Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions
of the inhabitants on this subject.—Concepcion, earthquake and
elevation of.—VALPARAISO, great elevation of, upraised shells, earth of
marine origin, gradual rise of the land within the historical
period.—COQUIMBO, elevation of, in recent times; terraces of marine
origin, their inclination, their escarpments not horizontal.—Guasco,
gravel terraces of.—Copiapo.—PERU.—Upraised shells of Cobija, Iquique,
and Arica.—Lima, shell-beds and sea-beach on San Lorenzo, human
remains, fossil earthenware, earthquake debacle, recent subsidence. On
the decay of upraised shells.—General summary.
Commencing at the south and proceeding northward, the first place at
which I landed, was at Cape Tres Montes, in lat. 46° 35′. Here, on the
shores of Christmas Cove, I observed in several places a beach of
pebbles with recent shells, about twenty feet above high-water mark.
Southward of Tres Montes (between lat. 47° and 48°), Byron[1] remarks,
“We thought it very strange, that upon the summits of the highest hills
were found beds of shells, a foot or two thick.” In the Chonos
Archipelago, the island of Lemus (lat. 44° 30′) was, according to M.
Coste,2 suddenly elevated eight feet, during the earthquake of 1829: he
adds, “Des roches jadis toujours couvertes par la mer, restant
aujourd’hui constamment decouvertes.” In other parts of this
archipelago, I observed two terraces of gravel, abutting to the foot of
each other: at Lowe’s Harbour (43° 48′), under a great mass of the
boulder formation, about three hundred feet in thickness, I found a
layer of sand, with numerous comminuted fragments of sea-shells, having
a fresh aspect, but too small to be identified.
[1] “Narrative of the Loss of the _Wager_.”
[2] “Comptes Rendus,” October 1838, p. 706.
_The Island of Chiloe._—The evidence of recent elevation is here more
satisfactory. The bay of San Carlos is in most parts bounded by
precipitous cliffs from about ten to forty feet in height, their bases
being separated from the present line of tidal action by a talus, a few
feet in height, covered with vegetation. In one sheltered creek (west
of P. Arena), instead of a loose talus, there was a bare sloping bank
of tertiary mudstone, perforated, above the line of the highest tides,
by numerous shells of a Pholas now common in the harbour. The upper
extremities of these shells, standing upright in their holes with grass
growing out of them, were abraded about a quarter of an inch, to the
same level with the surrounding worn strata. In other parts, I observed
(as at Pudeto) a great beach, formed of comminuted shells, twenty feet
above the present shore. In other parts again, there were small caves
worn into the foot of the low cliffs, and protected from the waves by
the talus with its vegetation: one such cave, which I examined, had its
mouth about twenty feet, and its bottom, which was filled with sand
containing fragments of shells and legs of crabs, from eight to ten
feet above high-water mark. From these several facts, and from the
appearance of the upraised shells, I inferred that the elevation had
been quite recent; and on inquiring from Mr. Williams, the Portmaster,
he told me he was convinced that the land had risen, or the sea fallen,
four feet within the last four years. During this period, there had
been one severe earthquake, but no particular change of level was then
observed; from the habits of the people who all keep boats in the
protected creeks, it is absolutely impossible that a rise of four feet
could have taken place suddenly and been unperceived. Mr. Williams
believes that the change has been quite gradual. Without the elevatory
movement continues at a quick rate, there can be no doubt that the sea
will soon destroy the talus of earth at the foot of the cliffs round
the bay, and will then reach its former lateral extension, but not of
course its former level: some of the inhabitants assured me that one
such talus, with a footpath on it, was even already sensibly decreasing
in width. I received several accounts of beds of shells, existing at
considerable heights in the inland parts of Chiloe; and to one of
these, near Catiman, I was guided by a countryman. Here, on the south
side of the peninsula of Lacuy, there was an immense bed of the _Venus
costellata_ and of an oyster, lying on the summit-edge of a piece of
tableland, 350 feet (by the barometer) above the level of the sea. The
shells were closely packed together, embedded in and covered by a very
black, damp, peaty mould, two or three feet in thickness, out of which
a forest of great trees was growing. Considering the nature and
dampness of this peaty soil, it is surprising that the fine ridges on
the outside of the Venus are perfectly preserved, though all the shells
have a blackened appearance. I did not doubt that the black soil, which
when dry, cakes hard, was entirely of terrestrial origin, but on
examining it under the microscope, I found many very minute rounded
fragments of shells, amongst which I could distinguish bits of Serpulæ
and mussels. The _Venus costellata_, and the Ostrea (_O. edulis_,
according to Captain King) are now the commonest shells in the
adjoining bays. In a bed of shells, a few feet below the 350 feet bed,
I found a horn of the little _Cervus humilis_, which now inhabits
Chiloe.
The eastern or inland side of Chiloe, with its many adjacent islets,
consists of tertiary and boulder deposits, worn into irregular plains
capped by gravel. Near Castro, and for ten miles southward, and on the
islet of Lemuy, I found the surface of the ground to a height of
between twenty and thirty feet above high-water mark, and in several
places apparently up to fifty feet, thickly coated by much comminuted
shells, chiefly of the _Venus costellata_ and _Mytilus Chiloensis_; the
species now most abundant on this line of coast. As the inhabitants
carry immense numbers of these shells inland, the continuity of the bed
at the same height was often the only means of recognising its natural
origin. Near Castro, on each side of the creek and rivulet of the
Gamboa, three distinct terraces are seen: the lowest was estimated at
about one hundred and fifty feet in height, and the highest at about
five hundred feet, with the country irregularly rising behind it;
obscure
traces, also, of these same terraces could be seen along other parts of
the coast. There can be no doubt that their three escarpments record
pauses in the elevation of the island. I may remark that several
promontories have the word Huapi, which signifies in the Indian tongue,
island, appended to them, such as Huapilinao, Huapilacuy, Caucahuapi,
etc.; and these, according to Indian traditions, once existed as
islands. In the same manner the term Pulo in Sumatra is appended[3] to
the names of promontories, traditionally said to have been islands; in
Sumatra, as in Chiloe, there are upraised recent shells. The Bay of
Carelmapu, on the mainland north of Chiloe, according to Agüerros,[4]
was in 1643 a good harbour; it is now quite useless, except for boats.
[3] Marsden’s “Sumatra,” p. 31.
[4] “Descripcion Hist. de la Provincia de Chiloé,” p. 78. From the
account given by the old Spanish writers, it would appear that several
other harbours, between this point and Concepcion, were formerly much
deeper than they now are.
_Valdivia._—I did not observe here any distinct proofs of recent
elevation; but in a bed of very soft sandstone, forming a fringe-like
plain, about sixty feet in height, round the hills of mica-slate, there
are shells of Mytilus, Crepidula, Solen, Novaculina, and Cytheræa, too
imperfect to be specifically recognised. At Imperial, seventy miles
north of Valdivia, Agüerros[5] states that there are large beds of
shells, at a considerable distance from the coast, which are burnt for
lime. The island of Mocha, lying a little north of Imperial, was
uplifted two feet,[6] during the earthquake of 1835.
[5] _Ibid.,_ p. 25.
[6] “Voyages of _Adventure_ and _Beagle_,” vol. ii, p. 415.
_Concepcion._—I cannot add anything to the excellent account by Captain
Fitzroy[7] of the elevation of the land at this place, which
accompanied the earthquake of 1835. I will only recall to the
recollection of geologists, that the southern end of the island of St.
Mary was uplifted eight feet, the central part nine, and the northern
end ten feet; and the whole island more than the surrounding districts.
Great beds of mussels, patellæ, and chitons still adhering to the rocks
were upraised above high-water mark; and some acres of a rocky flat,
which was formerly always covered by the sea, was left standing dry,
and exhaled an offensive smell, from the many attached and putrefying
shells. It appears from the researches of Captain Fitzroy that both the
island of St. Mary and Concepcion (which was uplifted only four or five
feet) in the course of some weeks subsided, and lost part of their
first elevation. I will only add as a lesson of caution, that round the
sandy shores of the great Bay of Concepcion, it was most difficult,
owing to the obliterating effects of the great accompanying wave, to
recognise any distinct
evidence of this considerable upheaval; one spot must be excepted,
where there was a detached rock which before the earthquake had always
been covered by the sea, but afterwards was left uncovered.
[7] _Ibid.,_ vol. ii, p. 412, _et seq._ In vol. v, p. 601 of the
“Geological Transactions” I have given an account of the remarkable
volcanic phenomena, which accompanied this earthquake. These phenomena
appear to me to prove that the action, by which large tracts of land
are uplifted, and by which volcanic eruptions are produced, is in
every respect identical.
On the island of Quiriquina (in the Bay of Concepcion), I found, at an
estimated height of four hundred feet, extensive layers of shells,
mostly comminuted, but some perfectly preserved and closely packed in
black vegetable mould; they consisted of Concholepas, Fissurella,
Mytilus, Trochus, and Balanus. Some of these layers of shells rested on
a thick bed of bright-red, dry, friable earth, capping the surface of
the tertiary sandstone, and extending, as I observed whilst sailing
along the coast, for 150 miles southward: at Valparaiso, we shall
presently see that a similar red earthy mass, though quite like
terrestrial mould, is really in chief part of recent marine origin. On
the flanks of this island of Quiriquina, at a less height than the 400
feet, there were spaces several feet square, thickly strewed with
fragments of similar shells. During a subsequent visit of the _Beagle_
to Concepcion, Mr. Kent, the assistant-surgeon, was so kind as to make
for me some measurements with the barometer: he found many marine
remains along the shores of the whole bay, at a height of about twenty
feet; and from the hill of Sentinella behind Talcahuano, at the height
of 160 feet, he collected numerous shells, packed together close
beneath the surface in black earth, consisting of two species of
Mytilus, two of Crepidula, one of Concholepas, of Fissurella, Venus,
Mactra, Turbo, Monoceros, and the _Balanus psittacus._ These shells
were bleached, and within some of the Balani other Balani were growing,
showing that they must have long lain dead in the sea. The above
species I compared with living ones from the bay, and found them
identical; but having since lost the specimens, I cannot give their
names: this is of little importance, as Mr. Broderip has examined a
similar collection, made during Captain Beechey’s expedition, and
ascertained that they consisted of ten recent species, associated with
fragments of Echini, crabs, and Flustræ; some of these remains were
estimated by Lieutenant Belcher to lie at the height of nearly a
thousand feet above the level of the sea.[8] In some places round the
bay, Mr. Kent observed that there were beds formed exclusively of the
_Mytilus Chiloensis_: this species now lives in parts never uncovered
by the tides. At considerable heights, Mr. Kent found only a few
shells; but from the summit of one hill, 625 feet high, he brought me
specimens of the Concholepas, _Mytilus Chiloensis_, and a Turbo. These
shells were softer and more brittle than those from the height of 164
feet; and these latter had obviously a much more ancient appearance
than the same species from the height of only twenty feet.
