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 Patellae, Mytili, and other species. I
found similar microscopical fragments in earth filling up the central
orifices of some large Fissurellae. 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. (In the “Proceedings of the Geological
Society” volume 2 page 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, which accompanied the earthquake of 1822 (Dr.
Meyen “Reise um Erde” Th. 1 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.): 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.
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, to a height of 500 feet, and at a
distance of three leagues from the coast (“Travels in Chile” volume 1
pages 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.): 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.
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,
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. (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.) 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.
(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:—
1. Venus opaca. 2. Mulinia Byronensis. 3. Pecten purpuratus. 4.
Mesodesma donaciforme. 5. Turritella cingulata. 6. Monoceros costatum.
7. Concholepas Peruviana. 8. Trochus (common Valparaiso species). 9.
Calyptraea 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 Calyptraea
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.
(DIAGRAM 8.—SECTION OF PLAIN OF COQUIMBO.
A Section through Plain B-B and Ravine A.
Surface of plain 252 feet above sea.
A. Stratified sand, with recent shells in same proportions as on the
beach, half filling up a ravine.
B. Surface of plain, with scattered shells in nearly same proportions
as on the beach.
C. Upper calcareous bed, and D. Lower calcareous sandy bed (Losa), both
with recent shells, but not in same proportions as on the beach.
E. Upper ferrugino-sandy old tertiary stratum, and F. Lower old
tertiary stratum, both with all, or nearly all, extinct shells.)
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:—
1. Calyptraea radians. 2. Turritella cingulata. 3. Oliva Peruviana. 4.
Murex labiosus, var. 5. Nassa (identical with a living species). 6.
Solen Dombeiana. 7. Pecten purpuratus. 8. Venus Chilensis. 9.
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.) 10. 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, that a rise has taken
place here within the last century and a half (“Proceedings of the
Geological Society” volume 2 page 446.); 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 Feuillee, were a 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 Feuillee 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.
VALLEY OF COQUIMBO.
(FIGURE 9. EAST AND WEST SECTION THROUGH THE TERRACES AT COQUIMBO,
WHERE THEY DEBOUCH FROM THE VALLEY, AND FRONT THE SEA.
Vertical scale 1/10 of inch to 100 feet: horizontal scale much
contracted.
Height of terrace in feet from east (high) to west (low): Terrace F.
364 Terrace E. 302 Terrace D. shown dotted, height not given. Terrace
C. 120 Terrace B. 70 Terrace A. 25 sloping down to level of sea at Town
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. (“Principles of Geology” 1st edition volume 3
page 131.) 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.
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.
(FIGURE 10. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF COQUIMBO.
From north F (high) through E?, D, C, B, A (low), B?, C, D?, E, F
(high).
Vertical scale 1/10 of inch to 100 feet: horizontal scale much
contracted.
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 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
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 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.)
We come now to the terraces on the opposite sides of the east and west
valley of Coquimbo: the section in Figure 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.
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 sections in Figures 9 and 10 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.
(FIGURE 11. DIAGRAM OF A BAY IN A 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 (“Philosophical Transactions” 1839 page 39.), 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.
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 (Mr. Place in the “Quarterly
Journal of Science” 1824 volume 17 page 42.), 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, 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 (“Voyages de la
Comm. du Nord” etc. also “Comptes Rendus” October 1842.), showing that
the proportional difference in the amount of elevation on the coast and
in the interior, varied at different periods.
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.
A north and south section across the valley in this part is represented
in Figure 12.
(FIGURE 12. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF GUASCO, AND OF
A PLAIN NORTH OF IT.
From left (north, high) to right (south, high) through plains B and A
and the River of Guasco at the Town of Ballenar.)
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
(“Reise um die Erde” th. 1 s. 372 et seq.) 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 Calyptraea 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.
Northward of Copiapo, in latitude 26 degrees S., the old voyager Wafer
found immense numbers of sea-shells some miles from the coast.
(Burnett’s “Collection of Voyages” volume 4 page 193.) At Cobija
(latitude 22 degrees 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, Calyptraea, 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 (“Voyage,
Part Geolog.” page 94. M. d’Orbigny (page 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 metres; il faudrait supposer encore que ce
soulevement n’a point ete graduel;...mais qu’il resulterait d’une seule
et meme 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, Calyptraea, 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.): 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 (latitude 20 degrees
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 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.” (“Silliman’s American
Journal of Science” volume 44 page 2.) 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 (latitude 18 degrees 28′), M. d’Orbigny 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. (“Voyage” etc. page 101.)
Lieutenant Freyer has given some more precise facts: he states (In a
letter to Mr. Lyell “Geological Proceedings” volume 2 page 179.) 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
Milleporae 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 found numerous recent sea shells
in sand, at a considerable distance from the sea. (“Edinburgh New
Philosophical Journal” volume 30 page 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. (M. Chevalier,
in the “Voyage of the ‘Bonite’” observed these shells; but his
specimens were lost.—“L’Institut” 1838 page 151.) 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 Serpulae. All,
according to Mr. G.B. Sowerby, are recent species: they consist of:—
1. Mytilus Magellanicus: same as that found at Valparaiso, and there
stated to be probably distinct from the true M. Magellanicus of the
east coast.
