Sulphate of Soda.... 3.00
Chloride of Soda... 28.69
Iodic Salts......... 0.63
Shells and Marl..... 2.60
99.90
The “mother-water” at some of the refineries is very rich in iodic
salts, and is supposed to contain much muriate of lime. (“Literary
Gazette” 1841 page 475.) In an unrefined specimen brought home by
myself, Mr. T. Reeks has ascertained that the muriate of lime is very
abundant. With respect to the origin of this saline mass, from the
manner in which the gently inclined, compact bed follows for so many
miles the sinuous margin of the plain, there can be no doubt that it
was deposited from a sheet of water: from the fragments of embedded
shells, from the abundant iodic salts, from the superficial saliferous
crust occurring at a higher level and being probably of marine origin,
and from the plain resembling in form those of Chile and that of
Uspallata, there can be little doubt that this sheet of water was, at
least originally, connected with the sea. (From an official document,
shown me by Mr. Belford Wilson, it appears that the first export of
nitrate of soda to Europe was in July 1830, on French account, in a
British ship:—
In year, the entire export was in Quintals.
1830............................ 17,300
1831............................ 40,885
1832............................ 51,400
1833............................ 91,335
1834........................... 149,538
The Spanish quintal nearly equals 100 English pounds.)
THIN, SUPERFICIAL, SALINE INCRUSTATIONS.
These saline incrustations are common in many parts of America:
Humboldt met with them on the tableland of Mexico, and the Jesuit
Falkner and other authors state that they occur at intervals over the
vast plains extending from the mouth of the Plata to Rioja and
Catamarca. (Azara “Travels” volume 1 page 55, considers that the Parana
is the eastern boundary of the saliferous region; but I heard of
“salitrales” in the Province of Entre Rios.) Hence it is that during
droughts, most of the streams in the Pampas are saline. I nowhere met
with these incrustations so abundantly as near Bahia Blanca: square
miles of the mud-flats, which near that place are raised only a few
feet above the sea, just enough to protect them from being overflowed,
appear, after dry weather, whiter than the ground after the thickest
hoar-frost. After rain the salts disappear, and every puddle of water
becomes highly saline; as the surface dries, the capillary action draws
the moisture up pieces of broken earth, dead sticks, and tufts of
grass, where the salt effloresces. The incrustation, where thickest,
does not exceed a quarter of an inch. M. Parchappe has analysed it (M.
d’Orbigny “Voyage” etc. Part. Hist. tome 1 page 664.); and finds that
the specimens collected at the extreme head of the low plain, near the
River Manuello, consist of 93 per cent of sulphate of soda, and 7 of
common salt; whilst the specimens taken close to the coast contain only
63 per cent of the sulphate, and 37 of the muriate of soda. This
remarkable fact, together with our knowledge that the whole of this low
muddy plain has been covered by the sea within the recent period, must
lead to the suspicion that the common salt, by some unknown process,
becomes in time changed into the sulphate. Friable, calcareous matter
is here abundant, and the case of the apparent double decomposition of
the shells and salt on San Lorenzo, should not be forgotten.
The saline incrustations, near Bahia Blanca, are not confined to,
though most abundant on, the low muddy flats; for I noticed some on a
calcareous plain between thirty and forty feet above the sea, and even
a little occurs in still higher valleys. Low alluvial tracts in the
valleys of the Rivers Negro and Colorado are also encrusted, and in the
latter valley such spaces appeared to be occasionally overflowed by the
river. I observed saline incrustations in some of the valleys of
Southern Patagonia. At Port Desire a low, flat, muddy valley was
thickly incrusted by salts, which on analysis by Mr. T. Reeks, are
found to consist of a mixture of sulphate and muriate of soda, with
carbonate of lime and earthy matter. On the western side of the
continent, the southern coasts are much too humid for this phenomenon;
but in Northern Chile I again met with similar incrustations. On the
hardened mud, in parts of the broad, flat-bottomed valley of Copiapo,
the saline matter encrusts the ground to the thickness of some inches:
specimens, sent by Mr. Bingley to Apothecaries’ Hall for analysis, were
said to consist of carbonate and sulphate of soda. Much sulphate of
soda is found in the desert of Atacama. In all parts of South America,
the saline incrustations occur most frequently on low damp surfaces of
mud, where the climate is rather dry; and these low surfaces have, in
almost every case, been upraised above the level of the sea, within the
recent period.
SALT-LAKES OF PATAGONIA AND LA PLATA.
Salinas, or natural salt-lakes, occur in various formations on the
eastern side of the continent,—in the argillaceo-calcareous deposit of
the Pampas, in the sandstone of the Rio Negro, where they are very
numerous, in the pumiceous and other beds of the Patagonian tertiary
formation, and in small primary districts in the midst of this latter
formation. Port S. Julian is the most southerly point (latitude 49
degrees to 50 degrees) at which salinas are known to occur. (According
to Azara “Travels” volume 1 page 56, there are salt-lakes as far north
as Chaco (latitude 25 degrees), on the banks of the Vermejo. The
salt-lakes of Siberia appear (Pallas “Travels” English Translation
volume 1 page 284) to occur in very similar depressions to those of
Patagonia.) The depressions, in which these salt-lakes lie, are from a
few feet to sixty metres, as asserted by M. d’Orbigny, below the
surface of the surrounding plains (“Voyage Geolog.” page 63.); and,
according to this same author, near the Rio Negro they all trend,
either in the N.E. and S.W. or in E. and W. lines, coincident with the
general slope of the plain. These depressions in the plain generally
have one side lower than the others, but there are no outlets for
drainage. Under a less dry climate, an outlet would soon have been
formed, and the salt washed away. The salinas occur at different
elevations above the sea; they are often several leagues in diameter;
they are generally very shallow, but there is a deep one in a
quartz-rock formation near C. Blanco. In the wet season, the whole, or
a part, of the salt is dissolved, being redeposited during the
succeeding dry season. At this period the appearance of the snow-white
expanse of salt crystallised in great cubes, is very striking. In a
large salina, northward of the Rio Negro, the salt at the bottom,
during the whole year, is between two and three feet in thickness.
