shells, resembling in every respect the Azara labiata,
d'Orbigny, as far as their worn condition permits of comparison, are
embedded in a reddish, softish, somewhat arenaceous marly rock: after
careful comparison, with the aid of a microscope and acids, I can perceive
no difference between the basis of this rock and the specimens collected by
me in many parts of the Pampas. I have also stated, on the authority of Sir
W. Parish, that northward of Buenos Ayres, on the highest parts of the
plain, about forty feet above the Plata, and two or three miles from it,
numerous shells of the Azara labiata (and I believe of Venus sinuosa) occur
embedded in a stratified earthy mass, including small marly concretions,
and said to be precisely like the great Pampean deposit. Hence we may
conclude that the mud of the Pampas continued to be deposited to within the
period of this existing estuary shell. Although this formation is of such
immense extent, I know of no other instance of the presence of shells in
it.
BUENOS AYRES TO THE RIO COLORADO.
With the exception of a few metamorphic ridges, the country between these
two points, a distance of 400 geographical miles, belongs to the Pampean
formation, and in the southern part is generally formed of the harder and
more calcareous varieties. I will briefly describe my route: about twenty-
five miles S.S.W. of the capital, in a well forty yards in depth, the upper
part, and, as I was assured, the entire thickness, was formed of dark red
Pampean mud without concretions. North of the River Salado, there are many
lakes; and on the banks of one (near the Guardia) there was a little cliff
similarly composed, but including many nodular and stalactiform
concretions: I found here a large piece of tessellated armour, like that of
the Glyptodon, and many fragments of bones. The cliffs on the Salado
consist of pale-coloured Pampean mud, including and passing into great
masses of tosca-rock: here a skeleton of the Megatherium and the bones of
other extinct quadrupeds (see the list at the end of this chapter) were
found. Large quantities of crystallised gypsum (of which specimens were
given me) occur in the cliffs of this river; and likewise (as I was assured
by Mr. Lumb) in the Pampean mud on the River Chuelo, seven leagues from
Buenos Ayres: I mention this because M. d'Orbigny lays some stress on the
supposed absence of this mineral in the Pampean formation.
Southward of the Salado the country is low and swampy, with tosca-rock
appearing at long intervals at the surface. On the banks, however, of the
Tapalguen (sixty miles south of the Salado) there is a large extent of
tosca-rock, some highly compact and even semi-crystalline, overlying pale
Pampean mud with the usual concretions. Thirty miles further south, the
small quartz-ridge of Tapalguen is fringed on its northern and southern
flank, by little, narrow, flat-topped hills of tosca-rock, which stand
higher than the surrounding plain. Between this ridge and the Sierra of
Guitru-gueyu, a distance of sixty miles, the country is swampy, with the
tosca-rock appearing only in four or five spots: this sierra, precisely
like that of Tapalguen, is bordered by horizontal, often cliff-bounded,
little hills of tosca-rock, higher than the surrounding plain. Here, also,
a new appearance was presented in some extensive and level banks of
alluvium or detritus of the neighbouring metamorphic rocks; but I neglected
to observe whether it was stratified or not. Between Guitru-gueyu and the
Sierra Ventana, I crossed a dry plain of tosca-rock higher than the country
hitherto passed over, and with small pieces of denuded tableland of the
same formation, standing still higher.
The marly or calcareous beds not only come up nearly horizontally to the
northern and southern foot of the great quartzose mountains of the Sierra
Ventana, but interfold between the parallel ranges. The superficial beds
(for I nowhere obtained sections more than twenty feet deep) retain, even
close to the mountains, their usual character: the uppermost layer,
however, in one place included pebbles of quartz, and rested on a mass of
detritus of the same rock. At the very foot of the mountains, there were
some few piles of quartz and tosca-rock detritus, including land-shells;
but at the distance of only half a mile from these lofty, jagged, and
battered mountains, I could not, to my great surprise, find on the
boundless surface of the calcareous plain even a single pebble. Quartz-
pebbles, however, of considerable size have at some period been transported
to a distance of between forty and fifty miles to the shores of Bahia
Blanca. (Schmidtmeyer "Travels in Chile" page 150, states that he first
noticed on the Pampas, very small bits of red granite, when fifty miles
distant from the southern extremity of the mountains of Cordova, which
project on the plain, like a reef into the sea.)
The highest peak of the St. Ventana is, by Captain Fitzroy's measurement,
3,340 feet, and the calcareous plain at its foot (from observations taken
by some Spanish officers) 840 feet above the sea-level. ("La Plata" etc. by
Sir W. Parish page 146.) On the flanks of the mountains, at a height of
three hundred or four hundred feet above the plain, there were a few small
patches of conglomerate and breccia, firmly cemented by ferruginous matter
to the abrupt and battered face of the quartz--traces being thus exhibited
of ancient sea-action. The high plain round this range sinks quite
insensibly to the eye on all sides, except to the north, where its surface
is broken into low cliffs. Round the Sierras Tapalguen, Guitru-gueyu, and
between the latter and the Ventana we have seen (and shall hereafter see
round some hills in Banda Oriental), that the tosca-rock forms low, flat-
topped, cliff-bounded hills, higher than the surrounding plains of similar
composition. From the horizontal stratification and from the appearance of
the broken cliffs, the greater height of the Pampean formation round these
primary hills ought not to be altogether or in chief part attributed to
these several points having been uplifted more energetically than the
surrounding country, but to the argillaceo-calcareous mud having collected
round them, when they existed as islets or submarine rocks, at a greater
height, than at the bottom of the adjoining open sea;--the cliffs having
been subsequently worn during the elevation of the whole country in mass.
