Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle, Vol. 3 [of 3] · Charles Darwin
ADDENDA.
Page 76. I have said that crystals of sulphate of magnesia, called by the Spanish inhabitants, _madre del sal_, are scattered on the borders of the Salinas in Patagonia. By a mistake a wrong bottle was examined; and the crystals, I now find, are the sulphate of soda; but it appears that some sulphate of magnesia is dissolved in the underlying fetid mud. Page 96. In enumerating the fossils, which I collected at Bahia Blanca, I mention a tusk like that of a boar, and some very flat grinders. These are now found to belong to the lower jaw of _Toxodon Platensis_. For an admirable description of this wonderful animal, and likewise of the great fossil (_Macrauchenia Patachonica_), mentioned at p. 208, which in some respects is allied to the Camelidæ, I must refer to the first part of the Zoology of the Voyage of the Beagle by Mr. Owen. Page 97. The remains just referred to, are said to be embedded with existing species of shells. This expression requires some qualification; the details are given in my geological introduction to Mr. Owen’s account of the Fossil Mammalia, above mentioned. Page 150.
With reference to what I have said, about the bones of horses having been several times brought to this country from North America, with those of the Mastodon, I see Mr. Rogers, in his Report to the British Association (vol. iii., p. 24) says, “It is possible that the horse ought to be added to this list of animals as indigenous to America;” and it is evident from a previous passage, that the bones have been found fossil. Mr. Rogers states, that the remains of two kinds of elephant and three kinds of oxen have been discovered there; as have, on two occasions, parts of the Megatherium. At Big Bone Lick, where the remains of the elephant, mastodon, and ox, are so extraordinarily numerous, the megalonyx has been found; and this is a parallel case to the contemporaneous embedment, in the southern hemisphere, of the mastodon, horse, megatherium, and the other Edentata. The more I reflect on the geographical distribution in the Old and New World of these gigantic mammifers, during the period antecedent to the present, in relation to the existing faunas of North and South America, now so strongly contrasted with each other, the more pregnant with interest the case appears to be.
I know of no other instance, in which we can thus almost mark the period of the splitting up of one great region into two well-characterized zoological provinces. With respect to the ancient range of the genus Equus, I may add to what has already been said, that its remains have been found from England in the west, to the Himalaya in the east, (_Buckland’s Reliquiæ Diluvianæ_, p. 222,) and from the western coast of North America, to the eastern plains of America in the southern hemisphere. We may, therefore, suspect that a very little research would discover the remains of the horse, embedded in the frozen soil of Kamtschatka, with those of the fossil ox and elephant; and thus render complete the evidence, that we there see the ancient, but perhaps temporary line of junction, since interrupted, between the fauna of what we call the New World with that of the Old. But I doubt not, that the snow-clad heights of Chimborazo, Illimani and Aconcagua have seen as many, and as strange forms of animals, pass by and become extinct, as ever did the Alpine pinnacles, or those loftiest ones of the Himalaya. Page 268. When contrasting the productions of the eastern coast of South America, with those of the western, and likewise with those of the corresponding parallels of latitude in Europe, I should have added (line 15) after the grape and fig, as flourishing in lat. 41°, the peach, and the nectarine (both of course standards), water and musk melons, batatas dulces (_Convolvulus batatas_), the olive and the orange; the latter, however, had only been lately introduced, but it promised to succeed well.
Page 272. I have spoken of the low latitudes in which tropical forms of vegetation are found in the southern hemisphere, and likewise of some mammalia and of birds. With respect to the parrot of the Strait of Magellan, Macquarrie Island in lat. 55° S. and long. 160° E. offers an analogous instance, in possessing a species of this genus. I am, however, now enabled to bring forward a more important observation, as directly bearing on the evidence by which geologists have chiefly judged of the climate of ancient Europe, namely, on the character of the marine productions of the southern hemisphere. In my journal I have remarked that the southern seas teem with life, under innumerable forms; and the truth of this remark is amply attested by the vast herds of great unwieldy seals with which the shores of Patagonia, the Falkland and the Antarctic islands, were, according to the narratives of all the early navigators, almost covered.
