Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle, Vol. 3 [of 3] · Charles Darwin
PATAGONIA.
The guanacoes appear to have favourite spots for dying in. On the banks of the St. Cruz, the ground was actually white with bones, in certain circumscribed spaces, which were generally bushy and all near the river. On one such spot I counted between ten and twenty heads. I particularly examined the bones; they did not appear, as some scattered ones which I had seen, gnawed or broken, as if dragged together by beasts of prey. The animals in most cases, must have crawled, before dying, beneath and amongst the bushes. Mr. Bynoe informs me that during the last voyage, he observed the same circumstance on the banks of the Rio Gallegos. I do not at all understand the reason of this, but I may observe, that the wounded guanacoes at the St. Cruz, invariably walked towards the river. At St. Jago in the Cape de Verd islands I remember having seen in a retired ravine a corner under a cliff, where numerous goats’ bones were collected: we at the time exclaimed, that it was the burial-ground of all the goats in the island. I mention these trifling circumstances, because in certain cases they might explain the occurrence of a number of uninjured bones in a cave, or buried under alluvial accumulations; and likewise the cause, why certain mammalia are more commonly embedded than others in sedimentary deposits. Any great flood of the St. Cruz., would wash down many bones of the guanaco, but probably not a single one of the puma, ostrich, or fox. I may also observe, that almost every kind of waterfowl when wounded takes to the shore to die; so that the remains of birds, from this cause alone and independently of other reasons, would but rarely be preserved in a fossil state.
One day the yawl was sent under the command of Mr. Chaffers with three days’ provisions to survey the upper part of the harbour. In the morning we searched for some watering-places, mentioned in an old Spanish chart. We found one creek, at the head of which there was a trickling rill (the first we had seen) of brackish water. Here the tide compelled us to wait several hours; and in the interval I walked some miles into the interior. The plain as usual, consisted of gravel, mingled with soil resembling chalk in appearance, but very different from it in nature. From the softness of these materials it was worn into many gullies. There was not a tree, and excepting the guanaco, which stood on the hill-top a watchful sentinel over its herd, scarcely an animal or a bird. All was stillness and desolation. One reflected how many ages the plain had thus lasted, and how many more it was doomed thus to continue. Yet in passing over these scenes, without one bright object near, an ill-defined but strong sense of pleasure is vividly excited.
In the evening we sailed a few miles further up, and then pitched the tents for the night. By the middle of the next day, the yawl was aground, and from the shoalness of the water could not proceed any higher. The water being found partly fresh Mr. Chaffers took the dingey, and went up two or three miles further, where she also grounded, but in a fresh-water river. The water was muddy, and though the stream was most insignificant in size, it would be difficult to account for its source, except from the melting snow on the Cordillera. At the spot where we bivouacked, we were surrounded by bold cliffs and steep pinnacles of porphyry. I do not think I ever saw a spot, which appeared more secluded from the rest of the world, than this rocky crevice in the wide plain.
The second day after our return to the anchorage, a party of officers and myself went to ransack an old Indian grave, which I had found on the summit of a neighbouring hill. Two immense stones, each probably weighing at least a couple of tons, had been placed in front of a ledge of rock, about six feet high. At the bottom of the grave on the hard rock, there was a layer of earth about a foot deep, which must have been brought up from the plain below. Above it a pavement of flat stones was placed, on which others were piled, so as to fill up the space between the ledge and the two great blocks. To complete the grave, the Indians had contrived to detach from the same ledge a huge fragment, and to throw it over the pile so as to rest on the two blocks. We undermined the grave on both sides, but could not find any relics, or even bones. The latter probably had decayed long since (in which case the grave must have been of extreme antiquity), for I found in another place some smaller heaps, beneath which a very few crumbling fragments could yet be distinguished, as having belonged to a man. Falconer states, that where an Indian dies he is buried, but that subsequently his bones are carefully taken up and carried, let the distance be ever so great, to be deposited near the sea-coast. This custom, I think, may be accounted for, by recollecting that before the introduction of horses, these Indians must have led nearly the same life as the Fuegians, and therefore generally resided in the neighbourhood of the sea. The common prejudice of lying where one’s ancestors have lain, would make the now roaming Indians bring the less perishable part of their dead to their ancient burial-grounds.