Southward of the Ventana, the plain extends farther than the eye can range;
its surface is not very level, having slight depressions with no drainage
exits; it is generally covered by a few feet in thickness of sandy earth;
and in some places, according to M. Parchappe, by beds of clay two yards
thick. (M. d'Orbigny "Voyage" Part Geolog. pages 47, 48.) On the banks of
the Sauce, four leagues S.E. of the Ventana, there is an imperfect section
about two hundred feet in height, displaying in the upper part tosca-rock
and in the lower part red Pampean mud. At the settlement of Bahia Blanca,
the uppermost plain is composed of very compact, stratified tosca-rock,
containing rounded grains of quartz distinguishable by the naked eye: the
lower plain, on which the fortress stands, is described by M. Parchappe as
composed of solid tosca-rock (Ibid.); but the sections which I examined
appeared more like a redeposited mass of this rock, with small pebbles and
fragments of quartz. I shall immediately return to the important sections
on the shores of Bahia Blanca. Twenty miles southward of this place, there
is a remarkable ridge extending W. by N. and E. by S., formed of small,
separate, flat-topped, steep-sided hills, rising between one hundred and
two hundred feet above the Pampean plain at its southern base, which plain
is a little lower than that to the north. The uppermost stratum in this
ridge consists of pale, highly calcareous, compact tosca-rock, resting (as
seen in one place) on reddish Pampean mud, and this again on a paler kind:
at the foot of the ridge, there is a well in reddish clay or mud. I have
seen no other instance of a chain of hills belonging to the Pampean
formation; and as the strata show no signs of disturbance, and as the
direction of the ridge is the same with that common to all the metamorphic
lines in this whole area, I suspect that the Pampean sediment has in this
instance been accumulated on and over a ridge of hard rocks, instead of, as
in the case of the above-mentioned Sierras, round their submarine flanks.
South of this little chain of tosca-rock, a plain of Pampean mud declines
towards the banks of the Colorado: in the middle a well has been dug in red
Pampean mud, covered by two feet of white, softish, highly calcareous
tosca-rock, over which lies sand with small pebbles three feet in
thickness--the first appearance of that vast shingle formation described in
the First Chapter. In the first section after crossing the Colorado, an old
tertiary formation, namely, the Rio Negro sandstone (to be described in the
next chapter), is met with: but from the accounts given me by the Gauchos,
I believe that at the mouth of the Colorado the Pampean formation extends a
little further southwards.
BAHIA BLANCA.
To return to the shores of this bay. At Monte Hermoso there is a good
section, about one hundred feet in height, of four distinct strata,
appearing to the eye horizontal, but thickening a little towards the N.W.
The uppermost bed, about twenty feet in thickness, consists of obliquely
laminated, soft sandstone, including many pebbles of quartz, and falling at
the surface into loose sand. The second bed, only six inches thick, is a
hard, dark-coloured sandstone. The third bed is pale-coloured Pampean mud;
and the fourth is of the same nature, but darker coloured, including in its
lower part horizontal layers and lines of concretions of not very compact
pinkish tosca-rock. The bottom of the sea, I may remark, to a distance of
several miles from the shore, and to a depth of between sixty and one
hundred feet, was found by the anchors to be composed of tosca-rock and
reddish Pampean mud. Professor Ehrenberg has examined for me specimens of
the two lower beds, and finds in them three Polygastrica and six
Phytolitharia.
(The following list is given in the "Monatsberichten der konig. Akad. zu
Berlin" April 1845:--
POLYGASTRICA.
Fragilaria rhabdosoma.
Gallionella distans.
Pinnularia?
PHYTOLITHARIA.
Lithodontium Bursa.
Lithodontium furcatum.
Lithostylidium exesum.
Lithostylidium rude.
Lithostylidium Serra.
Spongolithis Fustis?)
Of these, only one (Spongolithis Fustis?) is a marine form; five of them
are identical with microscopical structures of brackish-water origin,
hereafter to be mentioned, which form a central point in the Pampean
formation. In these two beds, especially in the lower one, bones of extinct
mammifers, some embedded in their proper relative positions and others
single, are very numerous in a small extent of the cliffs. These remains
consist of, first, the head of Ctenomys antiquus, allied to the living
Ctenomys Braziliensis; secondly, a fragment of the remains of a rodent;
thirdly, molar teeth and other bones of a large rodent, closely allied to,
but distinct from, the existing species of Hydrochoerus, and therefore
probably an inhabitant of fresh water; fourth and fifthly, portions of
vertebrae, limbs, ribs, and other bones of two rodents; sixthly, bones of
the extremities of some great megatheroid quadruped. (See "Fossil Mammalia"
page 109 by Professor Owen, in the "Zoology of the Voyage of the 'Beagle';"
and Catalogue page 36 of Fossil Remains in Museum of Royal College of
Surgeons.) The number of the remains of rodents gives to this collection a
peculiar character, compared with those found in any other locality. All
these bones are compact and heavy; many of them are stained red, with their
surfaces polished; some of the smaller ones are as black as jet.
Monte Hermoso is between fifty and sixty miles distant in a S.E. line from
the Ventana, with the intermediate country gently rising towards it, and
all consisting of the Pampean formation. What relation, then, do these
beds, at the level of the sea and under it, bear to those on the flanks of
the Ventana, at the height of 840 feet, and on the flanks of the other
neighbouring sierras, which, from the reasons already assigned, do not
appear to owe their greater height to unequal elevation? When the tosca-
rock was accumulating round the Ventana, and when, with the exception of a
few small rugged primary islands, the whole wide surrounding plains must
have been under water, were the strata at Monte Hermoso depositing at the
bottom of a great open sea, between eight hundred and one thousand feet in
depth? I much doubt this; for if so, the almost perfect carcasses of the
several small rodents, the remains of which are so very numerous in so
limited a space, must have been drifted to this spot from the distance of
many hundred miles. It appears to me far more probable, that during the
Pampean period this whole area had commenced slowly rising (and in the
cliffs, at several different heights we have proofs of the land having been
exposed to sea-action at several levels), and that tracts of land had thus
been formed of Pampean sediment round the Ventana and the other primary
ranges, on which the several rodents and other quadrupeds lived, and that a
stream (in which perhaps the extinct aquatic Hydrochoerus lived) drifted
their bodies into the adjoining sea, into which the Pampean mud continued
to be poured from the north. As the land continued to rise, it appears that
this source of sediment was cut off; and in its place sand and pebbles were
borne down by stronger currents, and conformably deposited over the Pampean
strata.
(FIGURE 15. SECTION OF BEDS WITH RECENT SHELLS AND EXTINCT MAMMIFERS, AT
PUNTA ALTA IN BAHIA BLANCA. (Showing beds from bottom to top: A, B, C, D.))