Having mentioned these facts to Mr. George B. Sowerby, he informs me, that the shells of the southern part of the southern hemisphere have some affinity in general character with those of the intertropical seas, or rather that they are of much larger size and of more vigorous growth than the analogous species (excepting the chitons of California) under corresponding zones in the northern hemisphere. Thus the immense size of the Patellæ, Fissurellæ, Chitons, and Barnacles of the Strait of Magellan, and the large size of the former at the Cape of Good Hope, may be taken as instances. On the east coast of South America (in lat. 39°) three species of Oliva (one of large size), a Voluta (and perhaps a second species), and a Terebra, are amongst the _most abundant_ shells on the mud-banks of Bahia Blanca. Another species of Voluta is found as far south as 45°, and there is some reason to believe, even much further. Oliva, Voluta, and Terebra are amongst the best-characterized tropical forms, that is, both the individuals and species of these genera are extremely abundant in the intertropical seas, whilst they are very rare, or do not exist, on the shores of temperate countries. It is doubtful even if one small species of these three genera, an Oliva, inhabits the southern shores of Europe; whilst in a _higher_ latitude, on the coast of South America, species of all three are the most _abundant_ kinds.
At Bahia Blanca many of these shells are embedded in gravel, and have been raised above the level of the sea. Now let us suppose that the climate of South America and of the surrounding seas were to undergo some change, so as to become in every respect like that of Europe; it can, I think, scarcely be doubted, that the shells of the above-mentioned genera would gradually become extinct, and be replaced by others better adapted to the new climate. What, then, would a geologist say, who entertained the generally-received opinions on the distribution of organic beings in accordance with our knowledge of the northern hemisphere (or, rather, in this imaginary case of both hemispheres), when he found these gravel beds abounding with Olivas, Volutas, and Terebras?--such shells not existing there. We may also suppose that he had discovered that the limits of the more tropical forms, both animal and vegetable, of the productions of the land, had likewise during this former period extended further south: what, then, would he say? Would he not at once infer, with the strongest appearance of truth, that the climate formerly had a more tropical character, properly so called, and therefore had a higher annual mean temperature than at present? Nevertheless, we know such an inference would have been absolutely erroneous.
To put the case in another point of view: should a geologist find, in lat. 39°, on the coast of Spain, a tertiary deposit, abounding with Olivas, Volutas, and Terebras; or in lat. 45°, on the coast of France, other beds, containing a large Voluta, and numerous Patellæ, Fissurellæ, Chitons, and Balani, larger and of stronger growth than the existing species, would he be justified, after what is now known, in pronouncing that the climate formerly had a higher mean temperature? I think it may be safely asserted that he would not be so justified, but rather would be bound to search for other evidence. In the actual case of Europe, we have knowledge (as will be shown in a following note[1]) of another element in the problem, namely of the lower descent during former times of the snow-line,--as is inferred from the former low descent of glaciers, on the same mountains, where they now occur only at great heights, and likewise from the congelation during this same period of the soil in a low latitude--and this new element, I believe, gives the key to the solution of the problem, which is, that the climate of Europe was formerly more equable, but so far from being strictly more tropical, that it probably even had a lower mean annual temperature than it now possesses. I need scarcely say that I here refer _only_ to the later tertiary periods: in the more ancient epochs, the plainest analogies tell us of an equatorial climate, whilst on the other hand, we are very far from having the smallest reason to suppose that the snow-line then descended low; and this is the key, as I have called it, to the problem of later times.