2. Venus costellata, Sowerby “Zoological Proceedings.”
3. Pecten purpuratus, Lam.
4. Chama, probably echinulata, Brod.
5. Calyptraea Byronensis, Gray.
6. Calyptraea radians (Trochus, Lam.)
7. Fissurella affinis, Gray.
8. Fissurella biradiata, Trembly.
9. Purpura chocolatta, Duclos.
10. Purpura Peruviana, Gray.
11. Purpura labiata, Gray.
12. Purpura buxea (Murex, Brod.).
13. Concholepas Peruviana.
14. Nassa, related to reticulata.
15. Triton rudis, Brod.
16. Trochus, not yet described, but well-known and very common.
17 and 18. Balanus, two species, both common on the coast.
These upraised shells appear to be nearly in the same proportional
numbers- -with the exception of the Crepidulae 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 Fissurellae 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. (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” volume 2 English
Translation page 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 degrees 46′); this rain
entirely ruined (“Ulloa” etc. page 18) the mud houses of the
inhabitants.) 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.
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 (Mr. Smith of
Jordan Hill found pieces of seaweed in an upraised pleistocene deposit
in Scotland. See his admirable Paper in the “Edinburgh New
Philosophical Journal” volume 25 page 384.), 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 of the earthquake of
1835 (I have described this in my “Journal of Researches” page 303 2nd
edition.), 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.
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 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 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.) 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 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. (“Observaciones sobre el Clima del Lima” par Dr. H. Unanue page
4.—Ulloa’s “Voyage” volume 2 English Translation page 97.—For Mr.
Cruikshank’s observations, see Mr. Lyell’s “Principles of Geology” 1st
edition volume 3 page 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. (See
“Volcanic Islands” etc. by the Author.) The other saline bodies may
perhaps have been partially thus derived, but chiefly, as I conclude
from the following facts, through a different means.
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, and slightly moistened, partially decompose each other (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’ Chronicle”
1845 page 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’
Chronicle” page 675 1844.): 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.
A 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 latitude 45 degrees 35′ to 12
degrees 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. (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.) 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.
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!
CHAPTER III.
ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS.
Basin-like plains of Chile; their drainage, their marine origin.—Marks
of sea-action on the eastern flanks of the Cordillera.—Sloping
terrace-like fringes of stratified shingle within the valleys of the
Cordillera; their marine origin.—Boulders in the valley of
Cachapual.—Horizontal elevation of the Cordillera.—Formation of
valleys.—Boulders moved by earthquake-waves.—Saline superficial
deposits.—Bed of nitrate of soda at Iquique.—Saline
incrustations.—Salt-lakes of La Plata and Patagonia; purity of the
salt; its origin.
The space between the Cordillera and the coast of Chile is on a rude
average from eighty to above one hundred miles in width; it is formed,
either of an almost continuous mass of mountains, or more commonly of
several nearly parallel ranges, separated by plains; in the more
southern parts of this province the mountains are quite subordinate to
the plains; in the northern part the mountains predominate.
The basin-like plains at the foot of the Cordillera are in several
respects remarkable; that on which the capital of Chile stands is
fifteen miles in width, in an east and west line, and of much greater
length in a north and south line; it stands 1,750 feet above the sea;
its surface appears smooth, but really falls and rises in wide gentle
undulations, the hollows corresponding with the main valleys of the
Cordillera: the striking manner in which it abruptly comes up to the
foot of this great range has been remarked by every author since the
time of Molina. (This plain is partially separated into two basins by a
range of hills; the southern half, according to Meyen (“Reise um Erde”
Th. 1 s. 274), falls in height, by an abrupt step, of between fifteen
and twenty feet.) Near the Cordillera it is composed of a stratified
mass of pebbles of all sizes, occasionally including rounded boulders:
near its western boundary, it consists of reddish sandy clay,
containing some pebbles and numerous fragments of pumice, and sometimes
passes into pure sand or into volcanic ashes. At Podaguel, on this
western side of the plain, beds of sand are capped by a calcareous
tuff, the uppermost layers being generally hard and substalagmitic, and
the lower ones white and friable, both together precisely resembling
the beds at Coquimbo, which contain recent marine shells. Abrupt, but
rounded, hummocks of rock rise out of this plain: those of Sta. Lucia
and S. Cristoval are formed of greenstone-porphyry almost entirely
denuded of its original covering of porphyritic claystone breccia; on
their summits, many fragments of rock (some of them kinds not found in
situ) are coated and united together by a white, friable, calcareous
tuff, like that found at Podaguel. When this matter was deposited on
the summit of S. Cristoval, the water must have stood 946 feet above
the surface of the surrounding plain. (Or 2,690 feet above the sea, as
measured barometrically by Mr. Eck. This tuff appears to the eye nearly
pure; but when placed in acid it leaves a considerable residue of sand
and broken crystals, apparently of feldspar. Dr. Meyen (“Reise” Th. 1
s. 269) says he found a similar substance on the neighbouring hill of
Dominico (and I found it also on the Cerro Blanco), and he attributes
it to the weathering of the stone. In some places which I examined, its
bulk put this view of its origin quite out of the question; and I
should much doubt whether the decomposition of a porphyry would, in any
case, leave a crust chiefly composed of carbonate of lime. The white
crust, which is commonly seen on weathered feldspathic rocks, does not
appear to contain any free carbonate of lime.)