[8] “Zoology of Captain Beechey’s Voyage,” p. 162.
_Coast north of Concepcion._—The first point examined was at the mouth
of the Rapel (160 miles north of Concepcion and sixty miles south of
Valparaiso), where I observed a few shells at the height of 100 feet,
and some barnacles adhering to the rocks three or four feet above the
highest tides: M. Gay[9] found here recent shells at the distance
of two leagues from the shore. Inland there are some wide,
gravel-capped plains, intersected by many broad, flat-bottomed valleys
(now carrying insignificant streamlets), with their sides cut into
successive wall-like escarpments, rising one above another, and in many
places, according to M. Gay, worn into caves. The one cave (C. del
Obispo) which I examined, resembled those formed on many sea-coasts,
with its bottom filled with shingle. These inland plains, instead of
sloping towards the coast, are inclined in an opposite direction
towards the Cordillera, like the successively rising terraces on the
inland or eastern side of Chiloe: some points of granite, which project
through the plains near the coast, no doubt once formed a chain of
outlying islands, on the inland shores of which the plains were
accumulated. At Bucalemu, a few miles northward of the Rapel, I
observed at the foot, and on the summit-edge of a plain, ten miles from
the coast, many recent shells, mostly comminuted, but some perfect.
There were, also, many at the bottom of the great valley of the Maypu.
At San Antonio, shells are said to be collected and burnt for lime. At
the bottom of a great ravine (Quebrada Onda, on the road to Casa
Blanca), at the distance of several miles from the coast, I noticed a
considerable bed, composed exclusively of _Mesodesma donaciforme_,
Desh., lying on a bed of muddy sand: this shell now lives associated
together in great numbers, on tidal-flats on the coast of Chile.
[9] “Annales des Scienc. Nat.,” Avril 1833.
_Valparaiso._
During two successive years I carefully examined, part of the time in
company with Mr. Alison, into all the facts connected with the recent
elevation of this neighbourhood. In very many parts a beach of broken
shells, about fourteen or fifteen feet above high-water mark, may be
observed; and at this level the coast-rocks, where precipitous, are
corroded in a band. At one spot, Mr. Alison, by removing some birds’
dung, found at this same level barnacles adhering to the rocks. For
several miles southward of the bay, almost every flat little headland,
between the heights of 60 and 230 feet (measured by the barometer), is
smoothly coated by a thick mass of comminuted shells, of the same
species, and apparently in the same proportional numbers with those
existing in the adjoining sea. The Concholepas is much the most
abundant, and the best preserved shell; but I extracted perfectly
preserved specimens of the _Fissurella biradiata_, a Trochus and
Balanus (both well-known, but according to Mr. Sowerby yet unnamed) and
parts of the _Mytilus Chiloensis._ Most of these shells, as well as an
encrusting Nullipora, partially retain their colour; but they are
brittle, and often stained red from the underlying brecciated mass of
primary rocks; some are packed together, either in black or reddish
moulds; some lie loose on the bare rocky surfaces. The total number of
these shells is immense; they are less numerous, though still far from
rare, up a height of 1,000 feet above the sea. On the summit of a hill,
measured 557 feet, there was a small horizontal band of comminuted
shells, of which _many_ consisted (and likewise from lesser heights) of
very young and small
specimens of the still living Concholepas, Trochus, Patellæ, Crepidulæ,
and of _Mytilus Magellanicus_ (?):[10] several of these shells were
under a quarter of an inch in their greatest diameter. My attention was
called to this circumstance by a native fisherman, whom I took to look
at these shell-beds; and he ridiculed the notion of such small shells
having been brought up for food; nor could some of the species have
adhered when alive to other larger shells. On another hill, some miles
distant, and 648 feet high, I found shells of the Concholepas and
Trochus, perfect, though very old, with fragments of _Mytilus
Chiloensis_, all embedded in reddish-brown mould: I also found these
same species, with fragments of an Echinus and of _Balanus psittacus_,
on a hill 1,000 feet high. Above this height, shells became very rare,
though on a hill 1,300 feet high,[11] I collected the Concholepas,
Trochus, Fissurella, and a Patella. At these greater heights the shells
are almost invariably embedded in mould, and sometimes are exposed only
by tearing up bushes. These shells obviously had a very much more
ancient appearance than those from the lesser heights; the apices of
the Trochi were often worn down; the little holes made by burrowing
animals were greatly enlarged; and the Concholepas was often perforated
quite through, owing to the inner plates of shell having scaled off.
[10] Mr. Cuming informs me that he does not think this species
identical with, though closely resembling, the true _M. Magellanicus_
of the southern and eastern coast of South America; it lives
abundantly on the coast of Chile.
[11] Measured by the barometer: the highest point in the range behind
Valparaiso I found to be 1,626 feet above the level of the sea.
Many of these shells, as I have said, were packed in, and were quite
filled with, blackish or reddish-brown earth, resting on the granitic
detritus. I did not doubt until lately that this mould was of purely
terrestrial origin, when with a microscope examining some of it from
the inside of a Concholepas from the height of about one hundred feet,
I found that it was in considerable part composed of minute fragments
of the spines, mouth-bones, and shells of Echini, and of minute
fragments, of chiefly very young Patellæ, Mytili, and other species. I
found similar microscopical fragments in earth filling up the central
orifices of some large Fissurellæ. This earth when crushed emits a
sickly smell, precisely like that from garden-mould mixed with guano.
The earth accidentally preserved within the shells, from the greater
heights, has the same general appearance, but it is a little redder; it
emits the same smell when rubbed, but I was unable to detect with
certainty any marine remains in it. This earth resembles in general
appearance, as before remarked, that capping the rocks of Quiriquina in
the Bay of Concepcion, on which beds of sea-shells lay. I have, also,
shown that the black, peaty soil, in which the shells at the height of
350 feet at Chiloe were packed, contained many minute fragments of
marine animals. These facts appear to me interesting, as they show that
soils, which would naturally be considered of purely terrestrial
nature, may owe their origin in chief part to the sea.
Being well aware from what I have seen at Chiloe and in Tierra del
Fuego, that vast quantities of shells are carried, during successive
ages, far inland, where the inhabitants chiefly subsist on these
productions, I am bound to state that at greater heights than 557 feet,
where the number of very young and small shells proved that they had
not been carried up for food, the only evidence of the shells having
been naturally left by the sea, consists in their invariable and
uniform appearance of extreme antiquity—in the distance of some of the
places from the coast, in others being inaccessible from the nearest
part of the beach, and in the absence of fresh water for men to
drink—in the shells _not lying in heaps_,—and, lastly, in the close
similarity of the soil in which they are embedded, to that which lower
down can be unequivocally shown to be in great part formed from the
debris of the sea animals.[12]
[12] In the “Proceedings of the Geolog. Soc.,” vol. ii, p. 446, I have
given a brief account of the upraised shells on the coast of Chile,
and have there stated that the proofs of elevation are not
satisfactory above the height of 230 feet. I had at that time
unfortunately overlooked a separate page written during my second
visit to Valparaiso, describing the shells now in my possession from
the 557 feet hill; I had not then unpacked my collections, and had not
reconsidered the obvious appearance of greater antiquity of the shells
from the greater heights, nor had I at that time discovered the marine
origin of the earth in which many of the shells are packed.
Considering these facts, I do not now feel a shadow of doubt that the
shells, at the height of 1,300 feet, have been upraised by natural
causes into their present position.
With respect to the position in which the shells lie, I was repeatedly
struck here, at Concepcion, and at other places, with the frequency of
their occurrence on the summits and edges either of separate hills, or
of little flat headlands often terminating precipitously over the sea.
The several above-enumerated species of mollusca, which are found
strewed on the surface of the land from a few feet above the level of
the sea up to the height of 1,300 feet, all now live either on the
beach, or at only a few fathoms’ depth: Mr. Edmondston, in a letter to
Professor E. Forbes, states that in dredging in the Bay of Valparaiso,
he found the common species of Concholepas, Fissurella, Trochus,
Monoceros, Chitons, etc., living in abundance from the beach to a depth
of seven fathoms; and dead shells occurred only a few fathoms deeper.
The common _Turritella cingulata_ was dredged up living at even from
ten to fifteen fathoms; but this is a species which I did not find here
amongst the upraised shells. Considering this fact of the species being
all littoral or sub-littoral, considering their occurrence at various
heights, their vast numbers, and their generally comminuted state,
there can be little doubt that they were left on successive beach-lines
during a gradual elevation of the land. The presence, however, of so
many whole and perfectly preserved shells appears at first a difficulty
on this view, considering that the coast is exposed to the full force
of an open ocean: but we may suppose, either that these shells were
thrown during gales on flat ledges of rock just above the level of
high-water mark, and that during the elevation of the land they are
never again touched by the waves, or, that during earthquakes, such as
those of
1822, 1835, and 1837, rocky reefs covered with marine-animals were it
one blow uplifted above the future reach of the sea. This latter
explanation is, perhaps, the most probable one with respect to the beds
at Concepcion entirely composed of the _Mytilus Chiloensis_, a species
which lives below the lowest tides; and likewise with respect to the
great beds occurring both north and south of Valparaiso, of the
_Mesodesma donaciforme_,—a shell which, as I am informed by Mr. Cuming,
inhabits sandbanks at the level of the lowest tides. But even in the
case of shells having the habits of this Mytilus and Mesodesma, beds of
them, wherever the sea gently throws up sand or mud, and thus protects
its own accumulations, might be upraised by the slowest movement, and
yet remain undisturbed by the waves of each new beach-line.