Punta Alta is situated about thirty miles higher up on the northern side of
this same bay: it consists of a small plain, between twenty and thirty feet
in height, cut off on the shore by a line of low cliffs about a mile in
length, represented in Figure 15 with its vertical scale necessarily
exaggerated. The lower bed (A) is more extensive than the upper ones; it
consists of stratified gravel or conglomerate, cemented by calcareo-
arenaceous matter, and is divided by curvilinear layers of pinkish marl, of
which some are precisely like tosca-rock, and some more sandy. The beds are
curvilinear, owing to the action of currents, and dip in different
directions; they include an extraordinary number of bones of gigantic
mammifers and many shells. The pebbles are of considerable size, and are of
hard sandstone, and of quartz, like that of the Ventana: there are also a
few well-rounded masses of tosca-rock.
The second bed B is about fifteen feet in thickness, but towards both
extremities of the cliff (not included in the diagram) it either thins out
and dies away, or passes insensibly into an overlying bed of gravel. It
consists of red, tough clayey mud, with minute linear cavities; it is
marked with faint horizontal shades of colour; it includes a few pebbles,
and rarely a minute particle of shell: in one spot, the dermal armour and a
few bones of a Dasypoid quadruped were embedded in it: it fills up furrows
in the underlying gravel. With the exception of the few pebbles and
particles of shells, this bed resembles the true Pampean mud; but it still
more closely resembles the clayey flats (mentioned in the First Chapter)
separating the successively rising parallel ranges of sand-dunes.
The bed C is of stratified gravel, like the lowest one; it fills up furrows
in the underlying red mud, and is sometimes interstratified with it, and
sometimes insensibly passes into it; as the red mud thins out, this upper
gravel thickens. Shells are more numerous in it than in the lower gravel;
but the bones, though some are still present, are less numerous. In one
part, however, where this gravel and the red mud passed into each other, I
found several bones and a tolerably perfect head of the Megatherium. Some
of the large Volutas, though embedded in the gravel-bed C, were filled with
the red mud, including great numbers of the little recent Paludestrina
australis. These three lower beds are covered by an unconformable mantle D
of stratified sandy earth, including many pebbles of quartz, pumice and
phonolite, land and sea-shells.
M. d'Orbigny has been so obliging as to name for me the twenty species of
Mollusca embedded in the two gravel beds: they consist of:--
1. Volutella angulata, d'Orbigny, "Voyage" Mollusq. and Pal.
2. Voluta Braziliana, Sol
3. Olicancilleria Braziliensis d'Orbigny.
4. Olicancilleria auricularia, d'Orbigny.
5. Olivina puelchana, d'Orbigny.
6. Buccinanops cochlidium, d'Orbigny.
7. Buccinanops globulosum, d'Orbigny.
8. Colombella sertulariarum, d'Orbigny.
9. Trochus Patagonicus, and var. of ditto, d'Orbigny.
10. Paludestrina Australis, d'Orbigny.
11. Fissurella Patagonica, d'Orbigny.
12. Crepidula muricata, Lam.
13. Venus purpurata, Lam.
14. Venus rostrata, Phillippi.
15. Mytilus Darwinianus, d'Orbigny.
16. Nucula semiornata, d'Orbigny.
17. Cardita Patagonica, d'Orbigny.
18. Corbula Patagonica (?), d'Orbigny.
19. Pecten tethuelchus, d'Orbigny.
20. Ostrea puelchana, d'Orbigny.
21. A living species of Balanus.
22 and 23. An Astrae and encrusting Flustra, apparently identical with
species now living in the bay.
All these shells now live on this coast, and most of them in this same bay.
I was also struck with the fact, that the proportional numbers of the
different kinds appeared to be the same with those now cast up on the
beach: in both cases specimens of Voluta, Crepidula, Venus, and Trochus are
the most abundant. Four or five of the species are the same with the
upraised shells on the Pampas near Buenos Ayres. All the specimens have a
very ancient and bleached appearance, and do not emit, when heated, an
animal odour: some of them are changed throughout into a white, soft,
fibrous substance; others have the space between the external walls, either
hollow, or filled up with crystalline carbonate of lime. (A Bulinus,
mentioned in the Introduction to the "Fossil Mammalia" in the "Zoology of
the Voyage of the 'Beagle'" has so much fresher an appearance, than the
marine species, that I suspect it must have fallen amongst the others, and
been collected by mistake.)
The remains of the extinct mammiferous animals, from the two gravel beds
have been described by Professor Owen in the "Zoology of the Voyage of the
'Beagle':" they consist of, 1st, one nearly perfect head and three
fragments of heads of the Megatherium Cuvierii; 2nd, a lower jaw of
Megalonyx Jeffersonii; 3rd, lower jaw of Mylodon Darwinii; 4th, fragments
of a head of some gigantic Edental quadruped; 5th, an almost entire
skeleton of the great Scelidotherium leptocephalum, with most of the bones,
including the head, vertebrae, ribs, some of the extremities to the claw-
bone, and even, as remarked by Professor Owen, the knee-cap, all nearly in
their proper relative positions; 6th, fragments of the jaw and a separate
tooth of a Toxodon, belonging either to T. Platensis, or to a second
species lately discovered near Buenos Ayres; 7th, a tooth of Equus
curvidens; 8th, tooth of a Pachyderm, closely allied to Palaeotherium, of
which parts of the head have been lately sent from Buenos Ayres to the
British Museum; in all probability this pachyderm is identical with the
Macrauchenia Patagonica from Port S. Julian, hereafter to be referred to.
Lastly, and 9thly, in a cliff of the red clayey bed B, there was a double
piece, about three feet long and two wide, of the bony armour of a large
Dasypoid quadruped, with the two sides pressed nearly close together: as
the cliff is now rapidly washing away, this fossil probably was lately much
more perfect; from between its doubled-up sides, I extracted the middle and
ungual phalanges, united together, of one of the feet, and likewise a
separate phalanx: hence one or more of the limbs must have been attached to
the dermal case, when it was embedded. Besides these several remains in a
distinguishable condition, there were very many single bones: the greater
number were embedded in a space 200 yards square. The preponderance of the
Edental quadrupeds is remarkable; as is, in contrast with the beds of Monte
Hermoso, the absence of Rodents. Most of the bones are now in a soft and
friable condition, and, like the shells, do not emit when burnt an animal
odour. The decayed state of the bones may be partly owing to their late
exposure to the air and tidal-waves. Barnacles, Serpulae, and corallines
are attached to many of the bones, but I neglected to observe whether these
might not have grown on them since being exposed to the present tidal
action (After having packed up my specimens at Bahia Blanca, this point
occurred to me, and I noted it; but forgot it on my return, until the
remains had been cleaned and oiled: my attention has been lately called to
the subject by some remarks by M. d'Orbigny.); but I believe that some of
the barnacles must have grown on the Scelidotherium, soon after being
deposited, and before being WHOLLY covered up by the gravel. Besides the
remains in the condition here described, I found one single fragment of
bone very much rolled, and as black as jet, so as perfectly to resemble
some of the remains from Monte Hermoso.
