Sidor som bilder
PDF
ePub

deposition of the tertiary strata had begun. Thus I shall explain, in the 4th Book,” my reasons for concluding that part of the chalk of England (the north and south downs, for example, together with the intervening secondary tracts) continued beneath the sea until the Eocene or earliest tertiary beds had begun to accumulate. A strait of the sea separating England and Wales has also been introduced, on the evidence afforded by shells of existing species found in a deposit of gravel, sand, loam, and clay, called the northern drift by Mr. , Murchison, who has traced it from Lancashire to the Bristol channel, over the space indicated in the map.t Mr. Trimmer has discovered similar recent marine shells on the northern coast of North Wales, and on Moel Trysane, near the Menai Straits, at the height of 1392 feet above the level of the sea 1 Some raised sea-beaches, one of them at the mouth of Carlingford Bay, Ireland, in which recent marine shells occur, lately observed by Professor Sedgwick and Mr. Murchison, have required an extension of the sea over part of the eastern shore of Ireland. A portion also of the primary district in Brittany is divided into islands, because it has been long known to be covered with patches of marine tertiary strata; and when I examined the disposition of these, in company with my friend, Captain S. E. Cook, R. N., in 1830, I was convinced that the sea must have covered much larger areas than are now occupied by these small and detached deposits. The former connexion of the White Sea and the Gulf of Finland is proved by the fact that a broad band of tertiary strata extends throughout part of the intervening space. The channel, it is true, is represented as somewhat broader than the tract now occupied by the tertiary formation; because the latter is bordered on the north-west by a part of Finland, which is extremely low, and so thickly interspersed with lakes as to be nearly half covered with fresh water. Certain portions of the western shores of Norway and Sweden have been left blank, because the discovery by Won Buch, Brongniart, and others of deposits of recent shells along the coasts of those countries, at several places and at various heights above the level of the sea, attests the comparatively recent date of the elevation of part of the gneiss and other primary rocks in that country, although we are unable as yet to determine how far the sea may have extended. On the other hand, a considerable space of low land along the shores on both sides of the Gulf of Bothnia, in the Baltic, is represented as sea, because the gradual rise of the land and the shoaling of the water on that coast, known to have taken place during the historical era, leave no room

* Ch. xxi. and xxii. * See Proceedings of Geol. Soc, vol. ii. p. 334.

for doubt that the boundaries of the gulf must have been greatly contracted within a comparatively modern period. Beds of sand and clay are also found far inland in these parts, containing fossil shells of species now inhabiting the neighbouring seas. A portion of Scania, and other tracts in the south of Sweden, have also been marked with ruled lines, because they are covered with clay, sand, and erratic blocks, which appeared to me, aster examining the district, to be tertiary. If the space overspread by such formations were more accurately known, the area represented as land in this part of Europe, would, doubtless, be much more circumscribed. I was anxious, even in the title of this map, to guard the reader against the supposition that it was intended to represent the state of the physical geography of part of Europe at any one point of time. The difficulty, or rather the impossibility, of restoring the geography of the globe as it may have existed at any former period, especially a remote one, consists in this, that we can only point out where part of the sea has been turned into land, and are almost always unable to determine what land may have become sea. All maps, therefore, pretending to represent the geography of remote geological epochs must be ideal. The map under consideration is not a restoration of a former state of things, at any particular moment of time, but a synoptical view of a certain amount of one kind of change (the conversion of sea into land) known to have been brought about within a given period. It may be stated that the movements of earthquakes occasion the subsidence as well as the upraising of the surface; and that, by the alternate rising and sinking of particular spaces at successive periods, a great area may have been entirely covered with marine deposits, although the whole may never have been beneath the waters at one time; nay, even though the relative proportion of land and sea may have continued unaltered throughout the whole period. I believe, however, that since the commencement of the tertiary period, the dry land in the northern hemisphere has been continually on the increase, both because it is now greatly in excess beyond the average proportion which land generally bears to water on the globe, and because a comparison of the secondary and tertiary strata assords indications, as I shall endeavour to show hereaster, of a passage from the condition of an ocean interspersed with islands to that of a large continent.” But supposing it were possible to represent all the vicissitudes in the distribution of land and sea that have occurred during the tertiary period, and to exhibit not only the actual existence of land where there was once sea, but also the extent of surface now submerged which may once have been land, the map would still fail to express all the important revolutions