It is worthy of remark, that nowhere near Valparaiso above the height
of twenty feet, or rarely of fifty feet, I saw any lines of erosion on
the solid rocks, or any beds of pebbles; this, I believe, may be
accounted for by the disintegrating tendency of most of the rocks in
this neighbourhood. Nor is the land here modelled into terraces: Mr.
Alison, however, informs me, that on both sides of one narrow ravine,
at the height of 300 feet above the sea, he found a succession of
rather indistinct step-formed beaches, composed of broken shells, which
together covered a space of about eighty feet vertical.
I can add nothing to the accounts already published of the elevation of
the land at Valparaiso,[13] which accompanied the earthquake of 1822:
but I heard it confidently asserted, that a sentinel on duty,
immediately after the shock, saw a part of a fort, which previously was
not within the line of his vision, and this would indicate that the
uplifting was not horizontal: it would even appear from some facts
collected by Mr. Alison, that only the eastern half of the bay was then
elevated. Through the kindness of this same gentleman, I am able to
give an interesting account of the changes of level, which have
supervened here within historical periods: about the year 1680 a long
sea-wall (or Prefil) was built, of which only a few fragments now
remain; up to the year 1817, the sea often broke over it, and washed
the houses on the opposite side of the road (where the prison now
stands); and even in 1819, Mr. J. Martin remembers walking at the foot
of this wall, and being often obliged to climb over it to escape the
waves. There now stands (1834) on the seaward side of this wall, and
between it and the beach, in one part a single row of houses, and in
another part two rows with a street between them. This great extension
of the beach in so short a time cannot be attributed simply to the
accumulation of detritus; for a resident engineer measured for me the
height between the lowest part of the wall visible, and the present
beach-line at spring-tides, and the difference was eleven feet six
inches. The church of S. Augustin is believed to have been built in
1614, and there is a tradition that the sea formerly flowed very near
it; by levelling, its foundations were found
to stand nineteen feet six inches above the highest beach-line; so that
we see in a period of 220 years, the elevation cannot have been as much
as nineteen feet six inches. From the facts given with respect to the
sea-wall, and from the testimony of the elder inhabitants, it appears
certain that the change in level began to be manifest about the year
1817. The only sudden elevation of which there is any record occurred
in 1822, and this seems to have been less than three feet. Since that
year, I was assured by several competent observers, that part of an old
wreck, which is firmly embedded near the beach, has sensibly emerged;
hence here, as at Chiloe, a slow rise of the land appears to be now in
progress. It seems highly probable that the rocks which are corroded in
a band at the height of fourteen feet above the sea were acted on
during the period, when by tradition the base of S. Augustin church,
now nineteen feet six inches above the highest water-mark, was
occasionally washed by the waves.
[13] Dr. Meyen (“Reise um Erde,” Th. I, s. 221) found in 1831 seaweed
and other bodies still adhering to some rocks which during the shock
of 1822 were lifted above the sea.
_Valparaiso to Coquimbo._—For the first seventy-five miles north of
Valparaiso I followed the coast-road, and throughout this space I
observed innumerable masses of upraised shells. About Quintero there
are immense accumulations (worked for lime) of the _Mesodesma
donaciforme_, packed in sandy earth; they abound chiefly about fifteen
feet above high-water, but shells are here found, according to Mr.
Miers,[14] to a height of 500 feet, and at a distance of three leagues
from the coast: I here noticed barnacles adhering to the rocks three or
four feet above the highest tides. In the neighbourhood of Plazilla and
Catapilco, at heights of between two hundred and three hundred feet,
the number of comminuted shells, with some perfect ones, especially of
the Mesodesma, packed in layers, was truly immense: the land at
Plazilla had evidently existed as a bay, with abrupt rocky masses
rising out of it, precisely like the islets in the broken bays now
indenting this coast. On both sides of the rivers Ligua, Longotomo,
Guachen, and Quilimari, there are plains of gravel about two hundred
feet in height, in many parts absolutely covered with shells. Close to
Conchalee, a gravel-plain is fronted by a lower and similar plain about
sixty feet in height, and this again is separated from the beach by a
wide tract of low land: the surfaces of all three plains or terraces
were strewed with vast numbers of the Concholepas, Mesodesma, an
existing Venus, and other still existing littoral shells. The two upper
terraces closely resemble in miniature the plains of Patagonia; and
like them are furrowed by dry, flat-bottomed, winding valleys.
Northward of this place I turned inward; and therefore found no more
shells: but the valleys of Chuapa, Illapel, and Limari, are bounded by
gravel-capped plains, often including a lower terrace within. These
plains send bay-like arms between and into the surrounding hills; and
they are continuously united with other extensive gravel-capped plains,
separating the coast mountain-ranges from the Cordillera.
[14] “Travels in Chile,” vol. i, pp. 395, 458. I received several
similar accounts from the inhabitants, and was assured that there are
many shells on the plain of Casa Blanca, between Valparaiso and
Santiago, at the height of 800 feet.
_Coquimbo._
A narrow fringe-like plain, gently inclined towards the sea, here
extends for eleven miles along the coast, with arms stretching up
between the coast-mountains, and likewise up the valley of Coquimbo: at
its southern extremity it is directly connected with the plain of
Limari, out of which hills abruptly rise like islets, and other hills
project like headlands on a coast. The surface of the fringe-like plain
appears level, but differs insensibly in height, and greatly in
composition, in different parts.
At the mouth of the valley of Coquimbo, the surface consists wholly of
gravel, and stands from 300 to 350 feet above the level of the sea,
being about one hundred feet higher than in other parts. In these other
and lower parts the superficial beds consist of calcareous matter, and
rest on ancient tertiary deposits hereafter to be described. The
uppermost calcareous layer is cream-coloured, compact,
smooth-fractured, sub-stalactiform, and contains some sand, earthy
matter, and recent shells. It lies on, and sends wedge-like veins
into,[15] a much more friable, calcareous, tuff-like variety; and both
rest on a mass about twenty feet in thickness, formed of fragments of
recent shells, with a few whole ones, and with small pebbles firmly
cemented together. This latter rock is called by the inhabitants
_losa_, and is used for building: in many parts it is divided into
strata, which dip at an angle of ten degrees seaward, and appear as if
they had originally been heaped in successive layers (as may be seen on
coral-reefs) on a steep beach. This stone is remarkable from being in
parts entirely formed of empty, pellucid capsules or cells of
calcareous matter, of the size of small seeds: a series of specimens
unequivocally showed that all these capsules once contained minute
rounded fragments of shells which have since been gradually dissolved
by water percolating through the mass.[16]
[15] In many respects this upper hard, and the underlying more
friable, varieties, resemble the great superficial beds at King
George’s Sound in Australia, which I have described in my “Geological
Observations on Volcanic Islands.” There could be little doubt that
the upper layers there have been hardened by the action of rain on the
friable, calcareous matter, and that the whole mass has originated in
the decay of minutely comminuted sea-shells and corals.
[16] I have incidentally described this rock in the above work on
Volcanic Islands.
The shells embedded in the calcareous beds forming the surface of this
fringe-like plain, at the height of from 200 to 250 feet above the sea,
consist of:—
Venus opaca.
Mulinia Byronensis.
Pecten purpuratus.
Mesodesma donaciforme.
Turritella cingulata.
Monoceros costatum.
Concholepas Peruviana.
Trochus (common Valparaiso species).
Calyptræa Byronensis.
Although these species are all recent, and are all found in the
neighbouring sea, yet I was particularly struck with the difference in
the
proportional numbers of the several species, and of those now cast up
on the present beach. I found only one specimen of the Concholepas, and
the Pecten was very rare, though both these shells are now the
commonest kinds, with the exception, perhaps, of the _Calyptræa
radians_, of which I did not find one in the calcareous beds. I will
not pretend to determine how far this difference in the proportional
numbers depends on the age of the deposit, and how far on the
difference in nature between the present sandy beaches and the
calcareous bottom, on which the embedded shells must have lived.
No. 8
Section of plain of Coquimbo.
[Illustration: Section of plain of Coquimbo.]
On the bare surface of the calcareous plain, or in a thin covering of
sand, there were lying, at a height from 200 to 252 feet, many recent
shells, which had a much fresher appearance than the embedded ones:
fragments of the Concholepas, and of the common Mytilus, still
retaining a tinge of its colour, were numerous, and altogether there
was manifestly a closer approach in proportional numbers to those now
lying on the beach. In a mass of stratified, slightly agglutinated
sand, which in some places covers up the lower half of the seaward
escarpment of the plain, the included shells appeared to be in exactly
the same proportional numbers with those on the beach. On one side of a
steep-sided ravine, cutting through the plain behind Herradura Bay, I
observed a narrow strip of stratified sand, containing similar shells
in similar proportional numbers; a section of the ravine is represented
in Diagram 8, which serves also to show the general composition of the
plain. I mention this case of the ravine chiefly because without the
evidence of the marine shells in the sand, any one would have supposed
that it had been hollowed out by simple alluvial action.
The escarpment of the fringe-like plain, which stretches for eleven
miles along the coast, is in some parts fronted by two or three narrow,
step-formed terraces, one of which at Herradura Bay expands into a
small plain. Its surface was there formed of gravel, cemented together
by calcareous matter; and out of it I extracted the following recent
shells, which are in a more perfect condition than those from the upper
plain:—
Calyptræa radians.
Turritella cingulata.
Oliva Peruviana.
Murex labiosus, var.
Nassa (identical with a living species).
Solen Dombeiana.
Pecten purpuratus.
Venus Chilensis.