JANUARY 9TH, 1834.--Before it was dark the Beagle anchored in the fine spacious harbour of Port St. Julian, situated about one hundred and ten miles to the south of Port Desire. We remained here eight days. The country is nearly similar to that of Port Desire, but, perhaps, rather more sterile. One day a party accompanied Captain FitzRoy on a long walk round the head of the harbour. We were eleven hours without tasting any water, and some of the party were quite exhausted. From the summit of a hill (since well named Thirsty Hill) a fine lake was spied, and two of the party proceeded with concerted signals to show whether it was fresh water. What was our disappointment to find a snow-white expanse of salt, crystallized in great cubes! We attributed our extreme thirst to the dryness of the atmosphere; but whatever the cause might be, we were exceedingly glad late in the evening to get back to the boats. Although we could nowhere find, during our whole visit, a single drop of fresh water, yet some must exist; for by an odd chance I found on the surface of the salt water, near the head of the bay, a Colymbetes not quite dead, which in all probability had lived in some not far distant pool. Three other kinds of insects,--a Cincindela, like _hybrida_, Cymindis, and a Harpalus, which all live on muddy flats occasionally overflowed by the sea, and one other beetle found dead on the plain,--completes the list of coleoptera. A good-sized fly (Tabanus) was extremely numerous, and tormented us by its painful bite. The common horsefly, which is so troublesome in the shady lanes of England, belongs to this genus. We here have the puzzle, that so frequently occurs in the case of musquitoes; on the blood of what animals do these insects commonly feed? The guanaco is nearly the only warmblooded quadruped, and they are present in numbers quite inconsiderable, compared to the multitude of flies.
The foundation of porphyry is not here present, as it was at Port Desire, and in consequence the tertiary deposits are arranged with greater regularity. Five successive plains of different altitudes are very distinct. The lower one is a mere fringe nearly on a level with the sea, but the upper one is elevated 950 feet. This latter is represented in this neighbourhood only by a few truncate conical hills, of exactly the same height. It was very interesting to stand on one of these flat patches of gravel, and viewing the wide surrounding country, to speculate on the enormous quantity of matter which must have been removed, thus to leave these mere points, as measures of the former table-land. I will now give a brief sketch of the geology of the grand tertiary formation of Patagonia, which extends from the Strait of Magellan to the Bay of S. Antonio. In Europe, deposits of the more recent eras have generally been accumulated in small basins or trough-shaped hollows. In South America, however, the entire plains of Patagonia extending seven hundred miles in length, and backed on the one hand by the chain of the Andes, and fronted on the other by the shores of the Atlantic, are thus constituted. Moreover the northern boundary is merely assumed in consequence of a mineralogical change in the strata: if organic remains were present, it probably would be found to be only an artificial limit. Again to the northward (1300 miles distant from the Strait of Magellan) we have the Pampas deposit, which though very different in composition, belongs to the same epoch with the superficial covering of the plains of Patagonia.
The cliffs on the coast give the following section: The lower part consists of a soft sandstone, containing large concretions of a harder nature. These strata contain many organic remains--immense oysters nearly a foot in diameter, curious pectens, echini, turritellæ, and other shells, of which the greater portion are extinct, but a few resemble those now existing on the coast.[10] Above these fossiliferous beds, a mass of soft friable stone or earth is superimposed, which, from its extreme whiteness, has been mistaken for chalk. It is, however, quite different; and closely resembles the less argillaceous varieties of decomposed felspar. This substance never contains organic remains. Lastly, the cliff is surmounted by a thick bed of gravel, almost exclusively derived from porphyritic rocks. For the sake of making the following description more easily intelligible, I have subjoined an imaginary section of the plains near the coast. It must be observed, that the width of each plain is in nature _very much_ greater in proportion to the height, than here represented. The whole series is horizontally stratified, and I do not recollect ever seeing any signs of violence, not even such as a fault. The gravel covers the entire surface of the land, from the Rio Colorado to the Strait of Magellan, a space of 800 miles, and is one chief cause of the desert character of Patagonia. Judging from a section across the continent at the St. Cruz river, and from some other reasons, I believe the gravel beds gradually thickening as they ascend, every where reach the base of the Cordillera. It is to these mountains we must look for the parent rocks, of at least a large portion of the well-rounded fragments. I apprehend so great an area covered by shingle, could scarcely be pointed out in any other part of the world.
Having said thus much of the constitution, let us look at the external configuration of the mass. The level plains are cut off along the whole line of coast by perpendicular cliffs, which are necessarily of different altitudes, because any one of the successive terraces, which, as I have already noticed, rise like steps one above the other, may form the sea cliff. These steps are often several miles broad; but from one point of view I have seen four very distinct lines of escarpment abutting one over the other. Having observed that the plains appeared to run for great distances along the coast at the same level, I measured barometrically the elevation of some of them, and compared these measurements, and took all those made by the officers employed in the survey. I was astonished to find at how great distances, even of 600 miles, plains occurred that had, within a few feet of difference, the same elevation. I believe I can distinguish seven or eight distinct terraces which occur along the line of coast, and which include heights between 1200 feet and the level of the sea. It will be understood that they are not always present, for the lower ones have in some parts been removed by the action of the sea sooner than in others. When any broad valley enters the country, the terraces sweep round and run up on each side; in which case, the correspondence on the opposite sides is beautifully illustrated.
