Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle, Vol. 3 [of 3] · Charles Darwin
ADDENDA.
Wherever the rounded pebbles which accompany the great blocks have not been removed by subsequent influences, it is remarked that small blocks, in other words pebbles of different sizes, form a bed of some inches, and sometimes even of many feet, upon which the great angular blocks repose. These pebbles are also much rounded, even polished, and are heaped up in such a way that the larger are above the smaller, and that the last often pass below into a fine sand, lying immediately over the polished surfaces. This order of superposition, which is constant, is opposed to all idea of a transport by currents; for in this latter case the order of the superposition of the pebbles would have been precisely reversed.” Further on (p. 379) he remarks that the action of the glaciers is immense; “for these masses, continually moving upon each other, and on the surface, bruise and grind down every thing moveable, and polish the solid surfaces on which they repose; at the same time that they push before them all that they encounter, with a force which is irresistible.
It is to these movements (of the great stratum of ice) we must attribute the strange superposition of the rolled pebbles, and of the sand, which immediately reposes upon the polished surfaces; and it is unquestionably to the grating of this sand upon these surfaces that the fine lines which we find (previously compared to the scratches made by a diamond on glass) are owing, and which would never have existed, if the sand had been acted upon by a current of water.” Now it may be demanded, by what possible means can such violent action arrange the large pebbles above the smaller ones, and these again above the sand? The fact appears to me utterly inexplicable on this view. Again it is said, that the surface of the rock is marked by furrows and gibbosities, as well as by scratches, and that these “never follow the direction of the slope of the mountain, but are oblique and longitudinal (that is, in the line of the mountain, and therefore nearly horizontal), a direction which excludes every idea of a stream of water being the cause of these erosions.” What explanation will it be believed is offered for this fact?--It is, that the fine lines and furrows “must have resulted from the much greater facility which the ice had in dilating itself in the direction of the great Swiss valley, than transversely, confined as it were between the Jura and the Alps.”!
I will now endeavour to show how far these very curious facts, which we owe to M. Agassiz’s observation, can be explained by the theory of floating ice: and the theory, I may add, if applicable to this case, solves that which presents far greater difficulties, than any other of the kind in Europe. I should first state that I make two _assumptions_, and if these be rejected, the theory is not applicable to the case of the erratic blocks of the Alps:--first, that an arm of the sea extended between the Jura and the Alps, during that period in which, as I have before shown, it is probable, that the proportional area of water in Europe was greater, and certain, that the productions of the land and water had a more tropical character, at the same time that the snow-line descended lower. The age of the Molasse, which occupies this area, between the Jura and Alps, has not been accurately determined; but it is supposed to be miocene, and is said to contain leaves of the Chamærops, a genus of palms, at present found further from the equator than any other kind. It is not, however, evident that the Molasse was deposited by the sea during the latest period, when it occupied a confined limit between the Alps and Jura; but even if this should be found to be the case, it would be rash in any one positively to conclude that glaciers could not have descended to the shores of a sea, on which the Chamærops flourished, after that we know they descend in the southern hemisphere so near the limits of several tropical forms.
My second _assumption_ is, that the elevation of this much of Switzerland, whenever it took place, was slow and gradual: this is supported by the strongest analogies of South America, Scandinavia, and other parts of the world; whilst on the other hand, the _assumption_ that it had been sudden, would be unsupported by a single ascertained fact in nature. Now, as the numerous masses of ice, which fall from the glaciers at the head of the sounds on the South American coast, are slowly drifted outwards (owing to the fresh water flowing in from the foot of the glaciers), and in the more open channels are left to be acted on by the winds and currents; so must it have been with the icebergs from the glaciers of the Alps, situated in the same latitude, and under similar conditions. These icebergs would in most cases be driven on some part of the surrounding shore; but from floating deep they would ground a little way from the beach, and then being packed together, and driven to and fro, as the winds changed, and as the tides rose and fell, would they not, like a glacier on the land, though in a lesser degree, “bruise and grind down every thing and polish the solid surface, on which they reposed?”
In the rapids of the North American rivers, over which large bodies of ice are driven, carrying with them pebbles and fragments of rocks, I am informed by Dr. Richardson, that the primitive rocks are scooped and hollowed, and have their surfaces polished and glossy. Dr. Richardson, however, is not prepared to say, whether this is caused by the passage of the ice or of the pebbles.[4] Although the icebergs might be drifted from side to side of the sound, if they were moved after having grounded, it would be along the shore by the set of the currents or wind, and perhaps slightly up and down by the tidal changes. Would not the necessary effect of this be, that the scratches formed by the sand grating between the rocks and the bottom of the icebergs should be, with some irregularities, longitudinal, or (from the effect of the tidal movement) oblique? And as the mountains slowly emerged during ages, every part would be thus acted on; and consequently the whole surface would be marked by longitudinal scratches.