* See Book iv. chap. iii.

in physical geography which have taken place within the epoch under consideration. For the oscillations of level, as was before stated, have not merely been such as to list up the land from below the waters, but in some cases to occasion a rise of several thousand feet above the sea. Thus the Alps have acquired an additional altitude of from 2000 to 4000 feet, and even in some places still more ; and the Apennines owe a considerable part of their height (from 1000 to 2000 feet and upwards) to subterranean convulsions which have happened within the tertiary epoch. On the other hand, some mountain-chains may have been lowered during the same series of ages, in an equal degree, and shoals may have been converted into deep abysses.” Concluding remarks on changes in physical geography—These observations, it may be said, are confined to Europe, and therefore to a space which constitutes but a small portion of the northern hemisphere; but it appeared from the remarks offered in the preceding chapter, that the great Lowland of Siberia, lying chiefly between the latitudes 55° and 75° N. (an area nearly equal to all Europe) is covered for the most part by marine strata, which, from the account given by Pallas, and other writers, may be considered as of tertiary formation. Upon a review of all the phenomena above enumerated, there appear grounds for inferring that the eras of the principal alterations in climate, as deduced from fossil remains, were coincident with the periods of the most remarkable changes in the former position of sea and land. A wide expanse of ocean interspersed with islands, seems to have pervaded the northern hemisphere at the periods when the transition and carboniserous rocks were formed, and the temperature was then hottest and most uniform. Subsequent modifications in climate accompanied the deposition of the secondary formations, when repeated changes were effected in the physical geography of our northern latitudes. Lastly, the refrigeration became most decided, and the climate most nearly assimilated to that now enjoyed, when the lands in Europe and northern Asia had attained their full extension, and the mountain-chains their actual height. It has becn objected to this theory of climate, that there are no geological proofs of the prevalence at any former period of a temperature lower than that now enjoyed; whereas, if the causes above assigned were the true ones, it might reasonably have been expected that fossil remains would sometimes indicate colder as well as hotter climates than those now established.t In answer to this objection, I may suggest, that our present climates are probably far more distant from the extreme of possible heat than from its opposite extreme of cold. A glance at the map (Pl. I. fig. 1. p. 130) will show that all the existing lands might be placed in the zone intervening between the 30th parallels of latitude on each side of the equator, and that even then they would by no means fill that space. In no other position would they give rise to so high a temperature. But in the present geographical condition of the earth, the land excluded from this zone, and lying between the poles and the parallels of 30, is in great excess; so much so that, instead of being to the sea in the proportion of one to three, which is as near as possible the average general ratio throughout the globe, it is as nine to twenty-three." Hence it ought not to surprise us is, in our geological retrospect, embracing, perhaps, a small part only of a complete cycle of change in the terrestrial climates, we should happen to discover every where the signs of a higher temperature. The strata hitherto examined may have originated when the quantity of equatorial land was always decreasing, and the land in regions nearer the poles augmenting in height and area, until at length it attained its present excess in high latitudes. There is nothing improbable in supposing that the geographical revolutions immediately preceding our times had this tendency; and in that case the refrigeration must have been constant, although, for reasons before explained, the rate of cooling may not have been uniform. Theory of Central Heat.—The gradual diminution of the supposed central heat of the globe has been resorted to by many geologists as the principal cause of alterations of climate. The matter of our planet is imagined, according to the conjecture of Leibnitz, to have been originally in an intensely heated state, and to have been parting ever since with portions of its heat, at the same time that it has contracted its dimensions. There are, undoubtedly, some grounds for inferring, srom recent observation and experiment, that the temperature of the earth increases as we descend srom the surface to that slight depth to which man can penetrate; but there are no proofs of a secular decrease of heat accompanied by contraction. On the contrary, La Place has shown, by reference to astronomical observations made in the time of Hipparchus, that in the last two thousand years there has been no sensible contraction of the globe by cooling; for had this been the case, even to an extremely small amount, the day would have been shortened, whereas its length has certainly not diminished during that period by or oth of a second. Baron Fourier, aster making a curious series of experiments on the cooling of incandescent bodies, has endeavoured, by profound mathematical calculations, to prove that the actual distribution of heat in the earth's envelope is precisely that which would have taken place if the globe had been formed in a medium of a very high temperature, and had asterwards been constantly cooled." Now this conclusion is appealed to by many as corroborating the theory of secular refrigeration, although the phenomenon might perhaps be ascribed, with equal propriety, to the action of volcanic heat, which we know has, in former ages, shifted its points of chief development over every part of the earth's crust. M. Cordier announces, as the result of his experiments and observations on the temperature of the interior of the earth, that the heat increases rapidly with the depth; but the increase does not follow the same law over the whole earth, being twice or three times as much in one country as in another, and these differences are not in constant relation either with the latitudes or longitudes of places.f All this is precisely what we should have expected to arise from variations in the intensity of volcanic heat, and from that change of position, which the principal theatres of volcanic action can be proved to have undergone. But the advocates of the doctrine of central heat contend, that although no contraction can be demonstrated to have taken place within the historical period (the operation being slow and the time of observation limited), yet it is no less certain that heat is annually passing out by radiation from the interior of the globe into the planetary spaces. Fourier even undertook to demonstrate that the quantity of heat thus transmitted into space in the course of every century, through every square metre of the earth's surface, would suffice to melt a column of ice having a square metre for its base, and being three metres (or nine feet ten inches) high. On the other hand, it is said, there is no assignable mode in which this heat can be again restored to the earth. Streams of incandescent lava arise up from unknown depths, flow out upon the surface, and, before they consolidate, emit much light and heat. In what manner does the igneous and luminous matter thus withdrawn from our planet return again from the celestial spaces? or, is lost, does it not imply a continual cooling of the central parts of the earth? This argument may appear plausible, until we reflect how ignorant we are of the sources of volcanic heat, or indeed of the nature of light and heat in general. It is doubtless true, that light and heat are continually