I have called these step-like plains level, because they appear to be absolutely so to the eye, but in truth they rise a little between the edge of one line of cliff and the base of the next above it. Their slope is about the same as that of the gradually shoaling bottom of the neighbouring sea. The elevation of 350 feet is gained by three steps; one of about 100 feet, the second 250, and the third 350. Over these three plains marine remains are frequently scattered, but they are especially abundant on the lower one. The shells are the same as the now existing littoral species, and the muscle and turbo yet partially retain their blue and purple colours. We have now stated the problem, which is to be explained so as to connect together these various phenomena. At first I could only understand the grand covering of gravel, by the supposition of some epoch of extreme violence, and the successive lines of cliff, by as many great elevations, the precise action of which I could not however follow out. Guided by the “Principles of Geology,” and having under my view the vast changes going on in this continent, which at the present day seems the great workshop of nature, I came to another, and I hope more satisfactory conclusion. The importance of any view which may explain the agency by which such vast beds of shingle have been transported over the surface of the successive plains, cannot be doubted. Whatever the cause may have been, it has determined the condition of this desert country, with respect to its form, nature, and capabilities of supporting life.
There are proofs, that the whole coast has been elevated to a considerable height within the recent period; and on the shores of the Pacific, where successive terraces likewise occur, we know that these changes have latterly been very gradual. There is indeed reason for believing, that the uplifting of the ground during the earthquakes in Chile, although only to the height of two or three feet, has been a disturbance which may be considered as a great one, in comparison to the series of lesser and scarcely sensible movements which are likewise in progress. Let us then imagine the consequence of the shoaling bed of an ocean, elevated at a perfectly equable rate, so that the same number of feet should be converted into dry land in each succeeding century. Every part of the surface would then have been exposed for an equal length of time to the action of the beach-line, and the whole in consequence equally modified. The shoaling bed of the ocean would thus be changed into a sloping land, with no marked line on it. If, however, there should occur a long period of repose in the elevations, and the currents of the sea should tend to wear away the land (as happens along this whole coast), then there would be formed a line of cliff. Accordingly as the repose was long, so would be the quantity of land consumed, and the consequent height of such cliffs. Let the elevations recommence, and another sloping bank (of shingle, or sand, or mud, according to the nature of the successive beach-lines) must be formed, which again will be broken by as many lines of cliff, as there shall be periods of rest in the action of the subterranean forces. Now this is the structure of the plains of Patagonia; and such gradual changes harmonize well with the undisturbed strata, extending over so many hundred miles.
I must here observe, that I am far from supposing that the entire coast of this part of the continent has ever been lifted up, to the height of even a foot, at any one moment of time; but, drawing our analogies from the shores of the Pacific, that the whole may have been insensibly rising, with every now and then a paroxysmal or accelerated movement in certain spots. With respect to the alternation of the periods of such continued rise and those of quiescence, we may grant that they are probable, because such alternation agrees with what we see in the action, not only of a single volcano, but likewise of the disturbances affecting whole regions of the earth. At the present day, to the north of the parallel 44°, the subterranean forces are constantly manifesting their power over a space of more than one thousand miles. But to the southward of that line, as far as Cape Horn, an earthquake is seldom or never experienced, and there is not a single point of eruption; yet in former ages, as we shall hereafter show, deluges of lava flowed from that very part. It is in conformity with our hypothesis that this southern region of repose, is at present suffering from the inroads of the ocean, as attested by the long line of cliff on the Patagonian coast. Such we believe to have been the causes of this singular configuration of the land. Nevertheless, we confess that it at first appears startling, that the most marked intervals between the heights of the successive plains should, instead of some great and sudden action of the subterranean forces, only indicate a longer period of repose.
In explaining the widely-spread bed of gravel, we must first suppose a great mass of shingle to be collected by the action of innumerable torrents, and the swell of an open ocean, at the submarine basis of the Andes, prior to the elevation of the plains of Patagonia. If such a mass should then be lifted up, and left exposed during one of the periods of subterranean repose; a certain breadth, for instance a mile, would be washed down, and spread out over the bottom of the invading waters. (That the sea near the coast can carry out pebbles, we may feel sure from the circumstance of their gradual decrease in size, according to the distance from the coast-line.) If this part of the sea should now be elevated, we should have a bed of gravel, but it would be of less thickness than in the first mass, both because it is spread over a larger area, and because it has been much reduced by attrition. This process being repeated, we might carry beds of gravel, always decreasing in thickness (as happens in Patagonia) to a considerable distance from the line of parent rock.[11] For instance, on the banks of the St. Cruz at the distance of one hundred miles above the mouth of the river, the bed of gravel is 212 feet thick, whereas, near the coast, it seldom exceeds 25 or 30 feet; the thickness being thus reduced to nearly one-eighth.