The icebergs on the South American coast sometimes transport angular fragments of rock, to the distance of many miles from the glacier whence they were detached; and as the winds and currents generally have sufficient steadiness to drive any floating object soon on shore, (as is known to be the case with a capsized boat, a barrel, or floating carcass, &c.), so the blocks of rock would be _generally_[5] landed on the shores of the channels between the Alpine ranges, and not dropped in the intervening spaces. If any pointed rock came so near the surface that a floating mass of ice, thus charged, grounded on it, the block would, when the ice melted, be there left. But it may be asked, would the blocks usually be deposited on the bare surface of the rocky bottom off the shore, or on an intervening layer of gravel or sediment? From what I have observed when passing in boats through the channels of Tierra del Fuego, and from frequent examinations of the armings of the lead used in sounding, I feel nearly sure that _absolutely bare_ submarine rock is not very common.
Moreover, where matter is depositing near a shore, the finer the particles are, the further they are drifted: in approaching a coast I have actually traced _every step in the series_, from the finest sand to large pebbles. But as the land in any case is slowly elevated, the same forces which carried the large pebbles to a certain distance from the beach, and the smaller ones to a still further distance, will, after each little elevation, carry them somewhat further:--a layer of little pebbles thus covering the sand, and a layer of large pebbles the smaller ones. Hence, when the part near the shore is converted into dry land, a section of the bed which was originally the bottom of the sea will necessarily show solid rock covered by sand, this by fine pebbles, and these again by others, gradually increasing in size. Such then, I conclude, must have been the nature of the sub-littoral deposits of the Alps, during their assumed slow elevation. Finally, as icebergs of large size would seldom be driven up on the beach of a sheet of water, if, like the channel between the Jura and the Alps, it were protected from the open sea, any fragments of rock transported by them would have been dropped some way outside, and therefore when upraised with the whole country, they would be found in most cases reposing on beds (where the loose matter had not been subsequently removed), characterized by the order of superposition just described.
Such is the explanation I would suggest of the very curious facts observed by M. Agassiz. I make no assumptions which are not supported by strong analogies and the foundation of the theory--namely, a change of climate of a peculiar kind--can be shown by reasoning, independent of the existence of erratic blocks, to be probable in a high degree: whether this is the case with the theory of M. Agassiz, I leave the reader to decide. Having said thus much on the scratched rocks of the Alps, I am tempted to make a few remarks on those of Scotland, described by Sir James Hall[6] in his celebrated paper (_Edinburgh Phil. Transact._, vol. vii.) on the Revolutions of the Earth’s Surface:--a case which has always appeared to me to be the strongest ever adduced in favour of the theory of an overwhelming debacle having rushed, at least in that country, over hill and valley. The furrows and scratches in the same district are parallel to each other, and hence run in the same direction:--thus, near Edinburgh, they extend in a line a little north of west and south of east, that is parallel to the valley of the estuary; but both to the eastward and westward they deviate from this line by more than half a right angle; and on the south-west part of Scotland they have no uniform direction. In the north of Scotland, however, near Brora, Mr. Murchison (_Geolog. Transact. 2d Series_, vol. ii., p. 357) found the hills marked in parallel lines, directed north-west and south-east.
The furrows and scratches near Edinburgh seem generally to traverse the less inclined surfaces, but Sir James, speaking of one part, says “the _perpendicular_ face as well as the rest is covered with lines, which are horizontal, or nearly so.” In these respects the case appears very similar to that of the Alps: the rocks, however, are not polished;[7] but this may be owing to their nature, sandstone and trap, and not to any difference in the cause; for Dr. Richardson tells me that in the same rivers in North America, in which the granitic rocks are much polished, those of laminated limestone are not at all so. Near Edinburgh, where the lines extend west and east, the western face of the hills (of which the highest mentioned is four hundred and seventy feet above the sea) is chiefly marked, whilst on the opposite or protected side, a long tail of (so called) _diluvium_ extends, which consists of blue clay, with large erratic boulders embedded in it. These boulders, as I am informed by Mr. James Hall, and by Mr. Smith of Jordanhill, are themselves marked with parallel lines, having one direction, which shows that they were held fast whilst drifted across the country, and not rolled over and over, like a pebble in a stream.