To the south this basin-like plain contracts, and rising scarcely
perceptibly with a smooth surface, passes through a remarkable level
gap in the mountains, forming a true land-strait, and called the
Angostura. It then immediately expands into a second basin-formed
plain: this again to the south contracts into another land-strait, and
expands into a third basin, which, however, falls suddenly in level
about forty feet. This third basin, to the south, likewise contracts
into a strait, and then again opens into the great plain of San
Fernando, stretching so far south that the snowy peaks of the distant
Cordillera are seen rising above its horizon as above the sea. These
plains, near the Cordillera, are generally formed of a thick stratified
mass of shingle (The plain of San Fernando has, according to MM. Meyen
and Gay “Reise” etc. Th. 1 ss. 295 and 298, near the Cordillera, an
upper step-formed plain of clay, on the surface of which they found
numerous blocks of rocks, from two to three feet long, either lying
single or piled in heaps, but all arranged in nearly straight lines.);
in other parts, of a red sandy clay, often with an admixture of
pumiceous matter. Although these basins are connected together like a
necklace, in a north and south line, by smooth land-straits, the
streams which drain them do not all flow north and south, but mostly
westward, through breaches worn in the bounding mountains; and in the
case of the second basin, or that of Rancagua, there are two distinct
breaches. Each basin, moreover, is not drained singly; thus, to give
the most striking instance, but not the only one, in proceeding
southward over the plain of Rancagua, we first find the water flowing
northward to and through the northern land-strait; then, without
crossing any marked ridge or watershed, we see it flowing
south-westward towards the northern one of the two breaches in the
western mountainous boundary; and lastly, again without any ridge, it
flows towards the southern breach in these same mountains. Hence the
surface of this one basin-like plain, appearing to the eye so level,
has been modelled with great nicety, so that the drainage, without any
conspicuous watersheds, is directed towards three openings in the
encircling mountains. ((It appears from Captain Herbert’s account of
the Diluvium of the Himalaya, “Gleanings of Science” Calcutta volume 2
page 164, that precisely similar remarks apply to the drainage of the
plains or valleys between those great mountains.) The streams flowing
from the southern basin-like plains, after passing through the breaches
to the west, unite and form the river Rapel, which enters the Pacific
near Navidad. I followed the southernmost branch of this river, and
found that the basin or plain of San Fernando is continuously and
smoothly united with those plains, which were described in the Second
Chapter, as being worn near the coast into successive cave-eaten
escarpments, and still nearer to the coast, as being strewed with
upraised recent marine remains.
I might have given descriptions of numerous other plains of the same
general form, some at the foot of the Cordillera, some near the coast,
and some halfway between these points. I will allude only to one other,
namely, the plain of Uspallata, lying on the eastern or opposite side
of the Cordillera, between that great range and the parallel lower
range of Uspallata. According to Miers, its surface is 6,000 feet above
the level of the sea: it is from ten to fifteen miles in width, and is
said to extend with an unbroken surface for 180 miles northwards: it is
drained by two rivers passing through breaches in the mountains to the
east. On the banks of the River Mendoza it is seen to be composed of a
great accumulation of stratified shingle, estimated at 400 feet in
thickness. In general appearance, and in numerous points of structure,
this plain closely resembles those of Chile.
The origin and manner of formation of the thick beds of gravel, sandy
clay, volcanic detritus, and calcareous tuff, composing these
basin-like plains, is very important; because, as we shall presently
show, they send arms or fringes far up the main valleys of the
Cordillera. Many of the inhabitants believe that these plains were once
occupied by lakes, suddenly drained; but I conceive that the number of
the separate breaches at nearly the same level in the mountains
surrounding them quite precludes this idea. Had not such distinguished
naturalists as MM. Meyen and Gay stated their belief that these
deposits were left by great debacles rushing down from the Cordillera,
I should not have noticed a view, which appears to me from many reasons
improbable in the highest degree—namely, from the vast accumulation of
WELL-ROUNDED PEBBLES—their frequent stratification with layers of
sand—the overlying beds of calcareous tuff—this same substance coating
and uniting the fragments of rock on the hummocks in the plain of
Santiago—and lastly even from the worn, rounded, and much denuded state
of these hummocks, and of the headlands which project from the
surrounding mountains. On the other hand, these several circumstances,
as well as the continuous union of the basins at the foot of the
Cordillera, with the great plain of the Rio Rapel which still retains
the marks of sea-action at various levels, and their general similarity
in form and composition with the many plains near the coast, which are
either similarly marked or are strewed with upraised marine remains,
fully convince me that the mountains bounding these basin-plains were
breached, their islet-like projecting rocks worn, and the loose
stratified detritus forming their now level surfaces deposited, by the
sea, as the land slowly emerged. It is hardly possible to state too
strongly the perfect resemblance in outline between these basin-like,
long, and narrow plains of Chile (especially when in the early morning
the mists hanging low represented water), and the creeks and fiords now
intersecting the southern and western shores of the continent. We can
on this view of the sea, when the land stood lower, having long and
tranquilly occupied the spaces between the mountain-ranges, understand
how the boundaries of the separate basins were breached in more than
one place; for we see that this is the general character of the inland
bays and channels of Tierra del Fuego; we there, also, see in the
sawing action of the tides, which flow with great force in the cross
channels, a power sufficient to keep the breaches open as the land
emerged. We can further see that the waves would naturally leave the
smooth bottom of each great bay or channel, as it became slowly
converted into land, gently inclined to as many points as there were
mouths, through which the sea finally retreated, thus forming so many
watersheds, without any marked ridges, on a nearly level surface. The
absence of marine remains in these high inland plains cannot be
properly adduced as an objection to their marine origin: for we may
conclude, from shells not being found in the great shingle beds of
Patagonia, though copiously strewed on their surfaces, and from many
other analogous facts, that such deposits are eminently unfavourable
for the embedment of such remains; and with respect to shells not being
found strewed on the surface of these basin-like plains, it was shown
in the last chapter that remains thus exposed in time decay and
disappear.