Page 282. With respect to ice transporting fragments of rock in the Antarctic regions, M. Cordier, in his instructions (_L’Institut_, 1837, p. 283) to the voyage of the Astrolabe and the Zélée, has this passage: “Les relations de l’expédition anglo-américaine de découverte exécutée en 1830, nous ont fait connaître que les plages des Nouvelles-Shetland sont couvertes de grands blocs erratiques formés de granite, et par conséquent d’une nature différente des autres roches du pays. M. James Eights, naturaliste de l’expédition, n’hésite pas à considérer ces blocs comme ayant été apportés par les glaces, qui viennent annuellement s’échouer et se fondre sur les plages dont il s’agit et comme étant les indices de terres inconnues situées plus près du pôle que la terre la Trinité.” I have not been able to find any account of this expedition. Lieutenant Kendall describes (_Geograph. Journal_, 1830) pinnacles of syenite in Smith’s Island, one of the South Shetland group; so that the inferences regarding the distances, from which the blocks are supposed to have come, probably are erroneous. In speaking (p. 272) of the rigour of the climate of Deception Island in South Shetland, I might have mentioned that Lieut. Kendall says (_Geograph. Journal_, 1830, p. 66), that on March the 8th, “We took the hint of the freezing over of the cove (lat. 62° 55′) and effected our retreat.” This is the same as if, in the northern hemisphere, the harbour of Christiansund in Norway, were to freeze on the 8th of September!
Page 285. I have described the dimensions of the great glacier which in lat. 46° 50′, sends down an arm to Kelly Harbour, and another to a flat swamp; I now find from information communicated to me by Captain FitzRoy, that it must communicate with the channels and bays northward, which extend behind the peninsula of Tres Montes. Aguëros, in giving an account of an expedition of the missionaries (_Descripcion Historial de la Provincia de Chiloe_, p. 227), says, they encountered in the Laguna de San Rafael (lat. 46° 33′ to 46° 48′) “many icebergs (_muchos farallones de nieve_), some great, some small, and others middle sized.” This was on the 22d of November, 1778. Captain FitzRoy also tells me, that in the account of another missionary voyage, it is said that the boats had difficulty, on account of the islands of ice, in passing through the Caño de Perdon, a strait connecting the Laguna de San Rafael, with the other bays behind Tres Montes. Transposing in imagination, as I have done at p. 291, the places in the southern hemisphere to corresponding ones in Europe, these facts are the same as if, in a channel of the sea stretching from the Mediterranean between the Alps and the Jura, a boat should encounter _in the latitude of the lake of Geneva_, and _on the 22d of June_, (but not on one occasion only,) so many icebergs, and of such dimensions, that the historian of the voyage should describe them as being “some great, some small, and others middle sized”!
Having insisted so strongly, in this part of my Journal, that it is in the southern hemisphere, where tropical forms encroach on the temperate zones, that solid glaciers descend to the sea in low latitudes; I might have added that it is in this same hemisphere, that the icebergs, which have been formed in the Polar Regions, are drifted furthest from their birthplace. Horsburgh (_Philosoph. Transact._, 1830) describes several great icebergs seen by a ship, in her passage to India, in 35° 50′ S.: that is, far to the northward of the latitude, where tree-ferns, arborescent grasses, parasitical orchideous plants, and even palm-trees grow; and within sixty miles of the land, where the rhinoceros, elephant, hippopotamus, lion, and hyena, are very numerous. Page 289.
Until lately I was not aware that there were sufficient data to speak with some precision of the southern limits of erratic blocks in the northern half of the New World. In Canada, and in the northern parts of the United States, innumerable great scattered fragments of rocks have been described by Bayfield, Bigsly, Hitchcock, and others. In parts of Massachusetts, according to Professor Hitchcock (_Report on the Geology of_), boulders seem to cover the whole face of the country. Further southward we hear from Mr. Rogers (_Report to Brit. Assoc._, vol. iii.) that boulders are common over the great valley which crosses Pennsylvania, Maryland, and Virginia (lat. 36° 30′ to 42°): and likewise in the states of Ohio, Kentucky, and Indiana, which are in nearly the same latitude. Mr. Rogers having described some blocks of sandstone at Washington and on the Susquehanna, which must have come from some distance northward, adds that “Drake in his picture of Cincinnati (39° 10′) mentions large masses of granite in that part of the Ohio, resting on the ordinary finer diluvium. The nearest granite to the north is at least one hundred leagues distant; while no primary rock occurs south or east, within even a much greater limit.”