It is admitted by all that the grooves on the solid rock were formed by the passage of these boulders over it. Although the minor inequalities of the surface of the land appear to have had no influence whatever on the action which produced the scratches, yet the larger features, as the general bearing of the main valleys, appear to have determined their direction. Sir James distinctly states that the scoopings and furrows have precisely that form which the long action of torrents tends to produce on a solid rock; but he adds, and I believe most truly, that the furrowed surface produced by such means is smooth, and not deeply scored and scratched. It is indeed utterly inconceivable that large stones should be carried along as if “_independently of their gravity_” by any ordinary means, with such velocity, as to mark with horizontal lines the _perpendicular_ face of a rock. From these facts,--from the presence of great erratic blocks, from the steepness of one face of the grooved hills, and the tail of sediment stretching out from the other, Sir James Hall, having in his mind the recorded cases of the great waves consequent on earthquakes, inferred that a vast deluge had burst over the country from the westward.
M. Brongniart, and lately M. Sefström (_L’Institut, February_ 22d, 1837), have described phenomena in Sweden almost identical with those of Scotland. The rocks are there grooved and scratched,[8] even to the height of 1500 feet, in north and south lines, parallel to the valley of the Baltic and of the Gulf of Bothnia; but they are considerably deflected by the larger inequalities of surface. The north side of the hills are most affected, whilst from the southern side, long ridges, called _oasars_, stretch out; they are composed of sand and water-worn materials, and appear to be similar, but on a much larger scale, to the _tails of diluvium_ in Scotland. In Sweden, however, the erratic blocks always lie on the surface of these ridges, and are not embedded within them: but M. Sefström says, that at the time when the grooves were formed, enormous masses of rock were torn from the mountains. In the United States, the phenomenon of the grooved rocks appears to be developed in an extraordinary manner. Professor Hitchcock (_Report on the Geolog. of Massachusetts_, p. 167) describes a tract about two hundred miles in width, over which nearly all the bare rock on the hills, even to the height of three thousand feet, is scored by parallel lines. In some parts boulders, weighing from fifty to one hundred tons, are yet lying on the surfaces, which bear the marks of their passage. The furrows are generally directed a little west of north, but in the western part of Massachusetts, and in the eastern of New York, they extend in a north-west and south-east line, and in one part even to W. 20° N. M. Sefström and Professor Hitchcock explain these appearances in their respective countries by the same agency, as Sir James Hall does in Scotland.
The theory of a great debacle is in these cases based on the united presence of erratic boulders, ridges of water-worn materials, forming tails to scarped hills, and parallel furrows and scratches on the surface of the rocks. 1st. With respect to the boulders, it would be superfluous to repeat the arguments in favour of the idea of their transportation by ice: and in the case of Sweden, it would be pre-eminently superfluous, as we know (see _Lyell on the Rising of the Land in Sweden, Phil. Transact._, 1835) that blocks are there transported yearly by this means. 2d. Every one who has examined a great estuary, or a channel where the tides run strongly, is aware that linear banks are formed behind any obstacle. Therefore these tails of diluvium might have been formed, as far as regards their external form, by ordinary means; and with respect to their internal structure, which appears extremely irregular, and without any stratification, it must be difficult for any one to speak with certainty, until the joint effects of ice transporting coarse fragments and gentle currents of water, fine mud, are better known. Mr. Lyell, indeed (_Phil. Transact._, 1835, p. 15) has advanced strong reasons, showing from the structure and composition of the _oasars_ that they could not have been formed by any sudden debacle. Whilst such linear banks were depositing on one side of the hills, the other, or exposed front, would almost necessarily become scarped. 3d. We have the admission of Sir James Hall, that the scoopings and grooves resemble those produced by the slow action of running water: therefore the scratches appear to be the only part of the phenomenon which remains unexplained.
In the Alps, we are told, that scratches are formed on rocks by glaciers grinding over them. According to the theory of floating ice, we have evidence in the erratic blocks near Edinburgh, that ice was formerly in action there; and, from the analogies given in this volume, it might well have been so, since the scene of supposed action lies two degrees nearer the Pole than Georgia, in the southern ocean, “almost wholly covered with everlasting snow.” What then would be the effect of the tides and gales of wind, driving packed icebergs with irresistible force, through channels, and over rocky shoals;--each part of the surface being exposed for centuries, as the country was elevated, to this action? Would not the fragments of rock embedded in the ice grate in a direct path over the surface, regardless of _minor inequalities_? and would not the fragments themselves be grooved and scored in one direction? Can we for one moment believe it possible that boulders, either in water or in the thickest mud, could be driven over a rugged surface, or along a perpendicular face of solid rock, with such enormous velocity as with their points to groove and scratch it, and nevertheless not to be rolled over and over, like a stone descending a mountain, but to be marked with parallel lines of abrasion, equally with the fixed, underlying mass? It appears to me that we assuredly can make no such admission. Travellers in the Arctic regions tell us that the drift-ice, with its irresistible power, can force up the gravel and sand into mounds (see _Geograph. Journal_, vol. viii., p. 221), and drive before it great boulders, and even ships, and masses of ice, high and dry on the beach. What then would be the effect of a few pebbles, or a single fragment, between such masses of ice and a steep coast-wall of rock? Would not scratches “horizontal, or nearly so” be formed, “indicating (to use Sir James Hall’s words) that grinders had been pressed against the rock;” as if “independently of their gravity”?