The salt rests almost always on a thick bed of black muddy sand, which
is fetid, probably from the decay of the burrowing worms inhabiting it.
(Professor Ehrenberg examined some of this muddy sand, but was unable
to find in it any infusoria.) In a salina, situated about fifteen miles
above the town of El Carmen on the Rio Negro, and three or four miles
from the banks of that river, I observed that this black mud rested on
gravel with a calcareous matrix, similar to that spread over the whole
surrounding plains: at Port S. Julian the mud, also, rested on the
gravel: hence the depressions must have been formed anteriorly to, or
contemporaneously with, the spreading out of the gravel. I was informed
that one small salina occurs in an alluvial plain within the valley of
the Rio Negro, and therefore its origin must be subsequent to the
excavation of that valley. When I visited the salina, fifteen miles
above the town, the salt was beginning to crystallise, and on the muddy
bottom there were lying many crystals, generally placed crossways of
sulphate of soda (as ascertained by Mr. Reeks), and embedded in the
mud, numerous crystals of sulphate of lime, from one to three inches in
length: M. d’Orbigny states that some of these crystals are acicular
and more than even nine inches in length (“Voyage Geolog.” page 64.);
others are macled and of great purity: those I found all contained some
sand in their centres. As the black and fetid sand overlies the gravel,
and that overlies the regular tertiary strata, I think there can be no
doubt that these remarkable crystals of sulphate of lime have been
deposited from the waters of the lake. The inhabitants call the
crystals of selenite, the padre del sal, and those of the sulphate of
soda, the madre del sal; they assured me that both are found under the
same circumstances in several of the neighbouring salinas; and that the
sulphate of soda is annually dissolved, and is always crystallised
before the common salt on the muddy bottom. (This is what might have
been expected; for M. Ballard asserts “Acad. des Sciences” October 7,
1844, that sulphate of soda is precipitated from solution more readily
from water containing muriate of soda in excess, than from pure water.)
The association of gypsum and salt in this case, as well as in the
superficial deposits of Iquique, appears to me interesting, considering
how generally these substances are associated in the older stratified
formations.
Mr. Reeks has analysed for me some of the salt from the salina near the
Rio Negro; he finds it composed entirely of chloride of sodium, with
the exception of 0.26 of sulphate of lime and of 0.22 of earthy matter:
there are no traces of iodic salts. Some salt from the salina
Chiquitos, in the Pampean formation, is equally pure. It is a singular
fact, that the salt from these salinas does not serve so well for
preserving meat, as sea-salt from the Cape de Verde Islands; and a
merchant at Buenos Ayres told me that he considered it as 50 per cent
less valuable. The purity of the Patagonian salt, or absence from it of
those other saline bodies found in all sea- water, is the only
assignable cause for this inferiority; a conclusion which is supported
by the fact lately ascertained, that those salts answer best for
preserving cheese which contain most of the deliquescent chlorides.
(“Horticultural and Agricultural Gazette” 1845 page 93.) (It would
probably well answer for the merchants of Buenos Ayres (considering the
great consumption there of salt for preserving meat) to import the
deliquescent chlorides to mix with the salt from the salinas: I may
call attention to the fact, that at Iquique, a large quantity of
muriate of lime, left in the MOTHER-WATER during the refinement of the
nitrate of soda, is annually thrown away.)
With respect to the origin of the salt in the salinas, the foregoing
analysis seems opposed to the view entertained by M. d’Orbigny and
others, and which seems so probable considering the recent elevation of
this line of coast, namely, that it is due to the evaporation of
sea-water and to the drainage from the surrounding strata impregnated
with sea-salt. I was informed (I know not whether accurately) that on
the northern side of the salina on the Rio Negro, there is a small
brine spring which flows at all times of the year: if this be so, the
salt in this case at least, probably is of subterranean origin. It at
first appears very singular that fresh water can often be procured in
wells, and is sometimes found in small lakes, quite close to these
salinas. (Sir W. Parish states “Buenos Ayres” etc. pages 122 and 170,
that this is the case near the great salinas westward of the S.
Ventana. I have seen similar statements in an ancient MS. Journal
lately published by S. Angelis. At Iquique, where the surface is so
thickly encrusted with saline matter, I tasted water only slightly
brackish, procured in a well thirty-six yards deep; but here one feels
less surprise at its presence, as pure water might percolate under
ground from the not very distant Cordillera.) I am not aware that this
fact bears particularly on the origin of the salt; but perhaps it is
rather opposed to the view of the salt having been washed out of the
surrounding superficial strata, but not to its having been the residue
of sea-water, left in depressions as the land was slowly elevated.
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 Fé.—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 American division of the
Camelidae. Several of the vertebrae in a chain, and nearly all the
bones of one of the limbs, even to the smallest bones of the foot, were
embedded in their proper relative positions: hence the skeleton was
certainly united by its flesh or ligaments, when enveloped in the mud.
This earthy mass, with its concretions and mammiferous remains, filling
up furrows in the underlying gravel, certainly presents a very striking
resemblance to some of the sections (for instance, at P. Alta in B.