He then proceeds, “We are reminded here of the great detached blocks, which strew the plains of northern Europe, and the explanation suggested, that they have been carried there upon floating ice;” and concludes with the important remark, that Mr. Conrad, who has explored the state of Alabama (30° to 35°) was never once able to perceive a boulder upon its surface. It would hence appear that 36° 30′ is the southern limit of the dispersion of erratic blocks in the United States; and these are spoken of, as having come from the north. Therefore, there is no occasion to suppose that the ice, in which by the theory they are believed to have been embedded, was formed in so low a latitude as that here mentioned; and at present, in the southern hemisphere, icebergs are _drifted_ to latitudes, though not formed in them, nearer the tropic than 36° 30′. In Europe I cannot hear of erratic blocks having been found further south than the southern flanks of the Alps, in lat. 45°; and Humboldt has said (see _Cuvier’s Theory of the Earth, translated by Professor Jameson_, p. 346) that they do not occur in Lombardy.
I may here remark, that care should be taken to separate the phenomenon of great angular blocks, from that of rounded ones, although of considerable size; for torrents, and more especially the waves of the sea, during its slow oscillation of level, are agents sufficiently powerful to produce great effects. The lowest latitude in South America, in which I found large angular fragments, which must have been transported by ice there formed, or by some unknown means, was in latitude 41°. But as I did not examine the country immediately north of it, I am not prepared to say that this is their extreme limit; but between latitude 27° and 33°, I found no appearance, on either side of the Cordillera, which indicated a power of transportation of the kind required to remove boulders from a distance. Thus, we find that the limit of their dispersion in the two Americas is nearly the same; although they approach the warmer zones rather more closely in the northern than in the southern division of the continent, and in both, probably, more so than in Europe.
In the note, in which I have considered the apparent exceptions to the law, that erratic boulders are not found in the intertropical regions, I have said that the internal evidence of the Macao case led me to doubt its reality, and I now find it is distinctly stated by M. Chevalier that the rounded blocks result from the secular disintegration of the fundamental rock (_L’Institut_, 1838, p. 151--_Analysis of the Voyage of the Bonite_). I may here add, that M. Puillon Boblaye, in his description of Bone and Constantine on the northern coast of Africa (_L’Institut_, 1838, p. 248), says, “Je n’ai rien vu que pût indiquer le phénomène des blocs erratiques.” My statement that erratic boulders are not found in Australia, is fully borne out by information communicated to me by Major Mitchell, who, in his repeated expeditions, has traversed so much of the south-east division of that continent. With the several facts given here and in the Journal (p. 289), I can scarcely doubt that the law of the distribution of erratic blocks is finally determined; and it is needless to specify the great, not to say conclusive, importance of this law on the theory of the means of their transportation,--a problem which has so long perplexed geologists. Page 294.
In my discussion on the climate of the southern hemisphere, I have shown that a low altitude of the line of perpetual snow, and consequently the descent of glaciers to the level of the sea in latitudes relatively low to what occurs in the northern hemisphere, and likewise the perpetual congelation of the soil a little beneath the surface in countries without the frigid zone, are the results of a climate which appears favourable to the passage of tropical forms beyond their proper limits, and to a vigorous native vegetation. The climate is one of an equable nature; and this must, to a considerable degree, be the effect of the great area of ocean compared with the land of the southern hemisphere. In the northern hemisphere we have proofs, that the productions both of the land and water, during the period antecedent to the present, had a more tropical character than they now have, and there is, also, a high degree of probability that the proportional area of water was much greater.