(FIGURE 13. SECTION OF THE PLAIN AT THE EASTERN FOOT OF THE CHILEAN
CORDILLERA.
From Cordillera (left) through Talus-plain and Level surface, 2,700
feet above sea, to Gravel terraces (right).)
I observed some appearances on the plains at the eastern and opposite
foot of the Cordillera which are worth notice, as showing that the sea
there long acted at nearly the same level as on the basin-plains of
Chile. The mountains on this eastern side are exceedingly abrupt; they
rise out of a smooth, talus-like, very gentle, slope, from five to ten
miles in width (as represented in Figure 13), entirely composed of
perfectly rounded pebbles, often white-washed with an aluminous
substance like decomposed feldspar. This sloping plain or talus blends
into a perfectly flat space a few miles in width, composed of reddish
impure clay, with small calcareous concretions as in the Pampean
deposit,—of fine white sand with small pebbles in layers,—and of the
above-mentioned white aluminous earth, all interstratified together.
This flat space runs as far as Mendoza, thirty miles northward, and
stands probably at about the same height, namely, 2,700 feet (Pentland
and Miers) above the sea. To the east it is bounded by an escarpment,
eighty feet in height, running for many miles north and south, and
composed of perfectly round pebbles, and loose, white-washed, or
embedded in the aluminous earth: behind this escarpment there is a
second and similar one of gravel. Northward of Mendoza, these
escarpments become broken and quite obliterated; and it does not appear
that they ever enclosed a lake-like area: I conclude, therefore, that
they were formed by the sea, when it reached the foot of the
Cordillera, like the similar escarpments occurring at so many points on
the coasts of Chile and Patagonia.
The talus-like plain slopes up with a smooth surface into the great dry
valleys of the Cordillera. On each hand of the Portillo valley, the
mountains are formed of red granite, mica-slate, and basalt, which all
have suffered a truly astonishing amount of denudation; the gravel in
the valley, as well as on the talus-like plain in front of it, is
composed of these rocks; but at the mouth of the valley, in the middle
(height probably about three thousand five hundred feet above the sea),
a few small isolated hillocks of several varieties of porphyry project,
round which, on all sides, smooth and often white-washed pebbles of
these same porphyries, to the exclusion of all others, extend to a
circumscribed distance. Now, it is difficult to conceive any other
agency, except the quiet and long-continued action of the sea on these
hillocks, which could have rounded and whitewashed the fragments of
porphyry, and caused them to radiate from such small and quite
insignificant centres, in the midst of that vast stream of stones which
has descended from the main Cordillera.
SLOPING TERRACES OF GRAVEL IN THE VALLEYS OF THE CORDILLERA.
(FIGURE 14. GROUND-PLAN OF A BIFURCATING VALLEY IN THE CORDILLERA,
bordered by smooth, sloping gravel-fringes (AA), worn along the course
of the river into cliffs.)
All the main valleys on both flanks of the Chilean Cordillera have
formerly had, or still have, their bottoms filled up to a considerable
thickness by a mass of rudely stratified shingle. In Central Chile the
greater part of this mass has been removed by the torrents;
cliff-bounded fringes, more or less continuous, being left at
corresponding heights on both sides of the valleys. These fringes, or
as they may be called terraces, have a smooth surface, and as the
valleys rise, they gently rise with them: hence they are easily
irrigated, and afford great facilities for the construction of the
roads. From their uniformity, they give a remarkable character to the
scenery of these grand, wild, broken valleys. In width, the fringes
vary much, sometimes being only broad enough for the roads, and
sometimes expanding into narrow plains. Their surfaces, besides gently
rising up the valley, are slightly inclined towards its centre in such
a manner as to show that the whole bottom must once have been filled up
with a smooth and slightly concave mass, as still are the dry
unfurrowed valleys of Northern Chile. Where two valleys unite into one,
these terraces are particularly well exhibited, as is represented in
Figure 14. The thickness of the gravel forming these fringes, on a rude
average, may be said to vary from thirty to sixty or eighty feet; but
near the mouths of the valleys it was in several places from two to
three hundred feet. The amount of matter removed by the torrents has
been immense; yet in the lower parts of the valleys the terraces have
seldom been entirely worn away on either side, nor has the solid
underlying rock been reached: higher up the valleys, the terraces have
frequently been removed on one or the other side, and sometimes on both
sides; but in this latter case they reappear after a short interval on
the line, which they would have held had they been unbroken. Where the
solid rock has been reached, it has been cut into deep and narrow
gorges. Still higher up the valleys, the terraces gradually become more
and more broken, narrower, and less thick, until, at a height of from
seven to nine thousand feet, they become lost, and blended with the
piles of fallen detritus.
I carefully examined in many places the state of the gravel, and almost
everywhere found the pebbles equally and perfectly rounded,
occasionally with great blocks of rock, and generally distinctly
stratified, often with parting seams of sand. The pebbles were
sometimes coated with a white aluminous, and less frequently with a
calcareous, crust. At great heights up the valleys the pebbles become
less rounded; and as the terraces become obliterated, the whole mass
passes into the nature of ordinary detritus. I was repeatedly struck
with the great difference between this detritus high up the valleys,
and the gravel of the terraces low down, namely, in the greater number
of the quite angular fragments in the detritus,—in the unequal degree
to which the other fragments have been rounded,—in the quantity of
associated earth,—in the absence of stratification,—and in the
irregularity of the upper surfaces. This difference was likewise well
shown at points low down the valleys, where precipitous ravines,
cutting through mountains of highly coloured rock, have thrown down
wide, fan- shaped accumulations of detritus on the terraces: in such
cases, the line of separation between the detritus and the terrace
could be pointed out to within an inch or two; the detritus consisting
entirely of angular and only partially rounded fragments of the
adjoining coloured rocks; the stratified shingle (as I ascertained by
close inspection, especially in one case, in the valley of the River
Mendoza) containing only a small proportion of these fragments, and
those few well rounded.