Blanca, or at the Barrancas de S. Gregorio) in the Pampean formation;
but I must believe that this resemblance is only accidental. I suspect
that the mud which at the present day is accumulating in deep and
narrow gullies at the head of the harbour, would, after elevation,
present a very similar appearance. The southernmost part of the true
Pampean formation, namely, on the Colorado, lies 560 miles of latitude
north of this point. (In the succeeding chapter I shall have to refer
to a great deposit of extinct mammiferous remains, lately discovered by
Captain Sulivan, R.N., at a point still further south, namely, at the
R. Gallegos; their age must at present remain doubtful.)
With respect to the age of the Macrauchenia, the shells on the surface
prove that the mass in which the skeleton was enveloped has been
elevated above the sea within the recent period: I did not see any of
the shells embedded at a sufficient depth to assure me (though it be
highly probable) that the whole thickness of the mass was
contemporaneous with these INDIVIDUAL SPECIMENS. That the Macrauchenia
lived subsequently to the spreading out of the gravel on this plain is
certain; and that this gravel, at the height of ninety feet, was spread
out long after the existence of recent shells, is scarcely less
certain. For, it was shown in the First Chapter, that this line of
coast has been upheaved with remarkable equability, and that over a
vast space both north and south of S. Julian, recent species of shells
are strewed on (or embedded in) the surface of the 250 feet plain, and
of the 350 feet plain up to a height of 400 feet. These wide
step-formed plains have been formed by the denuding action of the
coast-waves on the old tertiary strata; and therefore, when the surface
of the 350 feet plain, with the shells on it, first rose above the
level of the sea, the 250 feet plain did not exist, and its formation,
as well as the spreading out of the gravel on its summit, must have
taken place subsequently. So also the denudation and the
gravel-covering of the 90 feet plain must have taken place subsequently
to the elevation of the 250 feet plain, on which recent shells are also
strewed. Hence there cannot be any doubt that the Macrauchenia, which
certainly was entombed in a fresh state, and which must have been alive
after the spreading out of the gravel on the 90 feet plain, existed,
not only subsequently to the upraised shells on the surface of the 250
feet plain, but also to those on the 350 to 400 feet plain: these
shells, eight in number (namely, three species of Mytilus, two of
Patella, one Fusus, Voluta, and Balanus), are undoubtedly recent
species, and are the commonest kinds now living on this coast. At Punta
Alta in B. Blanca, I remarked how marvellous it was, that the Toxodon,
a mammifer so unlike to all known genera, should have co-existed with
twenty- three still living marine animals; and now we find that the
Macrauchenia, a quadruped only a little less anomalous than the
Toxodon, also co-existed with eight other still existing Mollusca: it
should, moreover, be borne in mind, that a tooth of a pachydermatous
animal was found with the other remains at Punta Alta, which Professor
Owen thinks almost certainly belonged to the Macrauchenia.
Mr. Lyell has arrived at a highly important conclusion with respect to
the age of the North American extinct mammifers (many of which are
closely allied to, and even identical with, those of the Pampean
formation), namely, that they lived subsequently to the period when
erratic boulders were transported by the agency of floating ice in
temperate latitudes. (“Geological Proceedings” volume 4 page 36.) Now
in the valley of the Santa Cruz, only fifty miles of latitude south of
the spot where the Macrauchenia was entombed, vast numbers of gigantic,
angular boulders, which must have been transported from the Cordillera
on icebergs, lie strewed on the plain, at the height of 1,400 feet
above the level of the sea. In ascending to this level, several
step-formed plains must be crossed, all of which have necessarily
required long time for their formation; hence the lowest or ninety feet
plain, with its superficial bed containing the remains of the
Macrauchenia, must have been formed very long subsequently to the
period when the 1,400 feet plain was beneath the sea, and boulders were
dropped on it from floating masses of ice. (It must not be inferred
from these remarks, that the ice-action ceased in South America at this
comparatively ancient period; for in Tierra del Fuego boulders were
probably transported contemporaneously with, if not subsequently to,
the formation of the ninety feet plain at S. Julian, and at other parts
of the coast of Patagonia.) Mr. Lyell’s conclusion, therefore, is thus
far confirmed in the southern hemisphere; and it is the more important,
as one is naturally tempted to admit so simple an explanation, that it
was the ice-period that caused the extinction of the numerous great
mammifers which so lately swarmed over the two Americas.
A SUMMARY AND CONCLUDING REMARKS ON THE PAMPEAN FORMATION.
One of its most striking features is its great extent; I passed
continuously over it from the Colorado to St. Fe Bajada, a distance of
500 geographical miles; and M. d’Orbigny traced it for 250 miles
further north. In the latitude of the Plata, I examined this formation
at intervals over an east and west line of 300 miles from Maldonado to
the R. Carcarana; and M. d’Orbigny believes it extends 100 miles
further inland: from Mr. Caldcleugh’s travels, however, I should have
thought that it had extended, south of the Cordovese range, to near
Mendoza, and I may add that I heard of great bones having been found
high up the R. Quinto. Hence the area of the Pampean formation, as
remarked by M. d’Orbigny, is probably at least equal to that of France,
and perhaps twice or thrice as great. In a basin, surrounded by
gravel-cliff (at a height of nearly three thousand feet), south of
Mendoza, there is, as described in the Third Chapter, a deposit very
like the Pampean, interstratified with other matter; and again at S.