If then we judge from the analogy of the southern hemisphere, the first and simple inference from these facts, is, that the temperature of Europe was formerly more equable, though perhaps with a lower mean, than it now is. It may be asked, as a test of this inference, did the snow-line formerly descend lower than it now does? Was the soil formerly frozen a little beneath the surface in a low latitude? The congealed carcasses of the great Pachydermata of Siberia answer the second question; and in my journal, I have indirectly considered the first one as answered, by the fact of the many erratic boulders of Europe having travelled from mountains, situated in regions where great bodies of ice do not at present descend to the level of the sea. For on the theory that these boulders were transported by icebergs from glaciers, which formerly descended into the sea in latitudes where perpetual snow is not now found, or if so, only at great heights, the problem receives so simple a solution, that I did not hesitate, having the other data, to assume, that the snow-line in Europe formerly did descend much lower than it does at present. But, had I studied my subject more attentively, I might have taken a higher ground: in a note, indeed (p. 294), I have stated that according to Professor Esmark, it is certain, that the glaciers of Norway formerly descended to a lower level; and I now found that some time since, Messrs.
Venetz and Charpentier, and more lately M. Agassiz, have incontestably shown, from the presence of glacier-dikes or moraines, and from the polished and scratched surface of the rocks, that in the Alps enormous bodies of ice formerly descended to the borders even of the lake of Geneva, and therefore much lower than the line of present lowest descent.[2] With these several facts it might have been boldly asserted, that the climate of Europe formerly was like that of the southern hemisphere as it now is; and consequently, as we know, that the sea within recent tertiary periods stood at a higher level over a large portion of our continent, it might have been affirmed, had there been no record of the existence of erratic blocks on this side of the globe, that it would be an anomaly, difficult of explanation, should there not be found around the eminences of central and northern Europe great unrolled fragments, scattered at long distances from, their parent sources, and often separated from them by profound valleys.
M. Agassiz has lately (_Address to the Helvetic Society, July 1837, translated in Jameson’s New Philosophical Journal_, vol. xxiii., p. 364, _and in several communications in the French periodical L’Institut_) written on the subject of the glaciers and boulders of the Alps. He clearly proves, as it appears to me, that the presence of the boulders on the Jura cannot be explained by any debacle, or by the power of ancient glaciers driving before them moraines, or by the subsequent elevation of the surface on which the boulders now lie. M. Agassiz also denies that they were transported by floating ice, but he does not fully state his objections to this theory; nor does he oppose it, by the argument of the apparent anomaly of a low descent of glaciers, with the generally-received opinion of the more tropical character of the productions of the antecedent periods,--which was philosophical, until the effects of a temperate and equable climate were considered.[3] On the contrary, he assumes that, during the gradual cooling of the earth, there have been periods of excessive refrigeration. It is needless to state that such an hypothesis is not supported by a single fact--without, indeed, the _assumed_ sudden renewal of life on the surface of the world at successive periods be considered such.
During this imagined period of excessive refrigeration, the Alps and the greater part of Europe, and even of Asia, are supposed to have been covered by one immense sheet of ice, and during the _assumed_ sudden elevation of the Alps, fragments of rocks are supposed to have been shot over the frozen surface, and, when the ice melted, to have dropped on the surface where they now lie. M. Agassiz considers that this view explains the position of the boulders on pinnacles, and their absence in the valleys. I confess I should have thought, after the flexure and elevation of the ice, these would have been the least probable situations: but neither this, or some other facts (p. 381), are quite intelligible to me from the briefness, with which they are alluded to. M. Agassiz says (p. 375), “The erratic blocks of the Jura every where repose on polished surfaces, all those at least which have not been carried beyond the crest of our mountains, and which have not fallen to the bottom of our longitudinal valleys, as may be seen throughout the valleys of the Creux du Vent. But they do not repose immediately upon these polished surfaces.
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