Very many of the bones had been broken, abraded, and rolled, before being
embedded. Others, even some of those included in the coarsest parts of the
the now hard conglomerate, still retain all their minutest prominences
perfectly preserved; so that I conclude that they probably were protected
by skin, flesh, or ligaments, whilst being covered up. In the case of the
Scelidotherium, it is quite certain that the whole skeleton was held
together by its ligaments, when deposited in the gravel in which I found
it. Some cervical vertebrae and a humerus of corresponding size lay so
close together, as did some ribs and the bones of a leg, that I thought
that they must originally have belonged to two skeletons, and not have been
washed in single; but as remains were here very numerous, I will not lay
much stress on these two cases. We have just seen that the armour of the
Dasypoid quadruped was certainly embedded together with some of the bones
of the feet.
Professor Ehrenberg has examined for me specimens of the finer matter from
in contact with these mammiferous remains: he finds in them two
Polygastrica, decidedly marine forms; and six Phytolitharia, of which one
is probably marine, and the others either of fresh-water or terrestrial
origin. ("Monatsberichten der Akad. zu Berlin" April 1845. The list
consists of:--
POLYGASTRICA.
Gallionella sulcata.
Stauroptera aspera? fragm.
PHYTOLITHARIA.
Lithasteriscus tuberculatus.
Lithostylidium Clepsammidium.
Lithostylidium quadratum.
Lithostylidium rude.
Lithostylidium unidentatum.
Spongolithis acicularis.)
Only one of these eight microscopical bodies is common to the nine from
Monte Hermoso: but five of them are in common with those from the Pampean
mud on the banks of the Parana. The presence of any fresh-water infusoria,
considering the aridity of the surrounding country, is here remarkable: the
most probable explanation appears to be, that these microscopical organisms
were washed out of the adjoining great Pampean formation during its
denudation, and afterwards redeposited.
We will now see what conclusions may be drawn from the facts above
detailed. It is certain that the gravel-beds and intermediate red mud were
deposited within the period, when existing species of Mollusca held to each
other nearly the same relative proportions as they do on the present coast.
These beds, from the number of littoral species, must have been accumulated
in shallow water; but not, judging from the stratification of the gravel
and the layers of marl, on a beach. From the manner in which the red clay
fills up furrows in the underlying gravel, and is in some parts itself
furrowed by the overlying gravel, whilst in other parts it either
insensibly passes into, or alternates with, this upper gravel, we may infer
several local changes in the currents, perhaps caused by slight changes, up
or down, in the level of the land. By the elevation of these beds, to which
period the alluvial mantle with pumice-pebbles, land and sea-shells
belongs, the plain of Punta Alta, from twenty to thirty feet in height, was
formed. In this neighbourhood there are other and higher sea-formed plains
and lines of cliffs in the Pampean formation worn by the denuding action of
the waves at different levels. Hence we can easily understand the presence
of rounded masses of tosca-rock in this lowest plain; and likewise, as the
cliffs at Monte Hermoso with their mammiferous remains stand at a higher
level, the presence of the one much-rolled fragment of bone which was as
black as jet: possibly some few of the other much-rolled bones may have
been similarly derived, though I saw only the one fragment, in the same
condition with those from Monte Hermoso. M. d'Orbigny has suggested that
all these mammiferous remains may have been washed out of the Pampean
formation, and afterwards redeposited together with the recent shells.
("Voyage" Part. Geolog. page 49.) Undoubtedly it is a marvellous fact that
these numerous gigantic quadrupeds, belonging, with the exception of the
Equus curvidens, to seven extinct genera, and one, namely, the Toxodon, not
falling into any existing family, should have co-existed with Mollusca, all
of which are still living species; but analogous facts have been observed
in North America and in Europe. In the first place, it should not be
overlooked, that most of the co-embedded shells have a more ancient and
altered appearance than the bones. In the second place, is it probable that
numerous bones not hardened by silex or any other mineral, could have
retained their delicate prominences and surfaces perfect if they had been
washed out of one deposit, and re-embedded in another:--this later deposit
being formed of large, hard pebbles, arranged by the action of currents or
breakers in shallow water into variously curved and inclined layers? The
bones which are now in so perfect a state of preservation, must, I
conceive, have been fresh and sound when embedded, and probably were
protected by skin, flesh, or ligaments. The skeleton of the Scelidotherium
indisputably was deposited entire: shall we say that when held together by
its matrix it was washed out of an old gravel-bed (totally unlike in
character to the Pampean formation), and re-embedded in another gravel-bed,
composed (I speak after careful comparison) of exactly the same kind of
pebbles, in the same kind of cement? I will lay no stress on the two cases
of several ribs and bones of the extremities having APPARENTLY been
embedded in their proper relative position: but will any one be so bold as
to affirm that it is possible, that a piece of the thin tessellated armour
of a Dasypoid quadruped, at least three feet long and two in width, and now
so tender that I was unable with the utmost care to extract a fragment more
than two or three inches square, could have been washed out of one bed, and
re-embedded in another, together with some of the small bones of the feet,
without having been dashed into atoms? We must then wholly reject M.
d'Orbigny's supposition, and admit as certain, that the Scelidotherium and
the large Dasypoid quadruped, and as highly probable, that the Toxodon,
Megatherium, etc., some of the bones of which are perfectly preserved, were
embedded for the first time, and in a fresh condition, in the strata in
which they were found entombed. These gigantic quadrupeds, therefore,
though belonging to extinct genera and families, coexisted with the twenty
above-enumerated Mollusca, the barnacle and two corals, still living on
this coast. From the rolled fragment of black bone, and from the plain of
Punta Alta being lower than that of Monte Hermoso, I conclude that the
coarse sub-littoral deposits of Punta Alta, are of subsequent origin to the
Pampean mud of Monte Hermoso; and the beds at this latter place, as we have
seen, are probably of subsequent origin to the high tosca-plain round the
Sierra Ventana: we shall, however, return, at the end of this chapter, to
the consideration of these several stages in the great Pampean formation.