Julian’s, in Patagonia, 560 miles south of the Colorado, a small
irregular bed of a nearly similar nature contains, as we have just
seen, mammiferous remains. In the provinces of Moxos and Chiquitos
(1,000 miles northward of the Pampas), and in Bolivia, at a height of
4,000 metres, M. d’Orbigny has described similar deposits, which he
believes to have been formed by the same agency contemporaneously with
the Pampean formation. Considering the immense distances between these
several points, and their different heights, it appears to me
infinitely more probable, that this similarity has resulted not from
contemporaneousness of origin, but from the similarity of the rocky
framework of the continent: it is known that in Brazil an immense area
consists of gneissic rocks, and we shall hereafter see, over how great
a length the plutonic rocks of the Cordillera, the overlying purple
porphyries, and the trachytic ejections, are almost identical in
nature.
Three theories on the origin of the Pampean formation have been
propounded:—First, that of a great debacle by M. d’Orbigny; this seems
founded chiefly on the absence of stratification, and on the number of
embedded remains of terrestrial quadrupeds. Although the Pampean
formation (like so many argillaceous deposits) is not divided into
distinct and separate strata, yet we have seen that in one good section
it was striped with horizontal zones of colour, and that in several
specified places the upper and lower parts differed, not only
considerably in colour, but greatly in constitution. In the southern
part of the Pampas the upper mass (to a certain extent stratified)
generally consists of hard tosca-rock, and the lower part of red
Pampean mud, often itself divided into two or more masses, varying in
colour and in the quantity of included calcareous matter. In Western
Banda Oriental, beds of a similar nature, but of a greater age,
conformably underlie and are intercalated with the regularly stratified
tertiary formation. As a general rule, the marly concretions are
arranged in horizontal lines, sometimes united into irregular strata:
surely, if the mud had been tumultuously deposited in mass, the
included calcareous matter would have segregated itself irregularly,
and not into nodules arranged in horizontal lines, one above the other
and often far apart: this arrangement appears to me to prove that mud,
differing slightly in composition, was successively and quietly
deposited. On the theory of a debacle, a prodigious amount of mud,
without a single pebble, is supposed to have been borne over the wide
surface of the Pampas, when under water: on the other hand, over the
whole of Patagonia, the same or another debacle is supposed to have
borne nothing but gravel,—the gravel and the fine mud in the
neighbourhood of the Rios Negro and Colorado having been borne to an
equal distance from the Cordillera, or imagined line of disturbance:
assuredly directly opposite effects ought not to be attributed to the
same agency. Where, again, could a mass of fine sediment, charged with
calcareous matter in a fit state for chemical segregation, and in
quantity sufficient to cover an area at least 750 miles long, and 400
miles broad, to a depth of from twenty to thirty feet to a hundred
feet, have been accumulated, ready to be transported by the supposed
debacle? To my mind it is little short of demonstration, that a great
lapse of time was necessary for the production and deposition of the
enormous amount of mudlike matter forming the Pampas; nor should I have
noticed the theory of a debacle, had it not been adduced by a
naturalist so eminent as M. d’Orbigny.
A second theory, first suggested, I believe, by Sir W. Parish, is that
the Pampean formation was thrown down on low and marshy plains by the
rivers of this country before they assumed their present courses. The
appearance and composition of the deposit, the manner in which it
slopes up and round the primary ranges, the nature of the underlying
marine beds, the estuary and sea-shells on the surface, the overlying
sandstone beds at M. Hermoso, are all quite opposed to this view. Nor
do I believe that there is a single instance of a skeleton of one of
the extinct mammifers having been found in an upright position, as if
it had been mired.
The third theory, of the truth of which I cannot entertain the smallest
doubt, is that the Pampean formation was slowly accumulated at the
mouth of the former estuary of the Plata and in the sea adjoining it. I
have come to this conclusion from the reasons assigned against the two
foregoing theories, and from simple geographical considerations. From
the numerous shells of the Azara labiata lying loose on the surface of
the plains, and near Buenos Ayres embedded in the tosca-rock, we know
that this formation not only was formerly covered by, but that the
uppermost parts were deposited in, the brackish water of the ancient La
Plata. Southward and seaward of Buenos Ayres, the plains were upheaved
from under water inhabited by true marine shells. We further know from
Professor Ehrenberg’s examination of the twenty microscopical organisms
in the mud round the tooth of the Mastodon high up the course of the
Parana, that the bottom- most part of this formation was of
brackish-water origin. A similar conclusion must be extended to the
beds of like composition, at the level of the sea and under it, at M.
Hermoso in Bahia Blanca. Dr. Carpenter finds that the harder varieties
of tosca-rock, collected chiefly to the south, contain marine spongoid
bodies, minute fragments of shells, corals, and Polythalamia; these
perhaps may have been drifted inwards by the tides, from the more open
parts of the sea. The absence of shells, throughout this deposit, with
the exception of the uppermost layers near Buenos Ayres, is a
remarkable fact: can it be explained by the brackish condition of the
water, or by the deep mud at the bottom? I have stated that both the
reddish mud and the concretions of tosca-rock are often penetrated by
minute, linear cavities, such as frequently may be observed in
fresh-water calcareous deposits:—were they produced by the burrowing of
small worms? Only on this view of the Pampean formation having been of
estuary origin, can the extraordinary numbers (presently to be alluded
to) of the embedded mammiferous remains be explained. (It is almost
superfluous to give the numerous cases (for instance, in Sumatra; Lyell
“Principles” volume 3 page 325 sixth edition, of the carcasses of
animals having been washed out to sea by swollen rivers; but I may
refer to a recent account by Mr. Bettington “Asiatic Society” 1845 June
21st, of oxen, deer, and bears being carried into the Gulf of Cambray;
see also the account in my “Journal” 2nd edition page 133, of the
numbers of animals drowned in the Plata during the great, often
recurrent, droughts.)