I particularly attended to the appearance of the terraces where the
valleys made abrupt and considerable bends, but I could perceive no
difference in their structure: they followed the bends with their usual
nearly equable inclination. I observed, also, in several valleys, that
wherever large blocks of any rock became numerous, either on the
surface of the terrace or embedded in it, this rock soon appeared
higher up in situ: thus I have noticed blocks of porphyry, of andesitic
syenite, of porphyry and of syenite, alternately becoming numerous, and
in each case succeeded by mountains thus constituted. There is,
however, one remarkable exception to this rule; for along the valley of
the Cachapual, M. Gay found numerous large blocks of white granite,
which does not occur in the neighbourhood. I observed these blocks, as
well as others of andesitic syenite (not occurring here in situ), near
the baths of Cauquenes at a height of between two and three hundred
feet above the river, and therefore quite above the terrace or fringe
which borders that river; some miles up the valleys there were other
blocks at about the same height. I also noticed, at a less height, just
above the terrace, blocks of porphyries (apparently not found in the
immediately impending mountains), arranged in rude lines, as on a
sea-beach. All these blocks were rounded, and though large, not
gigantic, like the true erratic boulders of Patagonia and Fuegia. M.
Gay states that the granite does not occur in situ within a distance of
twenty leagues (“Annales des Science Nat. “ 1 series tome 28. M. Gay,
as I was informed, penetrated the Cordillera by the great oblique
valley of Los Cupressos, and not by the most direct line.); I suspect,
for several reasons, that it will ultimately be found at a much less
distance, though certainly not in the immediate neighbourhood. The
boulders found by MM. Meyen and Gay on the upper plain of San Fernando
(mentioned in a previous note) probably belong to this same class of
phenomena.
These fringes of stratified gravel occur along all the great valleys of
the Cordillera, as well as along their main branches; they are
strikingly developed in the valleys of the Maypu, Mendoza, Aconcagua,
Cachapual, and according to Meyen, in the Tinguirica. (“Reise” etc. Th.
1 s. 302.) In the valleys, however, of Northern Chile, and in some on
the eastern flank of the Cordillera, as in the Portillo Valley, where
streams have never flowed, or are quite insignificant in volume, the
presence of a mass of stratified gravel can be inferred only from the
smooth slightly concave form of the bottom. One naturally seeks for
some explanation of so general and striking a phenomenon; that the
matter forming the fringes along the valleys, or still filling up their
entire beds, has not fallen from the adjoining mountains like common
detritus, is evident from the complete contrast in every respect
between the gravel and the piles of detritus, whether seen high up the
valleys on their sides, or low down in front of the more precipitous
ravines; that the matter has not been deposited by debacles, even if we
could believe in debacles having rushed down EVERY valley, and all
their branches, eastward and westward from the central pinnacles of the
Cordillera, we must admit from the following reasons,—from the distinct
stratification of the mass,—its smooth upper surface,—the well-rounded
and sometimes encrusted state of the pebbles, so different from the
loose debris on the mountains,—and especially from the terraces
preserving their uniform inclination round the most abrupt bends. To
suppose that as the land now stands, the rivers deposited the shingle
along the course of every valley, and all their main branches, appears
to me preposterous, seeing that these same rivers not only are now
removing and have removed much of this deposit, but are everywhere
tending to cut deep and narrow gorges in the hard underlying rocks.
I have stated that these fringes of gravel, the origin of which are
inexplicable on the notion of debacles or of ordinary alluvial action,
are directly continuous with the similarly-composed basin-like plains
at the foot of the Cordillera, which, from the several reasons before
assigned, I cannot doubt were modelled by the agency of the sea. Now if
we suppose that the sea formerly occupied the valleys of the Chilean
Cordillera, in precisely the same manner as it now does in the more
southern parts of the continent, where deep winding creeks penetrate
into the very heart of, and in the case of Obstruction Sound quite
through, this great range; and if we suppose that the mountains were
upraised in the same slow manner as the eastern and western coasts have
been upraised within the recent period, then the origin and formation
of these sloping, terrace-like fringes of gravel can be simply
explained. For every part of the bottom of each valley will, on this
view, have long stood at the head of a sea creek, into which the then
existing torrents will have delivered fragments of rocks, where, by the
action of the tides, they will have been rolled, sometimes encrusted,
rudely stratified, and the whole surface levelled by the blending
together of the successive beach lines. (Sloping terraces of precisely
similar structure have been described by me “Philosophical
Transactions” 1839 page 58, in the valleys of Lochaber in Scotland,
where, at higher levels, the parallel roads of Glen Roy show the marks
of the long and quiet residence of the sea. I have no doubt that these
sloping terraces would have been present in the valleys of most of the
European ranges, had not every trace of them, and all wrecks of
sea-action, been swept away by the glaciers which have since occupied
them. I have shown that this is the case with the mountains (“London
and Edinburgh Philosophical Journal” volume 21 page 187) of North
Wales.) As the land rose, the torrents in every valley will have tended
to have removed the matter which just before had been arrested on, or
near, the beach-lines; the torrents, also, having continued to gain in
force by the continued elevation increasing their total descent from
their sources to the sea. This slow rising of the Cordillera, which
explains so well the otherwise inexplicable origin and structure of the
terraces, judging from all known analogies, will probably have been
interrupted by many periods of rest; but we ought not to expect to find
any evidence of these periods in the structure of the gravel- terraces:
for, as the waves at the heads of deep creeks have little erosive
power, so the only effect of the sea having long remained at the same
level will be that the upper parts of the creeks will have become
filled up at such periods to the level of the water with gravel and
sand; and that afterwards the rivers will have thrown down on the
filled-up parts a talus of similar matter, of which the inclination (as
at the head of a partially filled-up lake) will have been determined by
the supply of detritus, and the force of the stream. (I have attempted
to explain this process in a more detailed manner, in a letter to Mr.