BUENOS AYRES TO ST. FE BAJADA, IN ENTRE RIOS.
For some distance northward of Buenos Ayres, the escarpment of the Pampean
formation does not approach very near to the Plata, and it is concealed by
vegetation: but in sections on the banks of the Rios Luxan, Areco, and
Arrecifes, I observed both pale and dark reddish Pampean mud, with small,
whitish concretions of tosca; at all these places mammiferous remains have
been found. In the cliffs on the Parana, at San Nicolas, the Pampean mud
contains but little tosca; here M. d'Orbigny found the remains of two
rodents (Ctenomys Bonariensis and Kerodon antiquus) and the jaw of a Canis:
when on the river I could clearly distinguish in this fine line of cliffs,
"horizontal lines of variation both in tint and compactness." (I quote
these words from my note-book, as written down on the spot, on account of
the general absence of stratification in the Pampean formation having been
insisted on by M. d'Orbigny as a proof of the diluvial origin of this great
deposit.) The plain northward of this point is very level, but with some
depressions and lakes; I estimated its height at from forty to sixty feet
above the Parana. At the A. Medio the bright red Pampean mud contains
scarcely any tosca-rock; whilst at a short distance the stream of the
Pabon, forms a cascade, about twenty feet in height, over a cavernous mass
of two varieties of tosca-rock; of which one is very compact and semi-
crystalline, with seams of crystallised carbonate of lime: similar compact
varieties are met with on the Salidillo and Seco. The absolute identity (I
speak after a comparison of my specimens) between some of these varieties,
and those from Tapalguen, and from the ridge south of Bahia Blanca, a
distance of 400 miles of latitude, is very striking.
At Rosario there is but little tosca-rock: near this place I first noticed
at the edge of the river traces of an underlying formation, which, twenty-
five miles higher up in the estancia of Gorodona, consists of a pale
yellowish clay, abounding with concretionary cylinders of a ferruginous
sandstone. This bed, which is probably the equivalent of the older tertiary
marine strata, immediately to be described in Entre Rios, only just rises
above the level of the Parana when low. The rest of the cliff at Gorodona,
is formed of red Pampean mud, with, in the lower part, many concretions of
tosca, some stalacti-formed, and with only a few in the upper part: at the
height of six feet above the river, two gigantic skeletons of the Mastodon
Andium were here embedded; their bones were scattered a few feet apart, but
many of them still held their proper relative positions: they were much
decayed and as soft as cheese, so that even one of the great molar teeth
fell into pieces in my hand. We here see that the Pampean deposit contains
mammiferous remains close to its base. On the banks of the Carcarana, a few
miles distant, the lowest bed visible was pale Pampean mud, with masses of
tosca-rock, in one of which I found a much decayed tooth of the Mastodon:
above this bed, there was a thin layer almost composed of small concretions
of white tosca, out of which I extracted a well preserved, but slightly
broken tooth of Toxodon Platensis: above this there was an unusual bed of
very soft impure sandstone. In this neighbourhood I noticed many single
embedded bones, and I heard of others having been found in so perfect a
state that they were long used as gate-posts: the Jesuit Falkner found here
the dermal armour of some gigantic Edental quadruped.
In some of the red mud scraped from a tooth of one of the Mastodons at
Gorodona, Professor Ehrenberg finds seven Polygastrica and thirteen
Phytolitharia, all of them, I believe, with two exceptions, already known
species. ("Monatsberichten der konig. Akad. zu Berlin" April 1845. The list
consists of:--
POLYGASTRICA.
Campylodiscus clypeus.
Coscinodiscus subtilis.
Coscinodiscus al. sp.
Eunotia.
Gallionella granulata.
Himantidium gracile.
Pinnularia borealis.)
Of these twenty, the preponderating number are of fresh-water origin; only
two species of Coscinodiscus and a Spongolithis show the direct influence
of the sea; therefore Professor Ehrenberg arrives at the important
conclusion that the deposit must have been of brackish-water origin. Of the
thirteen Phytolitharia, nine are met with in the two deposits in Bahia
Blanca, where there is evidence from two other species of Polygastrica that
the beds were accumulated in brackish water. The traces of coral, sponges,
and Polythalamia, found by Dr. Carpenter in the tosca-rock (of which I must
observe the greater number of specimens were from the upper beds in the
southern parts of the formation), apparently show a more purely marine
origin.
At ST. FE BAJADA, in Entre Rios, the cliffs, estimated at between sixty and
seventy feet in height, expose an interesting section: the lower half
consists of tertiary strata with marine shells, and the upper half of the
Pampean formation. The lowest bed is an obliquely laminated, blackish,
indurated mud, with distinct traces of vegetable remains. (M. d'Orbigny
"Voyage" Part. Geolog. page 37, has given a detailed description of this
section, but as he does not mention this lowest bed, it may have been
concealed when he was there by the river. There is a considerable
discrepancy between his description and mine, which I can only account for
by the beds themselves varying considerably in short distances.) Above this
there is a thick bed of yellowish sandy clay, with much crystallised gypsum
and many shells of Ostreae, Pectens, and Arcae: above this there generally
comes an arenaceous crystalline limestone, but there is sometimes
interposed a bed, about twelve feet thick, of dark green, soapy clay,
weathering into small angular fragments. The limestone, where purest, is
white, highly crystalline, and full of cavities: it includes small pebbles
of quartz, broken shells, teeth of sharks, and sometimes, as I was
informed, large bones: it often contains so much sand as to pass into a
calcareous sandstone, and in such parts the great Ostrea Patagonica chiefly
abounds. (Captain Sulivan, R.N., has given me a specimen of this shell,
which he found in the cliffs at Point Cerrito, between twenty and thirty
miles above the Bajada.) In the upper part, the limestone alternates with
layers of fine white sand. The shells included in these beds have been
named for me by M. d'Orbigny: they consist of:--
1. Ostrea Patagonica, d'Orbigny, "Voyage" Part. Pal.
2. Ostrea Alvarezii, d'Orbigny, "Voyage" Part. Pal.
3. Pecten Paranensis, d'Orbigny, "Voyage" Part. Pal.
4. Pecten Darwinianus, d'Orbigny, "Voyage" Part. Pal.
5. Venus Munsterii, d'Orbigny, "Voyage" Pal.
6. Arca Bonplandiana, d'Orbigny, "Voyage" Pal.
7. Cardium Platense, d'Orbigny, "Voyage" Pal.
8. Tellina, probably nov. species, but too imperfect for description.
PHYTOLITHARIA.