Amphidesma rugulosum. The small irregular wrinkles of the posterior
part of this shell are rather stronger than in the recent specimens of
this species from Coquimbo. (G. B. Sowerby.)
Balanus (identical with living species).
On the syenitic ridge, which forms the southern boundary of Herradura
Bay and Plain, I found the Concholepas and _Turritella cingulata_
(mostly in fragments), at the height of 242 feet above the sea. I could
not have told that these shells had not formerly been brought up by
man, if I had not found one very small mass of them cemented together
in a friable calcareous tuff. I mention this fact more particularly,
because I carefully looked, in many apparently favourable spots, at
lesser heights on the side of this ridge, and could not find even the
smallest fragment of a shell. This is only one instance out of many,
proving that the absence of sea-shells on the surface, though in many
respects inexplicable, is an argument of very little weight in
opposition to other evidence on the recent elevation of the land. The
highest point in this neighbourhood at which I found upraised shells of
existing species was on an inland calcareous plain, at the height of
252 feet above the sea.
It would appear from Mr. Caldcleugh’s researches,[17] that a rise has
taken place here within the last century and a half; and as no sudden
change of level has been observed during the not very severe
earthquakes, which have occasionally occurred here, the rising has
probably been slow, like that now, or quite lately, in progress at
Chiloe and at Valparaiso: there are three well-known rocks, called the
Pelicans, which in 1710, according to Feuillèe, were _à fleur d’eau_,
but now are said to stand twelve feet above low-water mark: the
spring-tides rise here only five feet. There is another rock, now nine
feet above high-water mark, which in the time of Frezier and Feuillèe
rose only five or six feet out of water. Mr. Caldcleugh, I may add,
also shows (and I received similar accounts) that there has been a
considerable decrease in the soundings during the last twelve years in
the Bays of Coquimbo, Concepcion, Valparaiso, and Guasco; but as in
these cases it is nearly impossible to distinguish between the
accumulation of sediment and the upheavement of the bottom, I have not
entered into any details.
[17] “Proceedings of the Geological Society,” vol. ii, p. 446.
_Valley of Coquimbo._—The narrow coast-plain sends, as before stated,
an arm, or more correctly a fringe, on both sides, but chiefly on the
southern side, several miles up the valley. These fringes are worn into
steps or terraces, which present a most remarkable appearance, and have
been compared (though not very correctly) by Captain Basil
Hall, to the parallel roads of Glen Roy in Scotland: their origin has
been ably discussed by Mr. Lyell.[18] The first section which I will
give (Figure 9), is not drawn across the valley, but in an east and
west line at its mouth, where the step-formed terraces debouch and
present their very gently inclined surfaces towards the Pacific.
[18] “Principles of Geology” (1st edit.), vol. iii, p. 131.
No. 9
East and west section through the terraces at Coquimbo, where they
debouch from the valley, and front the sea.
[Illustration: East and west section through terraces at Coquimbo.]
The bottom plain (A) is about a mile in width, and rises quite
insensibly from the beach to a height of twenty-five feet at the foot
of the next plain; it is sandy, and abundantly strewed with shells.
Plain or terrace B is of small extent, and is almost concealed by the
houses of the town, as is likewise the escarpment of terrace C. On both
sides of a ravine, two miles south of the town, there are two little
terraces, one above the other, evidently corresponding with B and C;
and on them marine remains of the species already enumerated were
plentiful. Terrace E is very narrow, but quite distinct and level; a
little southward of the town there were traces of a terrace D
intermediate between E and C. Terrace F is part of the fringe-like
plain, which stretches for the eleven miles along the coast; it is here
composed of shingle, and is 100 feet higher than where composed of
calcareous matter. This greater height is obviously due to the quantity
of shingle, which at some former period has been brought down the great
valley of Coquimbo.
Considering the many shells strewed over the terraces A, B, and C, and
a few miles southward on the calcareous plain, which is continuously
united with the upper step-like plain F, there cannot, I apprehend, be
any doubt, that these six terraces have been formed by the action of
the sea; and that their five escarpments mark so many periods of
comparative rest in the elevatory movement, during which the sea wore
into the land. The elevation between these periods may have been sudden
and on _an average_ not more than seventy-two feet each time, or it may
have been gradual and insensibly slow. From the shells on the three
lower terraces, and on the upper one, and I may add on the three
gravel-capped terraces at Conchalee, being all littoral and
sub-littoral species, and from the analogical facts given at
Valparaiso, and lastly from the evidence of a slow rising lately or
still in progress here, it appears to me far more probable that the
movement has been slow. The existence of these successive escarpments,
or old cliff-lines, is in another respect highly instructive, for they
show periods of comparative rest in the elevatory movement, and of
denudation, which would never even have been suspected from a close
examination of many miles of coast southward of Coquimbo.
We come now to the terraces on the opposite sides of the east and west
valley of Coquimbo: the section in figure No. 10 is taken in a north
and south line across the valley at a point about three miles from the
sea. The valley measured from the edges of the escarpments of the upper
plain FF is about a mile in width; but from the bases of the bounding
mountains it is from three to four miles wide. The terraces marked with
an interrogative do not exist on that side of the valley, but are
introduced merely to render the diagram more intelligible.
No. 10
North and south section across the valley of Coquimbo.
[Illustration: North and south section across the valley of Coquimbo.]
Terraces marked with ? do not occur on that side of the valley, and are
introduced only to make the diagram more intelligible. A river and
bottom-plain of valley C, E, and F, on the south side of valley, are
respectively, 197, 377, and 420 feet above the level of the sea.
AA. The bottom of the valley, believed to be 100 feet above the sea:
it is continuously united with the lowest plain A of figure No. 9.
B. This terrace higher up the valley expands considerably; seaward it
is soon lost, its escarpment being united with that of C: it is not
developed at all on the south side of the valley.
C. This terrace, like the last, is considerably expanded higher up the
valley. These two terraces apparently correspond with B and C of
figure No. 9.
D is not well developed in the line of this section; but seaward it
expands into a plain: it is not present on the south side of the
valley; but it is met with, as stated under the former section, a
little south of the town.
E is well developed on the south side, but absent on the north side of
the valley: though not continuously united with E of figure No. 9, it
apparently corresponds with it.
F. This is the surface-plain, and is continuously united with that
which stretches like a fringe along the coast. In ascending the valley
it gradually becomes narrower, and is at last, at the distance of
about ten miles from the sea, reduced to a row of flat-topped patches
on the sides of the mountains. None of the lower terraces extend so
far up the valley.
These five terraces are formed of shingle and sand; three of them, as
marked by Captain B. Hall (namely, B, C, and F), are much more
conspicuous than the others. From the marine remains copiously strewed
at the mouth of the valley on the lower terraces, and southward of the
town on the upper one, they are, as before remarked, undoubtedly of
marine origin; but within the valley, and this fact well deserves
notice, at a distance of from only a mile and a half to three or four
miles from the sea, I could not find even a fragment of a shell.
_On the inclination of the terraces of Coquimbo, and on the upper and
basal edges of their escarpments not being horizontal._—The surfaces of
these terraces slope in a slight degree, as shown by the two last
sections taken conjointly, both towards the centre of the valley, and
seawards towards its mouth. This double or diagonal inclination, which
is not the same in the several terraces, is, as we shall immediately
see, of simple explanation. There are, however, some other points which
at first appear by no means obvious,—namely, first, that each terrace,
taken in its whole breadth from the summit-edge of one escarpment to
the base of that above it, and followed up the valley, is not
horizontal; nor have the several terraces, when followed up the valley,
all the same inclination; thus I found the terraces C, E, and F,
measured at a point about two miles from the mouth of the valley, stood
severally between fifty-six to seventy-seven feet higher than at the
mouth. Again, if we look to any one line of cliff or escarpment,
neither its summit-edge nor its base is horizontal. On the theory of
the terraces having been formed during a slow and equable rise of the
land, with as many intervals of rest as there are escarpments, it
appears at first very surprising that horizontal lines of some kind
should not have been left on the land.
The direction of the diagonal inclination in the different terraces
being different,—in some being directed more towards the middle of the
valley, in others more towards its mouth,—naturally follows on the view
of each terrace, being an accumulation of successive beach-lines round
bays, which must have been of different forms and sizes when the land
stood at different levels: for if we look to the actual beach of a
narrow creek, its slope is directed towards the middle; whereas, in an
open bay, or slight concavity on a coast, the slope is towards the
mouth, that is, almost directly seaward; hence as a bay alters in form
and size, so will the direction of the inclination of its successive
beaches become changed.
[Illustration: A bay in the district which has begun slowly rising.]
If it were possible to trace any one of the many beach-lines, composing
each sloping terrace, it would of course be horizontal; but the only
lines of demarcation are the summit and basal edges of the escarpments.