Maclaren, published in the “Edinburgh New Philosophical Journal” volume
35 page 288.) Hence, after the final conversion of the creeks into
valleys, almost the only difference in the terraces at those points at
which the sea stood long, will be a somewhat more gentle inclination,
with river-worn instead of sea-worn detritus on the surface.
I know of only one difficulty on the foregoing view, namely, the far-
transported blocks of rock high on the sides of the valley of the
Cachapual: I will not attempt any explanation of this phenomenon, but I
may state my belief that a mountain-ridge near the Baths of Cauquenes
has been upraised long subsequently to all the other ranges in the
neighbourhood, and that when this was effected the whole face of the
country must have been greatly altered. In the course of ages,
moreover, in this and other valleys, events may have occurred like, but
even on a grander scale than, that described by Molina, when a slip
during the earthquake of 1762 banked up for ten days the great River
Lontue, which then bursting its barrier “inundated the whole country,”
and doubtless transported many great fragments of rock. (“Compendio de
la Hist.” etc. etc. tome 1 page 30. M. Brongniart, in his report on M.
Gay’s labours “Annales des Sciences” 1833, considers that the boulders
in the Cachapual belong to the same class with the erratic boulders of
Europe. As the blocks which I saw are not gigantic, and especially as
they are not angular, and as they have not been transported fairly
across low spaces or wide valleys, I am unwilling to class them with
those which, both in the northern and southern hemisphere “Geological
Transactions” volume 6 page 415, have been transported by ice. It is to
be hoped that when M. Gay’s long-continued and admirable labours in
Chile are published, more light will be thrown on this subject.
However, the boulders may have been primarily transported; the final
position of those of porphyry, which have been described as arranged at
the foot of the mountain in rude lines, I cannot doubt, has been due to
the action of waves on a beach. The valley of the Cachapual, in the
part where the boulders occur, bursts through the high ridge of
Cauquenes, which runs parallel to, but at some distance from, the
Cordillera. This ridge has been subjected to excessive violence;
trachytic lava has burst from it, and hot springs yet flow at its base.
Seeing the enormous amount of denudation of solid rock in the upper and
much broader parts of this valley where it enters the Cordillera, and
seeing to what extent the ridge of Cauquenes now protects the great
range, I could not help believing (as alluded to in the text) that this
ridge with its trachytic eruptions had been thrown up at a much later
period than the Cordillera. If this has been the case, the boulders,
after having been transported to a low level by the torrents (which
exhibit in every valley proofs of their power of moving great
fragments), may have been raised up to their present height, with the
land on which they rested.) Finally, notwithstanding this one case of
difficulty, I cannot entertain any doubt, that these terrace-like
fringes, which are continuously united with the basin-shaped plains at
the foot of the Cordillera, have been formed by the arrestment of
river-borne detritus at successive levels, in the same manner as we see
now taking place at the heads of all those many, deep, winding fiords
intersecting the southern coasts. To my mind, this has been one of the
most important conclusions to which my observations on the geology of
South America have led me; for we thus learn that one of the grandest
and most symmetrical mountain-chains in the world, with its several
parallel lines, has been together uplifted in mass between seven and
nine thousand feet, in the same gradual manner as have the eastern and
western coasts within the recent period. (I do not wish to affirm that
all the lines have been uplifted quite equally; slight differences in
the elevation would leave no perceptible effect on the terraces. It
may, however, be inferred, perhaps with one exception, that since the
period when the sea occupied these valleys, the several ranges have not
been dislocated by GREAT and ABRUPT faults or upheavals; for if such
had occurred, the terraces of gravel at these points would not have
been continuous. The one exception is at the lower end of a plain in
the Valle del Yeso (a branch of the Maypu), where, at a great height,
the terraces and valley appear to have been broken through by a line of
upheaval, of which the evidence is plain in the adjoining mountains;
this dislocation, perhaps, occurred AFTER THE ELEVATION of this part of
the valley above the level of the sea. The valley here is almost
blocked up by a pile about one thousand feet in thickness, formed, as
far as I could judge, from three sides, entirely, or at least in chief
part, of gravel and detritus. On the south side, the river has cut
quite through this mass; on the northern side, and on the very summit,
deep ravines, parallel to the line of the valley, are worn, as if the
drainage from the valley above had passed by these two lines before
following its present course.)
FORMATION OF VALLEYS.
The bulk of solid rock which has been removed in the lower parts of the
valleys of the Cordillera has been enormous. It is only by reflecting
on such cases as that of the gravel beds of Patagonia, covering so many
thousand square leagues of surface, and which, if heaped into a ridge,
would form a mountain-range almost equal to the Cordillera, that the
amount of denudation becomes credible. The valleys within this range
often follow anticlinal but rarely synclinal lines; that is, the strata
on the two sides more often dip from the line of valley than towards
it. On the flanks of the range, the valleys most frequently run neither
along anticlinal nor synclinal axes, but along lines of flexure or
faults: that is, the strata on both sides dip in the same direction,
but with different, though often only slightly different, inclinations.