Lithasteriscus tuberculatus.
Lithodontium bursa.
Lithodontium furcatum.
Lithodontium rostratum.
Lithostylidium Amphiodon.
Lithostylidium Clepsammidium.
Lithostylidium Hamus.
Lithostylidium polyedrum.
Lithostylidium quadratum.
Lithostylidium rude.
Lithostylidium Serra.
Lithostylidium unidentatum.
Spongolithis Fustis.
These species are all extinct: the six first were found by M. d'Orbigny and
myself in the formations of the Rio Negro, S. Josef, and other parts of
Patagonia; and therefore, as first observed by M. d'Orbigny, these beds
certainly belong to the great Patagonian formation, which will be described
in the ensuing chapter, and which we shall see must be considered as a very
ancient tertiary one. North of the Bajada, M. d'Orbigny found, in beds
which he considers as lying beneath the strata here described, remains of a
Toxodon, which he has named as a distinct species from the T. Platensis of
the Pampean formation. Much silicified wood is found on the banks of the
Parana (and likewise on the Uruguay), and I was informed that they come out
of these lower beds; four specimens collected by myself are dicotyledonous.
The upper half of the cliff, to a thickness of about thirty feet, consists
of Pampean mud, of which the lower part is pale-coloured, and the upper
part of a brighter red, with some irregular layers of an arenaceous variety
of tosca, and a few small concretions of the ordinary kind. Close above the
marine limestone, there is a thin stratum with a concretionary outline of
white hard tosca-rock or marl, which may be considered either as the
uppermost bed of the inferior deposits, or the lowest of the Pampean
formation; at one time I considered this bed as marking a passage between
the two formations: but I have since become convinced that I was deceived
on this point. In the section on the Parana, I did not find any mammiferous
remains; but at two miles distance on the A. Tapas (a tributary of the
Conchitas), they were extremely numerous in a low cliff of red Pampean mud
with small concretions, precisely like the upper bed on the Parana. Most of
the bones were solitary and much decayed; but I saw the dermal armour of a
gigantic Edental quadruped, forming a caldron-like hollow, four or five
feet in diameter, out of which, as I was informed, the almost entire
skeleton had been lately removed. I found single teeth of the Mastodon
Andium, Toxodon Platensis, and Equus curvidens, near to each other. As this
latter tooth approaches closely to that of the common horse, I paid
particular attention to its true embedment, for I did not at that time know
that there was a similar tooth hidden in the matrix with the other
mammiferous remains from Punta Alta. It is an interesting circumstance,
that Professor Owen finds that the teeth of this horse approach more
closely in their peculiar curvature to a fossil specimen brought by Mr.
Lyell from North America, than to those of any other species of Equus.
(Lyell "Travels in North America" volume 1 page 164 and "Proceedings of
Geological Society" volume 4 page 39.)
The underlying marine tertiary strata extend over a wide area: I was
assured that they can be traced in ravines in an east and west line across
Entre Rios to the Uruguay, a distance of about 135 miles. In a S.E.
direction I heard of their existence at the head of the R. Nankay; and at
P. Gorda in Banda Oriental, a distance of 170 miles, I found the same
limestone, containing the same fossil shells, lying at about the same level
above the river as at St. Fe. In a southerly direction, these beds sink in
height, for at another P. Gorda in Entre Rios, the limestone is seen at a
much less height; and there can be little doubt that the yellowish sandy
clay, on a level with the river, between the Carcarana and S. Nicholas,
belongs to this same formation; as perhaps do the beds of sand at Buenos
Ayres, which lie at the bottom of the Pampean formation, about sixty feet
beneath the surface of the Plata. The southerly declination of these beds
may perhaps be due, not to unequal elevation, but to the original form of
the bottom of the sea, sloping from land situated to the north; for that
land existed at no great distance, we have evidence in the vegetable
remains in the lowest bed at St. Fe; and in the silicified wood and in the
bones of Toxodon Paranensis, found (according to M. d'Orbigny) in still
lower strata.
BANDA ORIENTAL.
This province lies on the northern side of the Plata, and eastward of the
Uruguay: it has a gentle undulatory surface, with a basis of primary rocks;
and is in most parts covered up with an unstratified mass, of no great
thickness, of reddish Pampean mud. In the eastern half, near Maldonado,
this deposit is more arenaceous than in the Pampas, it contains many though
small concretions of marl or tosca-rock, and others of highly ferruginous
sandstone; in one section, only a few yards in depth, it rested on
stratified sand. Near Monte Video this deposit in some spots appears to be
of greater thickness; and the remains of the Glyptodon and other extinct
mammifers have been found in it. In the long line of cliffs, between fifty
and sixty feet in height, called the Barrancas de S. Gregorio, which extend
westward of the Rio S. Lucia, the lower half is formed of coarse sand of
quartz and feldspar without mica, like that now cast up on the beach near
Maldonado; and the upper half of Pampean mud, varying in colour and
containing honeycombed veins of soft calcareous matter and small
concretions of tosca-rock arranged in lines, and likewise a few pebbles of
quartz. This deposit fills up hollows and furrows in the underlying sand;
appearing as if water charged with mud had invaded a sandy beach. These
cliffs extend far westward, and at a distance of sixty miles, near Colonia
del Sacramiento, I found the Pampean deposit resting in some places on this
sand, and in others on the primary rocks: between the sand and the reddish
mud, there appeared to be interposed, but the section was not a very good
one, a thin bed of shells of an existing Mytilus, still partially retaining
their colour. The Pampean formation in Banda Oriental might readily be
mistaken for an alluvial deposit: compared with that of the Pampas, it is
often more sandy, and contains small fragments of quartz; the concretions
are much smaller, and there are no extensive masses of tosca-rock.