Now the summit-edge of one of these escarpments marks the furthest line
or point to which the sea has cut into a mass of gravel sloping
seaward; and as the sea will generally have greater power at the mouth
than at the protected head of the bay, so will the escarpment at the
mouth be cut deeper into the land, and its summit-edge be higher;
consequently it will not be horizontal. With respect to the basal or
lower edges of the escarpments, from picturing in one’s mind ancient
bays _ entirely_ surrounded at successive periods by cliff-formed
shores, one’s first impression is that they at least necessarily must
be horizontal, if the elevation has been horizontal. But here is a
fallacy: for after the sea has, during a cessation of the elevation,
worn cliffs all round the shores of a bay, when the movement
recommences, and especially if it recommences slowly, it might well
happen that, at the exposed mouth of the bay, the waves might continue
for some time wearing into the land, whilst in the protected and upper
parts
successive beach-lines might be accumulating in a sloping surface or
terrace at the foot of the cliffs which had been lately reached: hence,
supposing the whole line of escarpment to be finally uplifted above the
reach of the sea, its basal line or foot near the mouth will run at a
lower level than in the upper and protected parts of the bay;
consequently this basal line will not be horizontal. And it has already
been shown that the summit-edges of each escarpment will generally be
higher near the mouth (from the seaward sloping land being there most
exposed and cut into) than near the head of the bay; therefore the
total height of the escarpments will be greatest near the mouth; and
further up the old bay or valley they will on both sides generally thin
out and die away: I have observed this thinning out of the successive
escarpment at other places besides Coquimbo; and for a long time I was
quite unable to understand its meaning. The rude diagram in Figure 11
will perhaps render what I mean more intelligible; it represents a bay
in a district which has begun slowly rising. Before the movement
commenced, it is supposed that the waves had been enabled to eat into
the land and form cliffs, as far up, but with gradually diminishing
power, as the points AA: after the movement had commenced and gone on
for a little time, the sea is supposed still to have retained the
power, at the exposed mouth of the bay, of cutting down and into the
land as it slowly emerged; but in the upper parts of the bay it is
supposed soon to have lost this power, owing to the more protected
situation and to the quantity of detritus brought down by the river;
consequently low land was there accumulated. As this low land was
formed during a slow elevatory movement, its surface will gently slope
upwards from the beach on all sides. Now, let us imagine the bay, not
to make the diagram more complicated, suddenly converted into a valley:
the basal line of the cliffs will of course be horizontal, as far as
the beach is now seen extending in the diagram; but in the upper part
of the valley, this line will be higher, the level of the district
having been raised whilst the low land was accumulating at the foot of
the inland cliffs. If, instead of the bay in the diagram being suddenly
converted into a valley, we suppose with much more probability it to be
upraised slowly, then the waves in the upper parts of the bay will
continue very gradually to fail to reach the cliffs, which are now in
the diagram represented as washed by the sea, and which, consequently,
will be left standing higher and higher above its level; whilst at the
still exposed mouth, it might well happen that the waves might be
enabled to cut deeper and deeper, both down and into the cliffs, as the
land slowly rose.
The greater or lesser destroying power of the waves at the mouths of
successive bays, comparatively with this same power in their upper and
protected parts, will vary as the bays become changed in form and size,
and therefore at different levels, at their mouths and heads, more or
less of the surfaces between the escarpments (that is, the accumulated
beach-lines or terraces) will be left undestroyed: from what has gone
before we can see that, according as the elevatory movements after each
cessation recommence more or less slowly, according to the amount of
detritus delivered by the river at the heads of the successive bays,
and according to the degree of protection afforded by their altered
forms, so will a greater or less extent of terrace be accumulated in
the upper part, to which there will be no surface at a corresponding
level at the mouth: hence we can perceive why no one terrace, taken in
its whole breadth and followed up the valley, is horizontal, though
each separate beach-line must have been so; and why the inclination of
the several terraces, both transversely, and longitudinally up the
valley, is not alike.
I have entered into this case in some detail, for I was long perplexed
(and others have felt the same difficulty) in understanding how, on the
idea of an equable elevation with the sea at intervals eating into the
land, it came that neither the terraces nor the upper nor lower edges
of the escarpments were horizontal. Along lines of coast, even of great
lengths, such as that of Patagonia, if they are nearly uniformly
exposed, the corroding power of the waves will be checked and conquered
by the elevatory movement, as often as it recommences, at about the
same period; and hence the terraces, or accumulated beach-lines, will
commence being formed at nearly the same levels: at each succeeding
period of rest, they will, also, be eaten into at nearly the same rate,
and consequently there will be a much closer coincidence in their
levels and inclinations, than in the terraces and escarpments formed
round bays with their different parts very differently exposed to the
action of the sea. It is only where the waves are enabled, after a long
lapse of time, slowly to corrode hard rocks, or to throw up, owing to
the supply of sediment being small and to the surface being steeply
inclined, a narrow beach or mound, that we can expect, as at Glen Roy
in Scotland,[19] a distinct line marking an old sea-level, and which
will be strictly horizontal, if the subsequent elevatory movements have
been so: for in these cases no discernible effects will be produced,
except during the long intervening periods of rest; whereas in the case
of step-formed coasts, such as those described in this and the
preceding chapter, the terraces themselves are accumulated during the
slow elevatory process, the accumulation commencing sooner in protected
than in exposed situations, and sooner where there is copious supply of
detritus than where there is little; on the other hand, the steps or
escarpments are formed during the stationary periods, and are more
deeply cut down and into the coast-land in exposed than in protected
situations;—the cutting action, moreover, being prolonged in the most
exposed parts, both during the beginning and ending, if slow, of the
upward movement.
[19] “Philosophical Transactions,” 1839, p. 39.
Although in the foregoing discussion I have assumed the elevation to
have been horizontal, it may be suspected, from the considerable
seaward slope of the terraces, both up the valley of S. Cruz and up
that of Coquimbo, that the rising has been greater inland than nearer
the coast. There is reason to believe,[20] from the effects produced on
the water-course of a mill during the earthquake of 1822 in Chile, that
the upheaval one mile inland was nearly double, namely, between five
and seven feet, to what it was on the Pacific. We know, also, from the
admirable researches of M. Bravais,[21] that in Scandinavia the ancient
sea-beaches gently slope from the interior mountain-ranges towards the
coast, and that they are not parallel one to the other showing that the
proportional difference in the amount of elevation on the coast and in
the interior, varied at different periods.
[20] Mr. Place in the _Quarterly Journal of Science,_ 1824, vol. xvii,
p. 42.
[21] “Voyages de la Comm. du Nord,” etc., also “Comptes Rendus,” Oct.
1842.
_Coquimbo to Guasco._—In this distance of ninety miles, I found in
almost every part marine shells up to a height of apparently from two
hundred to three hundred feet. The desert plain near Choros is thus
covered; it is bounded by the escarpment of a higher plain, consisting
of pale-coloured, earthy, calcareous stone, like that of Coquimbo, with
the same recent shells embedded in it. In the valley of Chaneral, a
similar bed occurs in which, differently from that of Coquimbo, I
observed many shells of the Concholepas: near Guasco the same
calcareous bed is likewise met with.
In the valley of Guasco, the step-formed terraces of gravel are
displaced in a more striking manner than at any other point. I followed
the valley for thirty-seven miles (as reckoned by the inhabitants) from
the coast to Ballenar; in nearly the whole of this distance, five grand
terraces, running at corresponding heights on both sides of the broad
valley, are more conspicuous than the three best-developed ones at
Coquimbo. They give to the landscape the most singular and formal
aspect; and when the clouds hung low, hiding the neighbouring
mountains, the valley resembled in the most striking manner that of
Santa Cruz. The whole thickness of these terraces or plains seems
composed of gravel, rather firmly aggregated together, with occasional
parting seams of clay: the pebbles on the upper plain are often
whitewashed with an aluminous substance, as in Patagonia. Near the
coast I observed many sea-shells on the lower plains. At Freyrina
(twelve miles up the valley), there are six terraces beside the
bottom-surface of the valley: the two lower ones are here only from two
hundred to three hundred yards in width, but higher up the valley they
expand into plains; the third terrace is generally narrow; the fourth I
saw only in one place, but there it was distinct for the length of a
mile; the fifth is very broad; the sixth is the summit-plain, which
expands inland into a great basin. Not having a barometer with me, I
did not ascertain the height of these plains, but they appeared
considerably higher than those at Coquimbo. Their width varies much,
sometimes being very broad, and sometimes contracting into mere fringes
of separate flat-topped projections, and then quite disappearing: at
the one spot, where the fourth terrace was visible, the whole six
terraces were cut off for a short space by one single bold escarpment.
Near Ballenar (thirty-seven miles from the mouth of the river), the
valley between the summit-edges of the highest escarpments is several
miles in width, and the five terraces on both sides are broadly
developed: the highest cannot be less than six hundred feet above the
bed of the river, which itself must, I conceive, be some hundred feet
above the sea.
No. 12
North and south section across the valley of Guasco, and of a plain
north of it.
[Illustration: North and south section across the valley of Guasco.]
On the northern side of the valley the summit-plain of gravel (A) has
two escarpments, one facing the valley, and the other a great
basin-like plain (B), which stretches for several leagues northward.
This narrow plain (A) with the double escarpment, evidently once formed
a spit or promontory of gravel, projecting into and dividing two great
bays, and subsequently was worn on both sides into steep cliffs.
Whether the several escarpments in this valley were formed during the
same stationary periods with those of Coquimbo, I will not pretend to
conjecture; but if so the intervening and subsequent elevatory
movements must have been here much more energetic, for these plains
certainly stand at a much higher level than do those of Coquimbo.
_Copiapo._—From Guasco to Copiapo, I followed the road near the foot of
the Cordillera, and therefore saw no upraised remains. At the mouth,
however, of the valley of Copiapo there is a plain, estimated by
Meyen[22] between fifty and seventy feet in height, of which the upper
part consists chiefly of gravel, abounding with recent shells, chiefly
of the Concholepas, _Venus Dombeyi_, and _Calyptræa trochiformis._ A
little
inland, on a plain estimated by myself at nearly three hundred feet,
the upper stratum was formed of broken shells and sand cemented by
white calcareous matter, and abounding with embedded recent shells, of
which the _Mulinia Byronensis_ and _Pecten purpuratus_ were the most
numerous. The lower plain stretches for some miles southward, and for
an unknown distance northward, but not far up the valley; its seaward
face, according to Meyen, is worn into caves above the level of the
present beach. The valley of Copiapo is much less steeply inclined and
less direct in its course than any other valley which I saw in Chile;
and its bottom does not generally consist of gravel: there are no
step-formed terraces in it, except at one spot near the mouth of the
great lateral valley of the Despoblado where there are only two, one
above the other: lower down the valley, in one place I observed that
the solid rock had been cut into the shape of a beach, and was smoothed
over with shingle.