With respect to the first origin of the reddish mud, I will only
remark, that the enormous area of Brazil consists in chief part of
gneissic and other granitic rocks, which have suffered decomposition,
and been converted into a red, gritty, argillaceous mass, to a greater
depth than in any other country which I have seen. The mixture of
rounded grains, and even of small fragments and pebbles of quartz, in
the Pampean mud of Banda Oriental, is evidently due to the neighbouring
and underlying primary rocks. The estuary mud was drifted during the
Pampean period in a much more southerly course, owing probably to the
east and west primary ridges south of the Plata not having been then
elevated, than the mud of the Plata at present is; for it was formerly
deposited as far south as the Colorado. The quantity of calcareous
matter in this formation, especially in those large districts where the
whole mass passes into tosca-rock, is very great: I have already
remarked on the close resemblance in external and microscopical
appearance, between this tosca-rock and the strata at Coquimbo, which
have certainly resulted from the decay and attrition of recent shells:
I dare not, however, extend this conclusion to the calcareous rocks of
the Pampas, more especially as the underlying tertiary strata in
western Banda Oriental show that at that period there was a copious
emission of carbonate of lime, in connection with volcanic action. (I
may add, that there are nearly similar superficial calcareous beds at
King George’s Sound in Australia; and these undoubtedly have been
formed by the disintegration of marine remains see “Volcanic Islands”
etc. page 144. There is, however, something very remarkable in the
frequency of superficial, thin beds of earthy calcareous matter, in
districts where the surrounding rocks are not calcareous. Major
Charters, in a Paper read before the Geographical Society April 13,
1840 and abstracted in the “Athenaeum” page 317, states that this is
the case in parts of Mexico, and that he has observed similar
appearances in many parts of South Africa. The circumstance of the
uppermost stratum round the ragged Sierra Ventana, consisting of
calcareous or marly matter, without any covering of alluvial matter,
strikes me as very singular, in whatever manner we view the deposition
and elevation of the Pampean formation.)
The Pampean formation, judging from its similar composition, and from
the apparent absolute specific identity of some of its mammiferous
remains, and from the generic resemblance of others, belongs over its
vast area— throughout Banda Oriental, Entre Rios, and the wide extent
of the Pampas as far south as the Colorado,—to the same geological
epoch. The mammiferous remains occur at all depths from the top to the
bottom of the deposit; and I may add that nowhere in the Pampas is
there any appearance of much superficial denudation: some bones which I
found near the Guardia del Monte were embedded close to the surface;
and this appears to have been the case with many of those discovered in
Banda Oriental: on the Matanzas, twenty miles south of Buenos Ayres, a
Glyptodon was embedded five feet beneath the surface; numerous remains
were found by S. Muniz, near Luxan, at an average depth of eighteen
feet; in Buenos Ayres a skeleton was disinterred at sixty feet depth,
and on the Parana I have described two skeletons of the Mastodon only
five or six feet above the very base of the deposit. With respect to
the age of this formation, as judged of by the ordinary standard of the
existence of Mollusca, the only evidence within the limits of the true
Pampas which is at all trustworthy, is afforded by the still living
Azara labiata being embedded in tosca-rock near Buenos Ayres. At Punta
Alta, however, we have seen that several of the extinct mammifers, most
characteristic of the Pampean formation, co-existed with twenty species
of Mollusca, a barnacle and two corals, all still living on this same
coast;— for when we remember that the shells have a more ancient
appearance than the bones; that many of the bones, though embedded in a
coarse conglomerate, are perfectly preserved; that almost all the parts
of the skeleton of the Scelidotherium, even to the knee-cap, were lying
in their proper relative positions; and that a large piece of the
fragile dermal armour of a Dasypoid quadruped, connected with some of
the bones of the foot, had been entombed in a condition allowing the
two sides to be doubled together, it must assuredly be admitted that
these mammiferous remains were embedded in a fresh state, and therefore
that the living animals co-existed with the co-embedded shells.
Moreover, the Macrauchenia Patachonica (of which, according to
Professor Owen, remains also occur in the Pampas of Buenos Ayres, and
at Punta Alta) has been shown by satisfactory evidence of another kind,
to have lived on the plains of Patagonia long after the period when the
adjoining sea was first tenanted by its present commonest molluscous
animals. We must, therefore, conclude that the Pampean formation
belongs, in the ordinary geological sense of the word, to the Recent
Period. (M. d’Orbigny believes “Voyage” Part. Geolog. page 81, that
this formation, though “tres voisine de la notre, est neanmoins de
beaucoup anterieure a notre creation.”)
At St. Fe Bajada, the Pampean estuary formation, with its mammiferous
remains, conformably overlies the marine tertiary strata, which (as
first shown by M. d’Orbigny) are contemporaneous with those of
Patagonia, and which, as we shall hereafter see, belong to a very
ancient tertiary stage. When examining the junction between these two
formations, I thought that the concretionary layer of marl marked a
passage between the marine and estuary stages. M. d’Orbigny disputes
this view (as given in my “Journal”), and I admit that it is erroneous,
though in some degree excusable, from their conformability and from
both abounding with calcareous matter. It would, indeed, have been a
great anomaly if there had been a true passage between a deposit
contemporaneous with existing species of mollusca, and one in which all
the mollusca appear to be extinct. Northward of Santa Fe, M. d’Orbigny
met with ferruginous sandstones, marly rocks, and other beds, which he
considers as a distinct and lower formation; but the evidence that they
are not parts of the same with an altered mineralogical character, does
not appear to me quite satisfactory.