In the extreme western parts of this province, between the Uruguay and a
line drawn from Colonia to the R. Perdido (a tributary of the R. Negro),
the formations are far more complicated. Besides primary rocks, we meet
with extensive tracts and many flat-topped, horizontally stratified, cliff-
bounded, isolated hills of tertiary strata, varying extraordinarily in
mineralogical nature, some identical with the old marine beds of St. Fe
Bajada, and some with those of the much more recent Pampean formation.
There are, also, extensive LOW tracts of country covered with a deposit
containing mammiferous remains, precisely like that just described in the
more eastern parts of the province. Although from the smooth and unbroken
state of the country, I never obtained a section of this latter deposit
close to the foot of the higher tertiary hills, yet I have not the least
doubt that it is of quite subsequent origin; having been deposited after
the sea had worn the tertiary strata into the cliff-bounded hills. This
later formation, which is certainly the equivalent of that of the Pampas,
is well seen in the valleys in the estancia of Berquelo, near Mercedes; it
here consists of reddish earth, full of rounded grains of quartz, and with
some small concretions of tosca-rock arranged in horizontal lines, so as
perfectly to resemble, except in containing a little calcareous matter, the
formation in the eastern parts of Banda Oriental, in Entre Rios, and at
other places: in this estancia the skeleton of a great Edental quadruped
was found. In the valley of the Sarandis, at the distance of only a few
miles, this deposit has a somewhat different character, being whiter,
softer, finer-grained, and full of little cavities, and consequently of
little specific gravity; nor does it contain any concretions or calcareous
matter: I here procured a head, which when first discovered must have been
quite perfect, of the Toxodon Platensis, another of a Mylodon (This head
was at first considered by Professor Owen (in the "Zoology of the
'Beagle's' Voyage") as belonging to a distinct genus, namely,
Glossotherium.), perhaps M. Darwinii, and a large piece of dermal armour,
differing from that of the Glyptodon clavipes. These bones are remarkable
from their extraordinarily fresh appearance; when held over a lamp of
spirits of wine, they give out a strong odour and burn with a small flame;
Mr. T. Reeks has been so kind as to analyse some of the fragments, and he
finds that they contain about 7 per cent of animal matter, and 8 per cent
of water. (Liebig "Chemistry of Agriculture" page 194 states that fresh dry
bones contain from 32 to 33 per cent of dry gelatine. See also Dr. Daubeny,
in "Edinburgh New Philosophical Journal" volume 37 page 293.)
The older tertiary strata, forming the higher isolated hills and extensive
tracts of country, vary, as I have said, extraordinarily in composition:
within the distance of a few miles, I sometimes passed over crystalline
limestone with agate, calcareous tuffs, and marly rocks, all passing into
each other,--red and pale mud with concretions of tosca-rock, quite like
the Pampean formation,--calcareous conglomerates and sandstones,--bright
red sandstones passing either into red conglomerate, or into white
sandstone,--hard siliceous sandstones, jaspery and chalcedonic rocks, and
numerous other subordinate varieties. I was unable to mark out the
relations of all these strata, and will describe only a few distinct
sections:--in the cliffs between P. Gorda on the Uruguay and the A. de
Vivoras, the upper bed is crystalline cellular limestone often passing into
calcareous sandstone, with impressions of some of the same shells as at St.
Fe Bajada; at P. Gorda, this limestone is interstratified with and rests
on, white sand, which covers a bed about thirty feet thick of pale-coloured
clay, with many shells of the great Ostrea Patagonica (In my "Journal" page
171 1st edition, I have hastily and inaccurately stated that the Pampean
mud, which is found over the eastern part of B. Oriental, lies OVER the
limestone at P. Gorda; I should have said that there was reason to infer
that it was a subsequent or superior deposit.): beneath this, in the
vertical cliff, nearly on a level with the river, there is a bed of red mud
absolutely like the Pampean deposit, with numerous often large concretions
of perfectly characterised white, compact tosca-rock. At the mouth of the
Vivoras, the river flows over a pale cavernous tosca-rock, quite like that
in the Pampas, and this APPEARED to underlie the crystalline limestone; but
the section was not unequivocal like that at P. Gorda. These beds now form
only a narrow and much denuded strip of land; but they must once have
extended much further; for on the next stream, south of the S. Juan,
Captain Sulivan, R.N., found a little cliff, only just above the surface of
the river, with numerous shells of the Venus Munsterii, D'Orbigny,--one of
the species occurring at St. Fe, and of which there are casts at P. Gorda:
the line of cliffs of the subsequently deposited true Pampean mud, extend
from Colonia to within half a mile of this spot, and no doubt once covered
up this denuded marine stratum. Again at Colonia, a Frenchman found, in
digging the foundations of a house, a great mass of the Ostrea Patagonica
(of which I saw many fragments), packed together just beneath the surface,
and directly superimposed on the gneiss. These sections are important: M.
d'Orbigny is unwilling to believe that beds of the same nature with the
Pampean formation ever underlie the ancient marine tertiary strata; and I
was as much surprised at it as he could have been; but the vertical cliff
at P. Gorda allowed of no mistake, and I must be permitted to affirm, that
after having examined the country from the Colorado to St. Fe Bajada, I
could not be deceived in the mineralogical character of the Pampean
deposit.
Moreover, in a precipitous part of the ravine of Las Bocas, a red sandstone
is distinctly seen to overlie a thick bed of pale mud, also quite like the
Pampean formation, abounding with concretions of true tosca-rock. This
sandstone extends over many miles of country: it is as red as the brightest
volcanic scoriae; it sometimes passes into a coarse red conglomerate
composed of the underlying primary rocks; and often passes into a soft
white sandstone with red streaks. At the Calera de los Huerfanos, only a
quarter of a mile south of where I first met with the red sandstone, the
crystalline white limestone is quarried: as this bed is the uppermost, and
as it often passes into calcareous sandstone, interstratified with pure
sand; and as the red sandstone likewise passes into soft white sandstone,
and is also the uppermost bed, I believe that these two beds, though so
different, are equivalents. A few leagues southward of these two places, on
each side of the low primary range of S. Juan, there are some flat-topped,
cliff-bounded, separate little hills, very similar to those fringing the
primary ranges in the great plain south of Buenos Ayres: they are composed-
-1st, of calcareous tuff with many particles of quartz, sometimes passing
into a coarse conglomerate; 2nd, of a stone undistinguishable on the
closest inspection from the compacter varieties of tosca-rock; and 3rd, of
semi-crystalline limestone, including nodules of agate: these three
varieties pass insensibly into each other, and as they form the uppermost
stratum in this district, I believe that they, also, are the equivalents of
the pure crystalline limestone, and of the red and white sandstones and
conglomerates.