[22] “Reise um die Erde,” Th. I, s. 372, _et seq._
Northward of Copiapo, in lat. 26° S., the old voyager Wafer[23] found
immense numbers of sea-shells some miles from the coast. At Cobija
(lat. 22° 34′) M. d’Orbigny observed beds of gravel and broken shells,
containing ten species of recent shells; he also found, on projecting
points of porphyry, at a height of 300 feet, shells of Concholepas,
Chiton, Calyptræa, Fissurella, and Patella, still attached to the spots
on which they had lived. M. d’Orbigny argues from this fact, that the
elevation must have been great and sudden:[24] to me it appears far
more probable that the movement was gradual, with small starts as
during the earthquakes of 1822 and 1835, by which whole beds of shells
attached to the rocks were lifted above the subsequent reach of the
waves. M. d’Orbigny also found rolled pebbles extending up the mountain
to a height of at least six hundred feet. At Iquique (lat. 20° 12′ S.),
in a great accumulation of sand, at a height estimated between one
hundred and fifty and two hundred feet, I observed many large
sea-shells which I thought could not have been blown up by the wind to
that height. Mr. J. H. Blake has lately[25] described these
shells: he states that “inland toward the mountains they form a compact
uniform bed, scarcely a trace of the original shells being discernible;
but as we approach the shore, the forms become gradually more distinct
till we meet with the living shells on the coast.” This interesting
observation, showing by the gradual decay of the shells how slowly and
gradually the coast must have been uplifted, we shall presently see
fully confirmed at Lima. At Arica (lat. 18° 28′), M. d’Orbigny[26]
found a great range of sand-dunes, fourteen leagues in length,
stretching towards Tacna, including recent shells and bones of Cetacea,
and reaching up to a height of 300 feet above the sea. Lieutenant
Freyer has given some more precise facts: he states[27] that the Morro
of Arica is about four hundred feet high; it is worn into obscure
terraces, on the bare rock of which he found Balini and Milleporæ
adhering. At the height of between twenty and thirty feet the shells
and corals were in a quite fresh state, but at fifty feet they were
much abraded; there were, however, traces of organic remains at greater
heights. On the road from Tacna to Arequipa, between Loquimbo and
Moquegua, Mr. M. Hamilton[28] found numerous recent sea shells in sand,
at a considerable distance from the sea.
[23] Burnett’s “Collection of Voyages,” vol. iv, p. 193.
[24] “Voyage, Part Géolog.,” p. 94. M. d’Orbigny (p. 98), in summing
up, says: “S’il est certain (as he believes) que tous les terrains en
pente, compris entre la mer et les montagnes sont l’ancien rivage de
la mer, on doit supposer, pour l’ensemble, un exhaussement que ce ne
serait pas moindre de deux cent mètres; il faudrait supposer encore
que ce soulèvement n’a point été graduel; . . . mais qu’il résulterait
d’une seule et même cause fortuite,” etc. Now, on this view, when the
sea was forming the beach at the foot of the mountains, many shells of
Concholepas, Chiton, Calyptræa, Fissurella, and Patella (which are
known to live close to the beach), were attached to rocks at a depth
of 300 feet, and at a depth of 600 feet several of these same shells
were accumulating in great numbers in horizontal beds. From what I
have myself seen in dredging, I believe this to be improbable in the
highest degree, if not impossible; and I think everyone who has read
Professor E. Forbes’s excellent researches on the subject, will
without hesitation agree in this conclusion.
[25] _Silliman’s Amer. Journ. of Science,_ vol. xliv, p. 2.
[26] “Voyage,” etc., p. 101.
[27] In a letter to Mr. Lyell, “Geolog. Proc.,” vol. ii, p. 179.
[28] _Edin. New Phil. Journ.,_ vol. xxx, p. 155.
_Lima._
Northward of Arica, I know nothing of the coast for about a space of
five degrees of latitude; but near Callao, the port of Lima, there is
abundant and very curious evidence of the elevation of the land. The
island of San Lorenzo is upwards of one thousand feet high; the basset
edges of the strata composing the lower part are worn into three
obscure, narrow, sloping steps or ledges, which can be seen only when
standing on them: they probably resemble those described by Lieutenant
Freyer at Arica. The surface of the lower ledge, which extends from a
low cliff overhanging the sea to the foot of the next upper escarpment,
is covered by an enormous accumulation of recent shells.[29] The bed is
level, and in some parts more than two feet in thickness; I traced it
over a space of one mile in length, and heard of it in other places:
the uppermost part is eighty-five feet by the barometer above
high-water mark. The shells are packed together, but not stratified:
they are mingled with earth and stones, and are generally covered by a
few inches of detritus; they rest on a mass of nearly angular fragments
of the underlying sandstone, sometimes cemented together by common
salt. I collected eighteen species of shells of all ages and sizes.
Several of the univalves had evidently long lain dead at the bottom of
the sea, for their _ insides_ were incrusted with Balani and Serpulæ.
All, according to Mr. G.B. Sowerby, are recent species: they consist
of:—
Mytilus Magellanicus: same as that found at Valparaiso, and there
stated to be probably distinct from the true _M. Magellanicus_ of the
east coast.
Venus costellata, Sowerby “Zoological Proceedings.”
Pecten purpuratus, Lam.
Chama, probably echinulata, Brod.
Calyptræa Byronensis, Gray.
Calyptræa radians (Trochus, Lam.)
Fissurella affinis, Gray.
Fissurella biradiata, Trembly.
Purpura chocolatta, Duclos.
Purpura Peruviana, Gray.
Purpura labiata, Gray.
Purpura buxea (Murex, Brod.).
Concholepas Peruviana.
Nassa, related to reticulata.
Triton rudis, Brod.
Trochus, not yet described, but well-known and very common.
and 18. Balanus, two species, both common on the coast.
[29] M. Chevalier, in the “Voyage of the _Bonite_,” observed these
shells; but his specimens were lost.—“L’Institut,” 1838, p. 151.
These upraised shells appear to be nearly in the same proportional
numbers—with the exception of the Crepidulæ being more numerous—with
those on the existing beach. The state of preservation of the different
species differed much; but most of them were much corroded, brittle,
and bleached: the upper and lower surfaces of the Concholepas had
generally quite scaled off: some of the Trochi and Fissurellæ still
partially retain their colours. It is remarkable that these shells,
taken all together, have fully as ancient an appearance, although the
extremely arid climate appears highly favourable for their
preservation, as those from 1,300 feet at Valparaiso, and certainly a
more ancient appearance than those from five to six hundred feet from
Valparaiso and Concepcion; at which places I have seen grass and other
vegetables actually growing out of the shells. Many of the univalves
here at San Lorenzo were filled with, and united together by, pure
salt, probably left by the evaporation of the sea-spray, as the land
slowly emerged.[30] On the highest parts of the ledge, small fragments
of the shells were mingled with, and evidently in process of reduction
into, a yellowish-white, soft, calcareous powder, tasting strongly of
salt, and in some places as fine as prepared medicinal chalk.
[30] The underlying sandstone contains true layers of salt; so that
the salt may possibly have come from the beds in the higher parts of
the island; but I think more probably from the sea-spray. It is
generally asserted that rain never falls on the coast of Peru; but
this is not quite accurate; for, on several days, during our visit,
the so-called Peruvian dew fell in sufficient quantity to make the
streets muddy, and it would certainly have washed so deliquescent a
substance as salt into the soil. I state this because M. d’Orbigny, in
discussing an analogous subject, supposes that I had forgotten that it
never rains on this whole line of coast. See Ulloa’s “Voyage” (vol.
ii, Eng. Trans., p. 67) for an account of the muddy streets of Lima,
and on the continuance of the mists during the whole winter. Rain,
also, falls at rare intervals even in the driest districts, as, for
instance, during forty days, in 1726, at Chocope (7° 46′); this rain
entirely ruined (“Ulloa,” etc., p. 18) the mud houses of the
inhabitants.
_Fossil-remains of human art._—In the midst of these shells on San
Lorenzo, I found light corallines, the horny ovule-cases of Mollusca,
roots of seaweed,[31] bones of birds, the heads of Indian corn and
other vegetable matter, a piece of woven rushes, and another of nearly
decayed _cotton_ string. I extracted these remains by digging a hole,
on a level spot; and they had all indisputably been embedded with the
shells. I compared the plaited rush, the _cotton_ string, and Indian
corn, at the house of an antiquary, with similar objects, taken from
the Huacas or burial-grounds of the ancient Peruvians, and they were
undistinguishable; it should be observed that the Peruvians used string
only of cotton. The small quantity of sand or gravel with the shells,
the absence of large stones, the width and thickness of the bed, and
the time requisite for a ledge to be cut into the sandstone, all show
that these remains were not thrown high up by an earthquake-wave: on
the other hand, these facts, together with the number of dead shells,
and of floating objects, both marine and terrestrial, both natural and
human, render it almost certain that they were accumulated on a true
beach, since upraised eighty-five feet, and upraised this much since
_Indian man inhabited Peru._ The elevation may have been, either by
several small sudden starts, or quite gradual; in this latter case the
unrolled shells having been thrown up during gales beyond the reach of
the waves which afterwards broke on the slowly emerging land. I have
made these remarks, chiefly because I was at first surprised at the
complete difference in nature, between this broad, smooth, upraised bed
of shells, and the present shingle-beach at the foot of the low
sandstone-cliffs; but a beach formed, when the sea is cutting into the
land, as is shown now to be the case by the low bare sandstone-cliffs,
ought not to be compared with a beach accumulated on a gently inclined
rocky surface, at a period when the sea (probably owing to the
elevatory movement in process) was not able to eat into the land. With
respect to the mass of nearly angular, salt-cemented fragments of
sandstone, which lie under the shells, and which are so unlike the
materials of an ordinary sea-beach; I think it probable after having
seen the remarkable effects[32] of the earthquake of 1835, in
absolutely shattering as if by gunpowder the _surface_ of the primary
rocks near Concepcion, that a smooth bare surface of stone was left by
the sea covered by the shelly mass, and that afterwards when upraised,
it was superficially shattered by the severe shocks so often
experienced here.
[31] Mr. Smith of Jordan Hill found pieces of seaweed in an upraised
pleistocene deposit in Scotland. See his admirable Paper in the _Edin.
New Phil. Journal,_ vol. xxv, p. 384.
[32] I have described this in my “Journal of Researches,” p. 303, 2nd
edit.