As most of the nearly parallel ridges which together form the
Cordillera run approximately north and south, the east and west valleys
cross them in zig-zag lines, bursting through the points where the
strata have been least inclined. No doubt the greater part of the
denudation was affected at the periods when tidal- creeks occupied the
valleys, and when the outer flanks of the mountains were exposed to the
full force of an open ocean. I have already alluded to the power of the
tidal action in the channels connecting great bays; and I may here
mention that one of the surveying vessels in a channel of this kind,
though under sail, was whirled round and round by the force of the
current. We shall hereafter see, that of the two main ridges forming
the Chilean Cordillera, the eastern and loftiest one owes the greater
part of its ANGULAR upheaval to a period subsequent to the elevation of
the western ridge; and it is likewise probable that many of the other
parallel ridges have been angularly upheaved at different periods;
consequently many parts of the surfaces of these mountains must
formerly have been exposed to the full force of the waves, which, if
the Cordillera were now sunk into the sea, would be protected by
parallel chains of islands. The torrents in the valleys certainly have
great power in wearing the rocks; as could be told by the dull rattling
sound of the many fragments night and day hurrying downwards; and as
was attested by the vast size of certain fragments, which I was assured
had been carried onwards during floods; yet we have seen in the lower
parts of the valleys, that the torrents have seldom removed all the
sea-checked shingle forming the terraces, and have had time since the
last elevation in mass only to cut in the underlying rocks, gorges,
deep and narrow, but quite insignificant in dimensions compared with
the entire width and depth of the valleys.
Along the shores of the Pacific, I never ceased during my many and long
excursions to feel astonished at seeing every valley, ravine, and even
little inequality of surface, both in the hard granitic and soft
tertiary districts, retaining the exact outline, which they had when
the sea left their surfaces coated with organic remains. When these
remains shall have decayed, there will be scarcely any difference in
appearance between this line of coast-land and most other countries,
which we are accustomed to believe have assumed their present features
chiefly through the agency of the weather and fresh-water streams. In
the old granitic districts, no doubt it would be rash to attribute all
the modifications of outline exclusively to the sea-action; for who can
say how often this lately submerged coast may not previously have
existed as land, worn by running streams and washed by rain? This
source of doubt, however, does not apply to the districts superficially
formed of the modern tertiary deposits. The valleys worn by the sea,
through the softer formations, both on the Atlantic and Pacific sides
of the continent, are generally broad, winding, and flat-bottomed: the
only district of this nature now penetrated by arms of the sea, is the
island of Chiloe.
Finally, the conclusion at which I have arrived, with respect to the
relative powers of rain and sea water on the land, is, that the latter
is far the most efficient agent, and that its chief tendency is to
widen the valleys; whilst torrents and rivers tend to deepen them, and
to remove the wreck of the sea’s destroying action. As the waves have
more power, the more open and exposed the space may be, so will they
always tend to widen more and more the mouths of valleys compared with
their upper parts: hence, doubtless, it is, that most valleys expand at
their mouths,—that part, at which the rivers flowing in them, generally
have the least wearing power.
When reflecting on the action of the sea on the land at former levels,
the effect of the great waves, which generally accompany earthquakes,
must not be overlooked: few years pass without a severe earthquake
occurring on some part of the west coast of South America; and the
waves thus caused have great power. At Concepcion, after the shock of
1835, I saw large slabs of sandstone, one of which was six feet long,
three in breadth, and two in thickness, thrown high up on the beach;
and from the nature of the marine animals still adhering to it, it must
have been torn up from a considerable depth. On the other hand, at
Callao, the recoil-wave of the earthquake of 1746 carried great masses
of brickwork, between three and four feet square, some way out seaward.
During the course of ages, the effect thus produced at each successive
level, cannot have been small; and in some of the tertiary deposits on
this line of coast, I observed great boulders of granite and other
neighbouring rocks, embedded in fine sedimentary layers, the
transportal of which, except by the means of earthquake-waves, always
appeared to me inexplicable.
SUPERFICIAL SALINE DEPOSITS.