Between these points and Mercedes on the Rio Negro, there are scarcely any
good sections, the road passing over limestone, tosca-rock, calcareous and
bright red sandstones, and near the source of the San Salvador over a wide
extent of jaspery rocks, with much milky agate, like that in the limestone
near San Juan. In the estancia of Berquelo, the separate, flat-topped,
cliff-bounded hills are rather higher than in the other parts of the
country; they range in a N.E. and S.W. direction; their uppermost beds
consist of the same bright red sandstone, passing sometimes into a
conglomerate, and in the lower part into soft white sandstone, and even
into loose sand: beneath this sandstone, I saw in two places layers of
calcareous and marly rocks, and in one place red Pampean-like earth; at the
base of these sections, there was a hard, stratified, white sandstone, with
chalcedonic layers. Near Mercedes, beds of the same nature and apparently
of the same age, are associated with compact, white, crystalline limestone,
including much botryoidal agate, and singular masses, like porcelain, but
really composed of a calcareo-siliceous paste. In sinking wells in this
district the chalcedonic strata seem to be the lowest. Beds, such as there
described, occur over the whole of this neighbourhood; but twenty miles
further up the R. Negro, in the cliffs of Perika, which are about fifty
feet in height, the upper bed is a prettily variegated chalcedony, mingled
with a pure white tallowy limestone; beneath this there is a conglomerate
of quartz and granite; beneath this many sandstones, some highly
calcareous; and the whole lower two-thirds of the cliff consists of earthy
calcareous beds of various degrees of purity, with one layer of reddish
Pampean-like mud.
When examining the agates, the chalcedonic and jaspery rocks, some of the
limestones, and even the bright red sandstones, I was forcibly struck with
their resemblance to deposits formed in the neighbourhood of volcanic
action. I now find that M. Isabelle, in his "Voyage a Buenos Ayres," has
described closely similar beds on Itaquy and Ibicuy (which enter the
Uruguay some way north of the R. Negro) and these beds include fragments of
red decomposed true scoriae hardened by zeolite, and of black retinite: we
have then here good evidence of volcanic action during our tertiary period.
Still further north, near S. Anna, where the Parana makes a remarkable
bend, M. Bonpland found some singular amygdaloidal rocks, which perhaps may
belong to this same epoch. (M. d'Orbigny "Voyage" Part. Geolog. page 29) I
may remark that, judging from the size and well-rounded condition of the
blocks of rock in the above-described conglomerates, masses of primary
formation probably existed at this tertiary period above water: there is,
also, according to M. Isabelle, much conglomerate further north, at Salto.
From whatever source and through whatever means the great Pampean formation
originated, we here have, I must repeat, unequivocal evidence of a similar
action at a period before that of the deposition of the marine tertiary
strata with extinct shells, at Santa Fe and P. Gorda. During also the
deposition of these strata, we have in the intercalated layers of red
Pampean-like mud and tosca-rock, and in the passage near S. Juan of the
semi-crystalline limestones with agate into tosca undistinguishable from
that of the Pampas, evidence of the same action, though continued only at
intervals and in a feeble manner. We have further seen that in this
district, at a period not only subsequent to the deposition of the tertiary
strata, but to their upheavement and most extensive denudation, true
Pampean mud with its usual characters and including mammiferous remains,
was deposited round and between the hills or islets formed of these
tertiary strata, and over the whole eastern and low primary districts of
Banda Oriental.
EARTHY MASS, WITH EXTINCT MAMMIFEROUS REMAINS, OVER THE PORPHYRITIC GRAVEL
AT S. JULIAN, LATITUDE 49 DEGREES 14' S., IN PATAGONIA.
(FIGURE 16. SECTION OF THE LOWEST PLAIN AT PORT S. JULIAN.
(Section through beds from top to bottom: A, B, C, D, E, F.)
AA. Superficial bed of reddish earth, with the remains of the Macrauchenia,
and with recent sea-shells on the surface.
B. Gravel of porphyritic rocks.
C. and D. Pumiceous mudstone.--Ancient tertiary formation.
E. and F. Sandstone and argillaceous beds.--Ancient tertiary formation.)
This case, though not coming strictly under the Pampean formation, may be
conveniently given here. On the south side of the harbour, there is a
nearly level plain (mentioned in the First Chapter) about seven miles long,
and three or four miles wide, estimated at ninety feet in height, and
bordered by perpendicular cliffs, of which a section is represented in
Figure 16.
The lower old tertiary strata (to be described in the next chapter) are
covered by the usual gravel bed; and this by an irregular earthy, sometimes
sandy mass, seldom more than two or three feet in thickness, except where
it fills up furrows or gullies worn not only through the underlying gravel,
but even through the upper tertiary beds. This earthy mass is of a pale
reddish colour, like the less pure varieties of Pampean mud in Banda
Oriental; it includes small calcareous concretions, like those of tosca-
rock but more arenaceous, and other concretions of a greenish, indurated
argillaceous substance: a few pebbles, also, from the underlying gravel-bed
are also included in it, and these being occasionally arranged in
horizontal lines, show that the mass is of sub-aqueous origin. On the
surface and embedded in the superficial parts, there are numerous shells,
partially retaining their colours, of three or four of the now commonest
littoral species. Near the bottom of one deep furrow (represented in Figure
16), filled up with this earthy deposit, I found a large part of the
skeleton of the Macrauchenia Patachonica--a gigantic and most extraordinary
pachyderm, allied, according to Professor Owen, to the Palaeotherium, but
with affinities to the Ruminants, especially to the Am
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