The very low land surrounding the town of Callao, is to the south
joined by an obscure escarpment to a higher plain (south of Bella
Vista), which stretches along the coast for a length of about eight
miles. This plain appears to the eye quite level; but the sea-cliffs
show that its height varies (as far as I could estimate) from seventy
to one hundred and twenty feet. It is composed of thin, sometimes
waving, beds of clay, often of bright red and yellow colours, of layers
of impure sand, and in one part with a great stratified mass of
granitic pebbles. These
beds are capped by a remarkable mass, varying from two to six feet in
thickness, of reddish loam or mud, containing many scattered and broken
fragments of recent marine shells, sometimes though rarely single large
round pebble, more frequently short irregular layers of fine gravel,
and very many pieces of red coarse earthenware, which from their
curvatures must once have formed parts of large vessels. The
earthenware is of Indian manufacture; and I found exactly similar
pieces accidentally included within the bricks, of which the
neighbouring ancient Peruvian burial-mounds are built. These fragments
abounded in such numbers in certain spots, that it appeared as if
waggon-loads of earthenware had been smashed to pieces. The broken
sea-shells and pottery are strewed both on the surface, and throughout
the whole thickness of this upper loamy mass. I found them wherever I
examined the cliffs, for a space of between two and three miles, and
for half a mile inland; and there can be little doubt that this same
bed extends with a smooth surface several miles further over the entire
plain. Besides the little included irregular layers of small pebbles,
there are occasionally very obscure traces of stratification.
At one of the highest parts of the cliff, estimated 120 feet above the
sea, where a little ravine came down, there were two sections, at right
angles to each other, of the floor of a shed or building. In both
sections or faces, two rows, one over the other, of large round stones
could be distinctly seen; they were packed close together on an
artificial layer of sand two inches thick, which had been placed on the
natural clay-beds; the round stones were covered by three feet in
thickness of the loam with broken sea-shells and pottery. Hence, before
this widely spread-out bed of loam was deposited, it is certain that
the plain was inhabited; and it is probable, from the broken vessels
being so much more abundant in certain spots than in others, and from
the underlying clay being fitted for their manufacture, that the kilns
stood here.
The smoothness and wide extent of the plain, the bulk of matter
deposited, and the obscure traces of stratification seem to indicate
that the loam was deposited under water; on the other hand, the
presence of sea-shells, their broken state, the pebbles of various
sizes, and the artificial floor of round stones, almost prove that it
must have originated in a rush of water from the sea over the land. The
height of the plain, namely, 120 feet, renders it improbable that an
earthquake-wave, vast as some have here been, could have broken over
the surface at its present level; but when the land stood eighty-five
feet lower, at the period when the shells were thrown up on the ledge
at S. Lorenzo, and when as we know man inhabited this district, such an
event might well have occurred; and if we may further suppose, that the
plain was at that time converted into a temporary lake, as actually
occurred, during the earthquakes of 1713 and 1746, in the case of the
low land round Callao owing to its being encircled by a high
shingle-beach, all the appearances above described will be perfectly
explained. I must add, that at a lower level near the point where the
present low land round Callao joins the higher plain, there are
appearances of two
distinct deposits both apparently formed by debacles: in the upper one,
a horse’s tooth and a dog’s jaw were embedded; so that both must have
been formed after the settlement of the Spaniards: according to Acosta,
the earthquake-wave of 1586 rose eighty-four feet.
The inhabitants of Callao do not believe, as far as I could ascertain,
that any change in level is now in progress. The great fragments of
brickwork, which it is asserted can be seen at the bottom of the sea,
and which have been adduced as a proof of a late subsidence, are, as I
am informed by Mr. Gill, a resident engineer, loose fragments; this is
probable, for I found on the beach, and not near the remains of any
building, masses of brickwork, three and four feet square, which had
been washed into their present places, and smoothed over with shingle
during the earthquake of 1746. The spit of land, on which the ruins of
_Old_ Callao stand, is so extremely low and narrow, that it is
improbable in the highest degree that a town should have been founded
on it in its present state; and I have lately heard[33] that M. Tschudi
has come to the conclusion, from a comparison of old with modern
charts, that the coast both south and north of Callao has subsided. I
have shown that the island of San Lorenzo has been upraised eighty-five
feet since the Peruvians inhabited this country; and whatever may have
been the amount of recent subsidence, by so much more must the
elevation have exceeded the eighty-five feet. In several places[34] in
this neighbourhood, marks of sea-action have been observed: Ulloa gives
a detailed account of such appearances at a point five leagues
northward of Callao: Mr. Cruikshank found near Lima successive lines of
sea-cliffs, with rounded blocks at their bases, at a height of 700 feet
above the present level of the sea.
[33] I am indebted for this fact to Dr. E. Dieffenbach. I may add that
there is a tradition, that the islands of San Lorenzo and Fronton were
once joined, and that the channel between San Lorenzo and the
mainland, now above two miles in width, was so narrow that cattle used
to swim over.
[34] “Observaciones sobre el Clima del Lima” par Dr. H. Unanùe, p.
4.—Ulloa’s “Voyage,” vol. ii, Eng. Trans., p. 97.—For Mr. Cruikshank’s
observations, see Mr. Lyell’s “Principles of Geology” (1st edition)
vol. iii, p. 130.
_On the decay of upraised sea-shells._—I have stated that many of the
shells on the lower inclined ledge or terrace of San Lorenzo are
corroded in a peculiar manner, and that they have a much more ancient
appearance than the same species at considerably greater heights on the
coast of Chile. I have, also, stated that these shells in the upper
part of the ledge, at the height of eighty-five feet above the sea, are
falling, and in some parts are quite changed into a fine, soft, saline,
calcareous powder. The finest part of this powder has been analysed for
me, at the request of Sir H. De la Beche, by the kindness of Mr.
Trenham Reeks of the Museum of Economic Geology; it consists of
carbonate of lime in abundance, of sulphate and muriate of lime, and of
muriate and sulphate of soda. The carbonate of lime is obviously
derived from the shells; and common salt is so abundant in parts of
the bed, that, as before remarked, the univalves are often filled with
it. The sulphate of lime may have been derived, as has probably the
common salt, from the evaporation of the sea-spray, during the
emergence of the land; for sulphate of lime is now copiously deposited
from the spray on the shores of Ascension.[35] The other saline bodies
may perhaps have been partially thus derived, but chiefly, as I
conclude from the following facts, through a different means.
[35] See “Volcanic Islands,” etc., by the Author.
On most parts of the second ledge or old sea-beach, at a height of 170
feet, there is a layer of white powder of variable thickness, as much
in some parts as two inches, lying on the angular, salt-cemented
fragments of sandstone and under about four inches of earth, which
powder, from its close resemblance in nature to the upper and most
decayed parts of the shelly mass, I can hardly doubt originally existed
as a bed of shells, now much collapsed and quite disintegrated. I could
not discover with the microscope a trace of organic structure in it;
but its chemical constituents, according to Mr. Reeks, are the same as
in the powder extracted from amongst the decaying shells on the lower
ledge, with the marked exception that the carbonate of lime is present
in only very small quantity. On the third and highest ledge, I observed
some of this powder in a similar position, and likewise occasionally in
small patches at considerably greater heights near the summit of the
island. At Iquique, where the whole face of the country is covered by a
highly saliferous alluvium, and where the climate is extremely dry, we
have seen that, according to Mr. Blake, the shells which are perfect
near the beach become, in ascending, gradually less and less perfect,
until scarcely a trace of their original structure can be discovered.
It is known that carbonate of lime and common salt left in a mass
together,[36] and slightly moistened, partially decompose each other:
now we have at San Lorenzo and at Iquique, in the shells and salt
packed together, and occasionally moistened by the so-called Peruvian
dew, the proper elements for this action. We can thus understand the
peculiar corroded appearance of the shells on San Lorenzo, and the
great decrease of quantity in the carbonate of lime in the powder on
the upper ledge. There is, however, a great difficulty on this view,
for the resultant salts should be carbonate of soda and muriate of
lime; the latter is present, but not the carbonate of soda. Hence I am
led to the perhaps unauthorised conjecture (which I shall hereafter
have to refer to) that the carbonate of soda, by some unexplained
means, becomes converted into a sulphate. If the above remarks be just,
we are led to the very unexpected conclusion, that a dry climate, by
leaving the salt from the sea-spray
undissolved, is much less favourable to the preservation of upraised
shells than a humid climate. However this may be, it is interesting to
know the manner in which masses of shells, gradually upraised above the
sea-level, decay and finally disappear.
[36] I am informed by Dr. Kane, through Mr. Reeks, that a manufactory
was established on this principle in France, but failed from the small
quantity of carbonate of soda produced. Sprengel (_Gardeners’ Chron.,_
1845, p. 157) states, that salt and carbonate of lime are liable to
mutual decomposition in the soil. Sir H. De la Beche informs me, that
calcareous rocks washed by the spray of the sea, are often corroded in
a peculiar manner; see also on this latter subject _Gardeners’
Chron.,_ p. 675, 1844.