This subject may be here conveniently treated of: I will begin with the
most interesting case, namely, the superficial saline beds near Iquique
in Peru. The porphyritic mountains on the coast rise abruptly to a
height of between one thousand nine hundred and three thousand feet:
between their summits and an inland plain, on which the celebrated
deposit of nitrate of soda lies, there is a high undulatory district,
covered by a remarkable superficial saliferous crust, chiefly composed
of common salt, either in white, hard, opaque nodules, or mingled with
sand, in this latter case forming a compact sandstone. This saliferous
superficial crust extends from the edge of the coast-escarpment, over
the whole face of the country; but never attains, as I am assured by
Mr. Bollaert (long resident here) any great thickness. Although a very
slight shower falls only at intervals of many years, yet small
funnel-shaped cavities show that the salt has been in some parts
dissolved. (It is singular how slowly, according to the observations of
M. Cordier on the salt-mountain of Cardona in Spain “Ann. des Mines,
Translation of Geolog. Mem.” by De la Beche page 60, salt is dissolved,
where the amount of rain is supposed to be as much as 31.4 of an inch
in the year. It is calculated that only five feet in thickness is
dissolved in the course of a century.) In several places I saw large
patches of sand, quite moist, owing to the quantity of muriate of lime
(as ascertained by Mr. T. Reeks) contained in them. From the compact
salt- cemented sand being either red, purplish, or yellow, according to
the colour of the rocky strata on which it rested, I imagined that this
substance had probably been derived through common alluvial action from
the layers of salt which occur interstratified in the surrounding
mountains (“Journal of Researches” page 444 first edition.): but from
the interesting details given by M. d’Orbigny, and from finding on a
fresh examination of this agglomerated sand, that it is not irregularly
cemented, but consists of thin layers of sand of different tints of
colour, alternating with excessively fine parallel layers of salt, I
conclude that it is not of alluvial origin. M. d’Orbigny observed
analogous saline beds extending from Cobija for five degrees of
latitude northward, and at heights varying from six hundred to nine
hundred feet (“Voyage” etc. page 102. M. d’Orbigny found this deposit
intersected, in many places, by deep ravines, in which there was no
salt. Streams must once, though historically unknown, have flowed in
them; and M. d’Orbigny argues from the presence of undissolved salt
over the whole surrounding country, that the streams must have arisen
from rain or snow having fallen, not in the adjoining country, but on
the now arid Cordillera. I may remark, that from having observed ruins
of Indian buildings in absolutely sterile parts of the Chilian
Cordillera (“Journal” 2nd edition page 357), I am led to believe that
the climate, at a time when Indian man inhabited this part of the
continent, was in some slight degree more humid than it is at
present.): from finding recent sea- shells strewed on these saliferous
beds, and under them, great well-rounded blocks, exactly like those on
the existing beach, he believes that the salt, which is invariably
superficial, has been left by the evaporation of the sea-water. This
same conclusion must, I now believe, be extended to the superficial
saliferous beds of Iquique, though they stand about three thousand feet
above the level of the sea.
Associated with the salt in the superficial beds, there are numerous,
thin, horizontal layers of impure, dirty-white, friable, gypseous and
calcareous tuffs. The gypseous beds are very remarkable, from abounding
with, so as sometimes to be almost composed of, irregular concretions,
from the size of an egg to that of a man’s head, of very hard, compact,
heavy gypsum, in the form of anhydrite. This gypsum contains some
foreign particles of stone; it is stained, judging from its action with
borax, with iron, and it exhales a strong aluminous odour. The surfaces
of the concretions are marked by sharp, radiating, or bifurcating
ridges, as if they had been (but not really) corroded: internally they
are penetrated by branching veins (like those of calcareous spar in the
septaria of the London clay) of pure white anhydrite. These veins might
naturally have been thought to have been formed by subsequent
infiltration, had not each little embedded fragment of rock been
likewise edged in a very remarkable manner by a narrow border of the
same white anhydrite: this shows that the veins must have been formed
by a process of segregation, and not of infiltration. Some of the
little included and CRACKED fragments of foreign rock are penetrated by
the anhydrite, and portions have evidently been thus mechanically
displaced: at St. Helena, I observed that calcareous matter, deposited
by rain water, also had the power to separate small fragments of rock
from the larger masses. (“Volcanic Islands” etc. page 87.) I believe
the superficial gypseous deposit is widely extended: I received
specimens of it from Pisagua, forty miles north of Iquique, and
likewise from Arica, where it coats a layer of pure salt. M. d’Orbigny
found at Cobija a bed of clay, lying above a mass of upraised recent
shells, which was saturated with sulphate of soda, and included thin
layers of fibrous gypsum. (“Voyage Geolog.” etc. page 95.) These widely
extended, superficial, beds of salt and gypsum, appear to me an
interesting geological phenomenon, which could be presented only under
a very dry climate.
The plain or basin, on the borders of which the famous bed of nitrate
of soda lies, is situated at the distance of about thirty miles from
the sea, being separated from it by the saliferous district just
described. It stands at a height of 3,300 feet; its surface is level,
and some leagues in width; it extends forty miles northward, and has a
total length (as I was informed by Mr. Belford Wilson, the
Consul-General at Lima) of 420 miles. In a well near the works,
thirty-six yards in depth, sand, earth, and a little gravel were found:
in another well, near Almonte, fifty yards deep, the whole consisted,
according to Mr. Blake, of clay, including a layer of sand two feet
thick, which rested on fine gravel, and this on coarse gravel, with
large rounded fragments of rock. (See an admirable paper “Geological
and Miscellaneous Notices of Tarapaca” in “Silliman’s American Journal”
volume 44 page 1.) In many parts of this now utterly desert plain,
rushes and large prostrate trees in a hardened state, apparently
Mimosas, are found buried, at a depth from three to six feet; according
to Mr. Blake, they have all fallen to the south-west. The bed of
nitrate of soda is said to extend for forty to fifty leagues along the
western margin of the plain, but is not found in its central parts: it
is from two to three feet in thickness, and is so hard that it is
generally blasted with gunpowder; it slopes gently upwards from the
edge of the plain to between ten and thirty feet above its level. It
rests on sand in which, it is said, vegetable remains and broken shells
have been found; shells have also been found, according to Mr. Blake,
both on and in the nitrate of soda. It is covered by a superficial mass
of sand, containing nodules of common salt, and, as I was assured by a
miner, much soft gypseous matter, precisely like that in the
superficial crust already described: certainly this crust, with its
characteristic concretions of anhydrite, comes close down to the edge
of the plain.
The nitrate of soda varies in purity in different parts, and often
contains nodules of common salt. According to Mr. Blake, the proportion
of nitrate of soda varies from 20 to 75 per cent. An analysis by Mr. A.
Hayes, of an average specimen, gave:—