In Western Banda Oriental, while the marine tertiary strata were
accumulating, there were volcanic eruptions, much silex and lime were
precipitated from solution, coarse conglomerates were formed, being
derived probably from adjoining land, and layers of red mud and marly
rocks, like those of the Pampean formation, were occasionally
deposited. The true Pampean deposit, with mammiferous remains, instead
of as at Santa Fe overlying conformably the tertiary strata, is here
seen at a lower level folding round and between the flat-topped,
cliff-bounded hills, formed by a upheaval and denudation of these same
tertiary strata. The upheaval, having occurred here earlier than at
Santa Fe, may be naturally accounted for by the contemporaneous
volcanic action. At the Barrancas de S. Gregorio, the Pampean deposit,
as we have seen, overlies and fills up furrows in coarse sand,
precisely like that now accumulating on the shores near the mouth of
the Plata. I can hardly believe that this loose and coarse sand is
contemporaneous with the old tertiary and often crystalline strata of
the more western parts of the province; and am induced to suspect that
it is of subsequent origin. If that section near Colonia could be
implicitly trusted, in which, at a height of only fifteen feet above
the Plata, a bed of fresh-looking mussels, of an existing littoral
species, appeared to lie between the sand and the Pampean mud, I should
conclude that Banda Oriental must have stood, when the coarse sand was
accumulating, at only a little below its present level, and had then
subsided, allowing the estuary Pampean mud to cover far and wide its
surface up to a height of some hundred feet; and that after this
subsidence the province had been uplifted to its present level.
In Western Banda Oriental, we know, from two unequivocal sections that
there is a mass, absolutely undistinguishable from the true Pampean
deposit, beneath the old tertiary strata. This inferior mass must be
very much more ancient than the upper deposit with its mammiferous
remains, for it lies beneath the tertiary strata in which all the
shells are extinct. Nevertheless, the lower and upper masses, as well
as some intermediate layers, are so similar in mineralogical character,
that I cannot doubt that they are all of estuary origin, and have been
derived from the same great source. At first it appeared to me
extremely improbable, that mud of the same nature should have been
deposited on nearly the same spot, during an immense lapse of time,
namely, from a period equivalent perhaps to the Eocene of Europe to
that of the Pampean formation. But as, at the very commencement of the
Pampean period, if not at a still earlier period, the Sierra Ventana
formed a boundary to the south,—the Cordillera or the plains in front
of them to the west,—the whole province of Corrientes probably to the
north, for, according to M. d’Orbigny, it is not covered by the Pampean
deposit,—and Brazil, as known by the remains in the caves, to the
north-east; and as again, during the older tertiary period, land
already existed in Western Banda Oriental and near St. Fe Bajada, as
may be inferred from the vegetable debris, from the quantities of
silicified wood, and from the remains of a Toxodon found, according to
M. d’Orbigny, in still lower strata, we may conclude, that at this
ancient period a great expanse of water was surrounded by the same
rocky framework which now bounds the plains of Pampean formation. This
having been the case, the circumstance of sediment of the same nature
having been deposited in the same area during an immense lapse of time,
though highly remarkable, does not appear incredible.
The elevation of the Pampas, at least of the southern parts, has been
slow and interrupted by several periods of rest, as may be inferred
from the plains, cliffs, and lines of sand-dunes (with shells and
pumice-pebbles) standing at different heights. I believe, also, that
the Pampean mud continued to be deposited, after parts of this
formation had already been elevated, in the same manner as mud would
continue to be deposited in the estuary of the Plata, if the mud-banks
on its shores were now uplifted and changed into plains: I believe in
this from the improbability of so many skeletons and bones having been
accumulated at one spot, where M. Hermoso now stands, at a depth of
between eight hundred and one thousand feet, and at a vast distance
from any land except small rocky islets,—as must have been the case, if
the high tosca-plain round the Ventana and adjoining Sierras, had not
been already uplifted and converted into land, supporting mammiferous
animals. At Punta Alta we have good evidence that the gravel- strata,
which certainly belong to the true Pampean period, were accumulated
after the elevation in that neighbourhood of the main part of the
Pampean deposit, whence the rounded masses of tosca-rock were derived,
and that rolled fragment of black bone in the same peculiar condition
with the remains at Monte Hermoso.
The number of the mammiferous remains embedded in the Pampas is, as I
have remarked, wonderful: it should be borne in mind that they have
almost exclusively been found in the cliffs and steep banks of rivers,
and that, until lately, they excited no attention amongst the
inhabitants: I am firmly convinced that a deep trench could not be cut
in any line across the Pampas, without intersecting the remains of some
quadruped. It is difficult to form an opinion in what part of the
Pampas they are most numerous; in a limited spot they could not well
have been more numerous than they were at P. Alta; the number, however,
lately found by Senor F. Muniz, near Luxan, in a central spot in the
Pampas, is extraordinarily great: at the end of this chapter I will
give a list of all the localities at which I have heard of remains
having been discovered. Very frequently the remains consist of almost
perfect skeletons; but there are, also, numerous single bones, as for
instance at St. Fe. Their state of preservation varies much, even when
embedded near each other: I saw none others so perfectly preserved as
the heads of the Toxodon and Mylodon from the white soft earthy bed on
the Sarandis in Banda Oriental. It is remarkable that in two limited
sections I found no less than five teeth separately embedded, and I
heard of teeth having been similarly found in other parts: may we
suppose that the skeletons or heads were for a long time gently drifted
by currents over the soft muddy bottom, and that the teeth
occasionally, here and there, dropped out?