_Summary on the recent elevation of the west coast of South
America._—We have seen that upraised marine remains occur at intervals,
and in some parts almost continuously, from lat. 45° 35′ to 12° S.,
along the shores of the Pacific. This is a distance, in a north and
south line, of 2,075 geographical miles. From Byron’s observations, the
elevation has no doubt extended sixty miles further south; and from the
similarity in the form of the country near Lima, it has probably
extended many leagues further north.[37] Along this great line of
coast, besides the organic remains, there are in very many parts, marks
of erosion, caves, ancient beaches, sand-dunes, and successive terraces
of gravel, all above the present level of the sea. From the steepness
of the land on this side of the continent, shells have rarely been
found at greater distances inland than from two to three leagues; but
the marks of sea-action are evident farther from the coast; for
instance, in the valley of Guasco, at a distance of between thirty and
forty miles. Judging from the upraised shells alone, the elevation in
Chiloe has been 350 feet, at Concepcion certainly 625 feet; and by
estimation 1,000 feet; at Valparaiso 1,300 feet; at Coquimbo 252 feet;
northward of this place, sea-shells have not, I believe, been found
above 300 feet; and at Lima they were falling into decay (hastened
probably by the salt) at 85 feet. Not only has this amount of elevation
taken place within the period of existing Mollusca and Cirripedes; but
their proportional numbers in the neighbouring sea have in most cases
remained the same. Near Lima, however, a small change in this respect
between the living and the upraised was observed: at Coquimbo this was
more evident, all the shells being existing species, but with those
embedded in the uppermost calcareous plain not approximating so closely
in proportional numbers, as do those that lie loose on its surface at
the height of 252 feet, and still less closely than those which are
strewed on the lower plains, which latter are identical in proportional
numbers with those now cast up on the beach. From this circumstance,
and from not finding, upon careful examination, near Coquimbo any
shells at a greater height than 252 feet, I believe that the recent
elevation there has been much less than at Valparaiso, where it has
been 1,300 feet, and I may add, than at Concepcion. This considerable
inequality in the amount of elevation at Coquimbo and Valparaiso,
places only 200 miles apart, is not improbable, considering, first, the
difference in the force and number of the shocks now yearly affecting
different parts of this coast; and, secondly, the fact of single areas,
such as that of the province of Concepcion, having been uplifted very
unequally during the same earthquake. It would, in most cases, be very
hazardous to infer an inequality
of elevation, from shells being found on the surface or in superficial
beds at different heights; for we do not know on what their rate of
decay depends; and at Coquimbo one instance out of many has been given,
of a promontory, which, from the occurrence of one very small
collection of lime-cemented shells, has indisputably been elevated 242
feet, and yet on which, not even a fragment of shell could be found on
careful examination between this height and the beach, although many
sites appeared very favourable for the preservation of organic remains:
the absence, also, of shells on the gravel-terraces a short distance up
the valley of Coquimbo, though abundant on the corresponding terraces
at its mouth, should be borne in mind.
[37] I may take this opportunity of stating that in a MS. in the
Geological Society by Mr. Weaver, it is stated that beds of oysters
and other recent shells are found thirty feet above the level of the
sea, in many parts of Tampico, in the Gulf of Mexico.
There are other epochs, besides that of the existence of recent
Mollusca, by which to judge of the changes of level on this coast. At
Lima, as we have just seen, the elevation has been at least eighty-five
feet, within the Indo-human period; and since the arrival of the
Spaniards in 1530, there has apparently been a sinking of the surface.
At Valparaiso, in the course of 220 years, the rise must have been less
than nineteen feet; but it has been as much as from ten to eleven feet
in the seventeen years subsequently to 1817, and of this rise only a
part can be attributed to the earthquake of 1822, the remainder having
been insensible and apparently still, in 1834, in progress. At Chiloe
the elevation has been gradual, and about four feet during four years.
At Coquimbo, also, it has been gradual, and in the course of 150 years
has amounted to several feet. The sudden small upheavals, accompanied
by earthquakes, as in 1822 at Valparaiso, in 1835 at Concepcion, and in
1837 in the Chonos Archipelago, are familiar to most geologists, but
the gradual rising of the coast of Chile has been hardly noticed; it
is, however, very important, as connecting together these two orders of
events.
The rise of Lima, having been eighty-five feet within the period of
man, is the more surprising if we refer to the eastern coast of the
continent, for at Port S. Julian, in Patagonia, there is good evidence
(as we shall hereafter see) that when the land stood ninety feet lower,
the Macrauchenia, a mammiferous beast, was alive; and at Bahia Blanca,
when it stood only a few feet lower than it now does, many gigantic
quadrupeds ranged over the adjoining country. But the coast of
Patagonia is some way distant from the Cordillera, and the movement at
Bahia Blanca is perhaps noways connected with this great range, but
rather with the tertiary volcanic rocks of Banda Oriental, and
therefore the elevation at these places may have been infinitely slower
than on the coast of Peru. All such speculations, however, must be
vague, for as we know with certainty that the elevation of the whole
coast of Patagonia has been interrupted by many and long pauses, who
will pretend to say that, in such cases, many and long periods of
subsidence may not also have been intercalated?
In many parts of the coast of Chile and Peru there are marks of the
action of the sea at successive heights on the land, showing that the
elevation has been interrupted by periods of comparative rest in the
upward movement, and of denudation in the action of the sea. These
are plainest at Chiloe, where, in a height of about five hundred feet,
there are three escarpments,—at Coquimbo, where in a height of 364
feet, there are five,—at Guasco, where there are six, of which five may
perhaps correspond with those at Coquimbo, but if so, the subsequent
and intervening elevatory movements have been here much more
energetic,—at Lima, where, in a height of about 250 feet there are
three terraces, and others, as it is asserted, at considerably greater
heights. The almost entire absence of ancient marks of sea-action at
defined levels along considerable spaces of coast, as near Valparaiso
and Concepcion, is highly instructive, for as it is improbable that the
elevation at these places alone should have been continuous, we must
attribute the absence of such marks to the nature and form of the
coast-rocks. Seeing over how many hundred miles of the coast of
Patagonia, and on how many places on the shores of the Pacific, the
elevatory process has been interrupted by periods of comparative rest,
we may conclude, conjointly with the evidence drawn from other quarters
of the world, that the elevation of the land is generally an
intermittent action. From the quantity of matter removed in the
formation of the escarpments, especially of those of Patagonia, it
appears that the periods of rest in the movement, and of denudation of
the land, have generally been very long. In Patagonia, we have seen
that the elevation has been equable, and the periods of denudation
synchronous over very wide spaces of coast; on the shores of the
Pacific, owing to the terraces chiefly occurring in the valleys, we
have not equal means of judging on this point; and the very different
heights of the upraised shells at Coquimbo, Valparaiso, and Concepcion
seem directly opposed to such a conclusion.
Whether on this side of the continent the elevation, between the
periods of comparative rest when the escarpments were formed, has been
by small sudden starts, such as those accompanying recent earthquakes,
or, as is most probable, by such starts conjointly with a gradual
upward movement, or by great and sudden upheavals, I have no direct
evidence. But as on the eastern coast, I was led to think, from the
analogy of the last hundred feet of elevation in La Plata, and from the
nearly equal size of the pebbles over the entire width of the terraces,
and from the upraised shells being all littoral species, that the
elevation had been gradual; so do I on this western coast, from the
analogy of the movements now in progress, and from the vast numbers of
shells now living exclusively on or close to the beach, which are
strewed over the whole surface of the land up to very considerable
heights, conclude, that the movement here also has been slow and
gradual, aided probably by small occasional starts. We know at least
that at Coquimbo, where five escarpments occur in a height of 364 feet,
the successive elevations, if they have been sudden, cannot have been
very great. It has, I think, been shown that the occasional
preservation of shells, unrolled and unbroken, is not improbable even
during a quite gradual rising of the land; and their preservation, if
the movement has been aided by small starts, is quite conformable with
what actually takes place during recent earthquakes.
Judging from the present action of the sea, along the shores of the
Pacific, on the deposits of its own accumulation, the present time
seems in most places to be one of comparative rest in the elevatory
movement, and of denudation of the land. Undoubtedly this is the case
along the whole great length of Patagonia. At Chiloe, however, we have
seen that a narrow sloping fringe, covered with vegetation, separates
the present sea-beach from a line of low cliffs, which the waves lately
reached; here, then, the land is gaining in breadth and height, and the
present period is not one of rest in the elevation and of contingent
denudation; but if the rising be not prolonged at a quick rate, there
is every probability that the sea will soon regain its former
horizontal limits. I observed similar low sloping fringes on several
parts of the coast, both northward of Valparaiso and near Coquimbo; but
at this latter place, from the change in form which the coast has
undergone since the old escarpments were worn, it may be doubted
whether the sea, acting for any length of time at its present level,
would eat into the land; for it now rather tends to throw up great
masses of sand. It is from facts such as these that I have generally
used the term _comparative rest_, as applied to the elevation of the
land; the rest or cessation in the movement being comparative both with
what has preceded it and followed it, and with the sea’s power of
corrosion at each spot and at each level. Near Lima, the cliff-formed
shores of San Lorenzo, and on the mainland south of Callao, show that
the sea is gaining on the land; and as we have here some evidence that
its surface has lately subsided or is still sinking, the periods of
comparative rest in the elevation and of contingent denudation, may
probably in many cases include periods of subsidence. It is only, as
was shown in detail when discussing the terraces of Coquimbo, when the
sea with difficulty and after a long lapse of time has either corroded
a narrow ledge into solid rock, or has heaped up on a steep surface a
_narrow_ mound of detritus, that we can confidently assert that the
land at that level and at that period long remained absolutely
stationary. In the case of terraces formed of gravel or sand, although
the elevation may have been strictly horizontal, it may well happen
that no one level beach-line may be traceable, and that neither the
terraces themselves nor the summit nor basal edges of their escarpments
may be horizontal.
Finally, comparing the extent of the elevated area, as deduced from the
upraised recent organic remains, on the two sides of the continent, we
have seen that on the Atlantic, shells have been found at intervals
from Eastern Tierra del Fuego for 1,180 miles northward, and on the
Pacific for a space of 2,075 miles. For a length of 775 miles, they
occur in the same latitudes on both sides of the continent. Without
taking this circumstance into consideration, it is probable from the
reasons assigned in the last chapter, that the entire breadth of the
continent in Central Patagonia has been uplifted in mass; but from
other reasons there given, it would be hazardous to extend this
conclusion to La Plata. From the continent being narrow in the
southern-most parts of Patagonia, and from the shells found at the
Inner Narrows of the Strait of Magellan, and likewise far up the valley
of the Santa Cruz,
it is probable that the southern part of the western coast, which was
not visited by me, has been elevated within the period of recent
Mollusca: if so, the shores of the Pacific have been continuously,
recently, and in a geological sense synchronously upraised, from Lima
for a length of 2,480 nautical miles southward,—a distance equal to
that from the Red Sea to the North Cape of Scandinavia!