* It may be observed, that the facts and inferences exhibited in this Map bear not merely on the theory of climate above proposed, but serve also to illustrate the views explained in the third book respecting the migrations of animals and plants, and the gradual extinction of species.

f Allgemeine Literatur Zeitung, No. cxxxix. July, 1833.

* In this estimate, the space within the antarctic circle, of which nothing certain is known, is not taken into account: if included, it would probably add to the excess of dry land: for the great accumulation of ice in the antarctic region seems to imply the presence of a certain quantity of terra firma. The number of $quare miles on the surface of the globe, are 148,522,000, the part occupied by the sea being 110,849,000, and that by land, 37,673,000; so that the land is very nearly to the sea as one part in four. I am informed by Mr. Gardner, that according to a rough approximation, the land between the 30° N. lat. and the pole occupies a space about equal to that of the sea, and the land between the 30° S. lat. and the antarctic circle about one-sixteenth of that zone.

Vol. 1.--T

* See a Memoir on the Temperature of the Terrestrial Globe, and the Planetary Spaces, Ann. de Chimie et Phys. tom. xxvii. p. 136. Oct. 1824. See M. Cordier's Memoir on the Temperature of the Interior of the Earth, read to the Academy of Sciences, 4th June, 1827.-Edin. New Phil. Journal, No. viii. p. 273.

« FöregåendeFortsätt »