It may be naturally asked, where did these numerous animals live? From
the remarkable discoveries of MM. Lund and Clausen, it appears that
some of the species found in the Pampas inhabited the highlands of
Brazil: the Mastodon Andium is embedded at great heights in the
Cordillera from north of the equator to at least as far south as Tarija
(Humboldt states that the Mastodon has been discovered in New Granada:
it has been found in Quito. When at Lima, I saw a tooth of a Mastodon
in the possession of Don M. Rivero, found at Playa Chica on the
Maranon, near the Guallaga. Every one has heard of the numerous remains
of Mastodon in Bolivia.); and as there is no higher land, there can be
little doubt that this Mastodon must have lived on the plains and
valleys of that great range. These countries, however, appear too far
distant for the habitation of the individuals entombed in the Pampas:
we must probably look to nearer points, for instance to the province of
Corrientes, which, as already remarked, is said not to be covered by
the Pampean formation, and may therefore, at the period of its
deposition, have existed as dry land. I have already given my reasons
for believing that the animals embedded at M. Hermoso and at P. Alta in
Bahia Blanca, lived on adjoining land, formed of parts of the already
elevated Pampean deposit. With respect to the food of these many great
extinct quadrupeds, I will not repeat the facts given in my “Journal”
(second edition page 85), showing that there is no correlation between
the luxuriance of the vegetation of a country and the size of its
mammiferous inhabitants. I do not doubt that large animals could now
exist, as far as the amount, not kind, of vegetation is concerned, on
the sterile plains of Bahia Blanca and of the R. Negro, as well as on
the equally, if not more sterile plains of Southern Africa. The
climate, however, may perhaps have somewhat deteriorated since the
mammifers embedded at Bahia Blanca lived there; for we must not infer,
from the continued existence of the same shells on the present coasts,
that there has been no change in climate; for several of these shells
now range northward along the shores of Brazil, where the most
luxuriant vegetation flourishes under a tropical temperature. With
respect to the extinction, which at first fills the mind with
astonishment, of the many great and small mammifers of this period, I
may also refer to the work above cited (second edition page 173), in
which I have endeavoured to show, that however unable we may be to
explain the precise cause, we ought not properly to feel more surprised
at a species becoming extinct than at one being rare; and yet we are
accustomed to view the rarity of any particular species as an ordinary
event, not requiring any extraordinary agency.
The several mammifers embedded in the Pampean formation, which mostly
belong to extinct genera, and some even to extinct families or orders,
and which differ nearly, if not quite, as much as do the Eocene
mammifers of Europe from living quadrupeds having existed
contemporaneously with mollusca, all still inhabiting the adjoining
sea, is certainly a most striking fact. It is, however, far from being
an isolated one; for, during the late tertiary deposits of Britain, an
elephant, rhinoceros, and hippopotamus co-existed with many recent land
and fresh-water shells; and in North America, we have the best evidence
that a mastodon, elephant, megatherium, megalonyx, mylodon, an extinct
horse and ox, likewise co- existed with numerous land, fresh-water, and
marine recent shells. (Many original observations, and a summary on
this subject, are given in Mr. Lyell’s paper in the “Geological
Proceedings” volume 4 page 3 and in his “Travels in North America”
volume 1 page 164 and volume 2 page 60. For the European analogous
cases see Mr. Lyell’s “Principles of Geology” 6th edition volume 1 page
37.) The enumeration of these extinct North American animals naturally
leads me to refer to the former closer relation of the mammiferous
inhabitants of the two Americas, which I have discussed in my
“Journal,” and likewise to the vast extent of country over which some
of them ranged: thus the same species of the Megatherium, Megalonyx,
Equus (as far as the state of their remains permits of identification),
extended from the Southern United States of North America to Bahia
Blanca, in latitude 39 degrees S., on the coast of Patagonia. The fact
of these animals having inhabited tropical and temperate regions, does
not appear to me any great difficulty, seeing that at the Cape of Good
Hope several quadrupeds, such as the elephant and hippopotamus, range
from the equator to latitude 35 degrees south. The case of the Mastodon
Andium is one of more difficulty, for it is found from latitude 36
degrees S., over, as I have reason to believe, nearly the whole of
Brazil, and up the Cordillera to regions which, according to M.
d’Orbigny, border on perpetual snow, and which are almost destitute of
vegetation: undoubtedly the climate of the Cordillera must have been
different when the mastodon inhabited it; but we should not forget the
case of the Siberian mammoth and rhinoceros, as showing how severe a
climate the larger pachydermata can endure; nor overlook the fact of
the guanaco ranging at the present day over the hot low deserts of
Peru, the lofty pinnacles of the Cordillera, and the damp forest-clad
land of Southern Tierra del Fuego; the puma, also, is found from the
equator to the Strait of Magellan, and I have seen its footsteps only a
little below the limits of perpetual snow in the Cordillera of Chile.
At the period, so recent in a geological sense, when these extinct
mammifers existed, the two Americas must have swarmed with quadrupeds,
many of them of gigantic size; for, besides those more particularly
referred to in this chapter, we must include in this same period those
wonderfully numerous remains, some few of them specifically, and others
generically related to those of the Pampas, discovered by MM. Lund and
Clausen in the caves of Brazil. Finally, the facts here given show how
cautious we ought to be in judging of the antiquity of a formation from
even a great amount of difference between the extinct and living
species in any one class of animals;—we ought even to be cautious in
accepting the general proposition, that change in organic forms and
lapse of time are at all, necessarily, correlatives.