In this explanation only _veræ causæ_ are introduced, and reasons can be assigned, for the belief that these causes have been in action in these districts. On the theory of debacles, it still remains to be proved that rocks can be thus _scooped_ and _furrowed_, or hills _scarped_; although I am far from affirming they cannot,--and scratched, I presume, they certainly would be. With respect to Sweden, where the land is now rising, and where ice even still is a transporting agent, it is undoubtedly the part of the geologist, to endeavour by long and laborious research to account for the phenomena by these real agencies. For to introduce, before it is absolutely forced on us, the hypothesis of a deluge of mud and stones, fifteen hundred feet deep in Sweden, or three thousand in North America, which rushing over the country, rounded the northern fronts of the hills, and rolling by their eastern and western flanks, left them marked with oblique furrows, is to violate, as it appears to me, every rule of inductive philosophy. Page 297.
With reference to the embedment of the Siberian animals with their flesh, I have mentioned in a note, the case of ice described as rising from the bottom of the sea, off the coast of Greenland. Messrs. Dease and Simpson, during their late memorable journey along the shores of the Arctic ocean, speaking of one part (_Geograph. Journal_, vol. viii., p. 218) say, “The ice lay much closer here; and numerous masses adhered to the bottom, under the water, which obliged us to search a passage out from the shore.” Further on (p. 220) they say, “But nowhere had the thaw penetrated more than two inches beneath the surface (of the land), while under water along the shore, the bottom was still _impenetrably frozen_.” This was on the second of August. It should, however, be observed, that the sea along this part of the American coast is extremely shallow. Page 321. I have given my reasons for believing that the temperature of the mineral springs of Cauquenes, was permanently changed by the earthquake of 1822. This inference is altogether false, for I find that Schmidtmeyer, in his Travels in Chile during the years 1820 and 1821, says (p. 311) that the temperature of the different springs was 83°, 103°, 106°, 112°, 117° and 118° of Fahrenheit. Now Mr. Caldcleugh says, after the earthquake of February, 1835, the temperature fell from 118° to 92°. Previously, therefore, to this shock, it had regained the temperature which it had in 1820.
Page 377. When I offered my views on the cause of the great waves, which follow earthquakes on certain coasts, I was not aware of the paper on this subject by Sir James Hall in the _Edinburgh Royal Transactions_, vol. vii., p. 154. I cannot, however, perceive the necessity of a sudden elevation of the bottom, to produce the observed effects, as supposed by that distinguished philosopher. Having read the abstract of a _Notice on the Resistance of Water_, by Mr. Russell, I perceive the subject is far more intricate, than I was at the time aware. Page 381. I have said that during the _few months_ subsequently to the great shock of February, 1835, at Concepcion, upwards of three hundred tremours were felt, but I should have said, within _twelve days_. (See _Geograph. Journal_, vol. vi., p. 322. _Sketch of Surveying Voyage of the Adventure and Beagle, by Captain FitzRoy_). From some additional information which I have met with since finishing this chapter, I find the train of volcanic phenomena, which followed this earthquake, affected a larger area than that mentioned (seven hundred by four hundred miles), and affected it in a manner which gives great additional weight to the argument that South America is in that part a mere crust resting over a sheet of fluid rock; and likewise to the generalization that the action of volcanoes, and the permanent elevation of the land (and consequently, as I believe, the elevation of mountain chains) are parts of the same phenomenon, and due to the same cause.
Page 446. When I put together the few and exceedingly imperfect remarks on the subject of Miasmata, I did not know of Dr. Ferguson’s remarkable dissertation--the result of his investigations in Holland, Spain, Portugal, and the West Indies--on the Nature and History of Marsh Poison (_Edinburgh Royal Transact._, vol. ix., p. 273). He there clearly proves the fact, which had struck me with so much surprise, namely, that the driest districts, which, according to common notions, would be considered as the most healthy, are often singularly the contrary. In his concluding remarks, Dr. Ferguson says (p. 290), “One only condition, then, seems to be indispensable to the production of the marsh poison, on all surfaces capable of absorption, and that is the _paucity_ of water, where it has previously or recently _abounded_. To this there is no exception in climates of high temperature; and from thence we may justly infer, that the poison is produced at a highly advanced stage of the _drying process_.” And, from facts previously stated, it would appear that even in barren hilly countries, the banks of mountain torrents, which had been overflowed, sometimes became extremely insalubrious.
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