I have already stated that the gravel is separated from the fossiliferous strata by some white beds of a friable substance, singularly resembling chalk, but which cannot be compared, as far as I am aware, with any formation in Europe. With respect to its origin, I may observe that the well-rounded pebbles all consist of various felspathic porphyries; and that, from their prolonged attrition, during the successive remodellings of the whole mass, much sediment must have been produced. I have already remarked that the white earthy matter more closely resembles decomposed felspar, than any other substance. If such is its origin, it would always, from its lightness, be carried further to seaward than the pebbles. But as the land was elevated, the beds would be brought nearer the coast-line, and so become covered by the fresh masses of gravel which were travelling outwards. When these white beds were themselves elevated, they would hold a position intermediate between the gravel and the common foundation, or the fossiliferous strata. To explain my meaning more clearly, let us suppose the bottom of the present sea covered to a certain distance from the coast-line, with pebbles gradually decreasing in size, and beyond it by the white sediment. Let the land rise, so that the beach-line, by the fall of the water, may be carried outwards; then likewise the gravel, by the same agency as before, will be transported so much further from the coast, and will cover the white sediment, and these beds again will invade the more distant parts of the bottom of the sea. By this outward progress, the order of superposition must always be gravel, white sediment, and the fossiliferous strata.
Such is the history of the changes by which the present condition of Patagonia has, I believe, been determined. These changes all result from the assumption of a steady but very gradual elevation, extending over a wide area, and interrupted at long intervals by periods of repose. But we must now return to Port St. Julian. On the south side of the harbour, a cliff of about ninety feet in height intersects a plain constituted of the formations above described; and its surface is strewed over with recent marine shells. The gravel, however, differently from that in every other locality, is covered by a very irregular and thin bed of a reddish loam, containing a few small calcareous concretions. The matter somewhat resembles that of the Pampas, and probably owes its origin either to a small stream having formerly entered the sea at that spot, or to a mud-bank similar to those now existing at the head of the harbour. In one spot this earthy matter filled up a hollow, or gully, worn quite through the gravel, and in this mass a group of large bones was embedded. The animal to which they belonged, must have lived, as in the case at Bahia Blanca, at a period long subsequent to the existence of the shells now inhabiting the coast.
We may feel sure of this, because the formation of the lower terrace or plain, must necessarily have been posterior to those above it, and on the surface of the two higher ones, sea-shells of recent species are scattered. From the small physical change, which the last one hundred feet elevation of the continent could have produced, the climate, as well as the general condition of Patagonia, probably was nearly the same, at the time when the animal was embedded, as it now is. This conclusion is moreover supported by the identity of the shells belonging to the two ages. Then immediately occurred the difficulty, how could any large quadruped have subsisted on these wretched deserts in lat. 49° 15′? I had no idea at the time, to what kind of animal these remains belonged. The puzzle, however, was soon solved when Mr. Owen examined them; for he considers that they formed part of an animal allied to the guanaco or llama, but fully as large as the true camel. As all the existing members of the family of Camelidæ are inhabitants of the most sterile countries, so may we suppose was this extinct kind. The structure of the cervical vertebræ, the transverse processes not being perforated for the vertebral artery, indicates its affinity: some other parts, however, of its structure, probably are anomalous.
The most important result of this discovery, is the confirmation of the law that existing animals have a close relation in form with extinct species. As the guanaco is the characteristic quadruped of Patagonia, and the vicuna of the snow-clad summits of the Cordillera, so in bygone days, this gigantic species of the same family must have been conspicuous on the southern plains. We see this same relation of type between the existing and fossil Ctenomys, between the capybara (but less plainly, as shown by Mr. Owen) and the gigantic Toxodon; and lastly, between the living and extinct Edentata. At the present day, in South America, there exist probably nineteen species of this order, distributed into several genera; while throughout the rest of the world there are but five. If, then, there is a relation between the living and the dead, we should expect that the Edentata would be numerous in the fossil state. I need only reply by enumerating the megatherium, and the three or four other great species, discovered at Bahia Blanca; the remains of some of which are also abundant over the whole immense territory of La Plata. I have already pointed out the singular relation between the armadilloes and their great prototypes, even in a point apparently of so little importance as their external covering. The order of rodents at the present day, is most conspicuous in South America, on account of the vast number[12] and size of the species, and the multitude of individuals: according to the same law, we should expect to find their representatives in a fossil state. Mr. Owen has shown how far the Toxodon is thus related; and it is moreover not improbable that another large animal has likewise a similar affinity.
Text from Project Gutenberg, public domain in the United States.