LOCALITIES WITHIN THE REGION OF THE PAMPAS WHERE GREAT BONES HAVE BEEN
FOUND.
The following list, which includes every account which I have hitherto
met with of the discovery of fossil mammiferous remains in the Pampas,
may be hereafter useful to a geologist investigating this region, and
it tends to show their extraordinary abundance. I heard of and saw many
fossils, the original position of which I could not ascertain; and I
received many statements too vague to be here inserted. Beginning to
the south:—we have the two stations in Bahia Blanca, described in this
chapter, where at P. Alta, the Megatherium, Megalonyx, Scelidotherium,
Mylodon, Holophractus (or an allied genus), Toxodon, Macrauchenia, and
an Equus were collected; and at M. Hermoso a Ctenomys, Hydrochaerus,
some other rodents and the bones of a great megatheroid quadruped.
Close north-east of the S. Tapalguen, we have the Rios ‘Huesos’ (i.e.
“bones”), which probably takes its name from large fossil bones. Near
Villa Nuevo, and at Las Averias, not far from the Salado, three nearly
perfect skeletons, one of the Megatherium, one of the Glyptodon
clavipes, and one of some great Dasypoid quadruped, were found by the
agent of Sir W. Parish (see his work “Buenos Ayres” etc. page 171). I
have seen the tooth of a Mastodon from the Salado; a little northward
of this river, on the borders of a lake near the G. del Monte, I saw
many bones, and one large piece of dermal armour; higher up the Salado,
there is a place called Monte “Huesos.” On the Matanzas, about twenty
miles south of Buenos Ayres, the skeleton (vide page 178 of “Buenos
Ayres” etc. by Sir W. Parish) of a Glyptodon was found about five feet
beneath the surface; here also (see Catalogue of Royal College of
Surgeons) remains of Glyptodon clavipes, G. ornatus, and G. reticulatus
were found. Signor Angelis, in a letter which I have seen, refers to
some great remains found in Buenos Ayres, at a depth of twenty varas
from the surface. Seven leagues north of this city the same author
found the skeletons of Mylodon robustus and Glyptodon ornatus. From
this neighbourhood he has lately sent to the British Museum the
following fossils:—Remains of three or four individuals of Megatherium;
of three species of Glyptodon; of three individuals of the Mastodon
Andium; of Macrauchenia; of a second species of Toxodon, different from
T. Platensis; and lastly, of the Machairodus, a wonderful large
carnivorous animal. M. d’Orbigny has lately received from the Recolate
“Voyage” Pal. page 144), near Buenos Ayres, a tooth of Toxodon
Platensis.
Proceeding northward, along the west bank of the Parana, we come to the
Rio Luxan, where two skeletons of the Megatherium have been found; and
lately, within eight leagues of the town of Luxan, Dr. F. X. Muniz has
collected (“British Packet” Buenos Ayres September 25, 1841), from an
average depth of eighteen feet, very numerous remains, of no less than,
as he believes, nine distinct species of mammifers. At Areco, large
bones have been found, which are believed, by the inhabitants, to have
been changed from small bones, by the water of the river! At Arrecifes,
the Glyptodon, sent to the College of Surgeons, was found; and I have
seen two teeth of a Mastodon from this quarter. At S. Nicolas, M.
d’Orbigny found remains of a Canis, Ctenomys, and Kerodon; and M.
Isabelle (“Voyage” page 332) refers to a gigantic Armadillo found
there. At S. Carlos, I heard of great bones. A little below the mouth
of the Carcarana, the two skeletons of Mastodon were found; on the
banks of this river, near S. Miguel, I found teeth of the Mastodon and
Toxodon; and “Falkner” (page 55) describes the osseous armour of some
great animal; I heard of many other bones in this neighbourhood. I have
seen, I may add, in the possession of Mr. Caldcleugh, the tooth of a
Mastodon Andium, said to have been found in Paraguay; I may here also
refer to a statement in this gentleman’s travels (volume 1 page 48), of
a great skeleton having been found in the province of Bolivia in
Brazil, on the R. de las Contas. The furthest point westward in the
Pampas, at which I have HEARD of fossil bones, was high up on the banks
of R. Quinto.
In Entre Rios, besides the remains of the Mastodon, Toxodon, Equus, and
a great Dasypoid quadruped near St. Fe Bajada, I received an account of
bones having been found a little S.E. of P. Gorda (on the Parana), and
of an entire skeleton at Matanzas, on the Arroyo del Animal.
In Banda Oriental, besides the remains of the Toxodon, Mylodon, and two
skeletons of great animals with osseous armour (distinct from that of
the Glyptodon), found on the Arroyos Sarandis and Berquelo, M. Isabelle
(“Voyage” page 322) says, many bones have been found near the R. Negro,
and on the R. Arapey, an affluent of the Paraguay, in latitude 30
degrees 40 minutes south. I heard of bones near the source of the A.
Vivoras. I saw the remains of a Dasypoid quadruped from the Arroyo
Seco, close to M. Video; and M. d’Orbigny refers (“Voyage” Geolog. page
24), to another found on the Pedernal, an affluent of the St. Lucia;
and Signor Angelis, in a letter, states that a third skeleton of this
family has been found, near Canelones. I saw a tooth of the Mastodon
from Talas, another affluent of the St. Lucia. The most eastern point
at which I heard of great bones having been found, was at Solis Grande,
between M. Video and Maldonado.
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.