Geological Observations on South America Part 4

With respect to the position in which the sh.e.l.ls 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 sh.e.l.ls 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 sh.e.l.ls. 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 sh.e.l.ls 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 sh.e.l.ls 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 sh.e.l.l which, as I am informed by Mr. c.u.ming, inhabits sandbanks at the level of the lowest tides. But even in the case of sh.e.l.ls 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 acc.u.mulations, 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 sh.e.l.ls, which together covered a s.p.a.ce 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 a.s.serted, 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 acc.u.mulation 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 a.s.sured 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 s.p.a.ce I observed innumerable ma.s.ses of upraised sh.e.l.ls. About Quintero there are immense acc.u.mulations (worked for lime) of the Mesodesma donaciforme, packed in sandy earth; they abound chiefly about fifteen feet above high-water, but sh.e.l.ls 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 a.s.sured that there are many sh.e.l.ls 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 sh.e.l.ls, 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 ma.s.ses 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 sh.e.l.ls. 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 sh.e.l.ls. 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 sh.e.l.ls: 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 sh.e.l.ls. It lies on, and sends wedge-like veins into, a much more friable, calcareous, tuff-like variety; and both rest on a ma.s.s about twenty feet in thickness, formed of fragments of recent sh.e.l.ls, 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 ma.s.s has originated in the decay of minutely comminuted sea-sh.e.l.ls 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 sh.e.l.ls which have since been gradually dissolved by water percolating through the ma.s.s. (I have incidentally described this rock in the above work on Volcanic Islands.)

The sh.e.l.ls 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 sh.e.l.ls 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 sh.e.l.ls must have lived.

(DIAGRAM 8.--SECTION OF PLAIN OF COQUIMBO.

Section through Plain B-B and Ravine A.

Surface of plain 252 feet above sea.

A. Stratified sand, with recent sh.e.l.ls in same proportions as on the beach, half filling up a ravine.

B. Surface of plain, with scattered sh.e.l.ls in nearly same proportions as on the beach.

C. Upper calcareous bed, and D. Lower calcareous sandy bed (Losa), both with recent sh.e.l.ls, 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 sh.e.l.ls.)

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 sh.e.l.ls, 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 ma.s.s of stratified, slightly agglutinated sand, which in some places covers up the lower half of the seaward escarpment of the plain, the included sh.e.l.ls 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 sh.e.l.ls 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 sh.e.l.ls 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 sh.e.l.ls, 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. Na.s.sa (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 sh.e.l.l are rather stronger than in the recent specimens of this species from Coquimbo. (G.B. Sowerby.) 10. Bala.n.u.s (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 sh.e.l.ls had not formerly been brought up by man, if I had not found one very small ma.s.s 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 sh.e.l.l. This is only one instance out of many, proving that the absence of sea-sh.e.l.ls 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 sh.e.l.ls 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 acc.u.mulation 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 sh.e.l.ls.

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 quant.i.ty of shingle, which at some former period has been brought down the great valley of Coquimbo.

Considering the many sh.e.l.ls 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 sh.e.l.ls 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 a.n.a.logical 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.