Imágenes de páginas
PDF
EPUB

rolled or violently distorted, or deprived of their natural contour, such remains have not been brought to their present beds from a distance; but that the deluge must have been sudden, and overtaken them in their natural resorts; and hence may, in many cases, have swept away all the individuals of a species in a common calamity.

There is, however, a great difficulty with some naturalists in conceiving that such animals as the elephant, the tapir, the rhinoceros, the hippopotamus, the mammoth, or mastodon, animals now only found in the torrid regions, could have existed in these northern parts of the globe. M. de Marschall endeavoured by one sweeping stroke of the fancy to solve this, as well as that of the extraordinary fragments in which they are often imbedded, and held out that the whole have fallen at different times, like meteoric stones, from heaven.* The real difficulty, however, vanishes, in a considerable degree, if not entirely, when we reflect, that although the torrid regions furnish us with some of these genera, they do not appear in any instance to contain the same precise species as are traced among the large fossil quadrupeds of the northern and colder parts: and hence it is no argument, that because the habits of the extant species do not qualify them for a residence in these latter regions, such situations might not have furnished a comfortable home to the species whose remains are found among us. The fossil species do not differ less from the living to which they make the nearest approach, than various animals that are familiar to us do from others that belong to the same tribes, and which are found under one species or other, over the whole world. The race of horses, of swine, or of sheep, furnishes us with abundant examples of this remark: and that of dogs affords perhaps a still more striking illustration; for while under one form, that of the isatis or Arctic fox, the canis Lagopus of Linnæus, we find it in the northernmost coast of America, and even the frozen sea, living in clefts, or burrowing on the naked mountains, and in that of the almost infinite varieties of the c. familiaris, or domestic dog, in the bosom of our own country, in the form of the c. aureus, chacal or jackal, we meet with it in the warmest parts of Asia and Barbary, prowling at night in flocks of one or two hundred individuals.

The extensive TURBARIES or PEAT-FIELDS, which are so common to many parts of Europe, are produced by an accumulation of the remains of sphagnum and other aquatic mosses. These surround and cover up the small knolls upon which they are formed; or, in many places, descend along the valleys after the manner of the glaciers of Switzerland; but while the latter melt away every year at their lower edges, the mosses are not checked by any obstacle in their regular increase; and as such increase takes place in determinate proportions, by sounding their depth to the solid ground we may form some estimate of their antiquity.

The ordinary rise of those extensive ranges of DOWNS, which are seen skirting the coats of many countries, and especially where the shore is not very bold, is a mixt effort of sea and wind. To produce this, however, the soil that the sea washes over must consist of sand. This is first pushed in successive tides towards the shore; it next becomes dry, by being left there at every reflux of the sea; and is then drifted up the beach, and to a considerable distance from the beach, by the winds which are almost always blowing from the sea, and often in whirls or eddies; and are at length fixed by the growth of wild plants, whose seeds are in like manner wafted about

* Recherches sur l'Origine, &c. Geissen, 1802.

[ocr errors]

on the wings of the breeze, or casually dropped with the excretions of birds or other animals that pass over them. In several parts, observes M. Cuvier, these proceed with a frightful rapidity, overwhelming forests, houses, and cultivated fields, in their irresistible progress. Those on the coast of the Bay of Biscay have actually buried a considerable number of villages whose existence is noticed in the records of the middle ages. And even in the present day they are threatening not fewer than ten distinct hamlets with almost inevitable destruction; one of which, named Mimigan, has been in perpetual danger for upwards of twenty years, from a sand-hill of more than sixty feet in perpendicular height, produced by the cause we are now contemplating, and which is very obviously augmenting.*

There are various forelands on the coasts of the North Sea, and particularly on those of the counties of Sleswigh and Holstein, which are formed in the same manner. But the most extraordinary inroads of sand-storms and sand-floods are, perhaps, those which have taken place in the Lybian Desert, and in Lower Egypt. M. Denon informs us, in his travels over this part of the world, that the summits of the ruins of ancient cities buried under mountains of drifted sands still appear externally; and that but for a ridge of mountains, called the Lybian Chain, which borders the left bank of the Nile, and forms a barrier against the invasion of these sands, the shores of the river, on that side, would long since have ceased to be habitable. "Nothing," says M. Denon, "can be more melancholy, than to walk over villages swallowed by the sand of the desert, to trample under foot the roofs of their houses, to strike against the tops of their minarets, and to reflect, that yonder, in days of yore, were cultivated fields, that hard by were groves of flourishing trees, and the dwellings of men close at hand; and that all has now vanished."

The various ISLANDS that spot the surface of the sea have arisen from different causes. Many of them have been merely separated from the adjoining continent by the inroad of the sea itself upon the main-land; others have been thrown up by volcanoes, which have at times disgorged prodigious blocks of granite among the mixt materials, such as are frequently found in the Danish archipelago, in the midst of the geest, or alluvial matter, which has collected around them. Other islands are altogether the masonry of madrepores, and other coral zoophytes of wonderful industry and perseverance, of which the South Sea furnishes us with the largest and most astonishing specimens. These islands are for the most part flat and low, and surrounded by enormous belts of coral reefs. Most of the calcareous zoophytes are employed in their construction, but the principal worm is the madrepora lubricata of Linnæus.

In so large an abundance, and with so much facility, is calcareous matter elaborated by these, as well as by various other animals, and especially the testaceous worms, that M. Cuvier is inclined to ascribe all the calcareous rocks that enter into the solid crust of the earth to an animal origin.§

Report concerning the downs of the Gulf of Gascony, or Bay of Biscay, by M. Tassin, Mont de Marsan. an. x. Cuvier, Theory of the Earth, § 31.

† De Luc, Voyages Geologiques, tom. i.

Jameson's Notes on Cuvier's Theory, &c. p. 217. Compare Dolomieu's Memoir on Egypt, in Journ. de Physique, tom. xlii.

Some writers have proceeded much farther than this, for they have resolved all the solid materials of the earth's crust into an organic origin. Such was the opinion of Demaillet and Lamarck; who suppose that every thing was originally fluid; that this universal fluid gave rise to plants and animals; that all clay or argillaceous earth is the produce of the former, all calcareous earth of the latter, and that siliceous earth has been the result of the two. Telliamid, p. 169. Philosophie Zoologique, passim.

But this is to suppose the earth of a far higher antiquity, and to have been the subject of more numerous general deluges, and inversions of sea and land, than are called for by the Wernerian system, or appear reconcileable with the Mosaic narrative. M. Cuvier apprehends, indeed, that such catastrophes may have occurred five or six times in succession, at a distance of four, five, or six thousand years from each other; and that even the chalk formation found in the basin of Paris originated in a revolution of this kind that occurred antecedently to that which is usually regarded as the flood of Noah. And, following up this idea, he conceives, towards the close of his Introductory Theory of the earth, that if the science of fossil organic productions could be carried to a much higher degree of perfection, we should be able to obtain far fuller information upon this subject; "and man, to whom only a short space of time is allotted upon the earth, would have the glory of restoring the history of thousands of ages which preceded the existence of the human race, and of thousands of animals that never were contemporaneous with his species."

LECTURE VII

ON GEOLOGY.

(The subject continued.)

In our last study I attempted a brief sketch of the chief phænomena that occur to the eye of the geologist upon a survey of the solid crust of the earth, as far as he is able to penetrate into it. The conclusion to which such phænomena lead us is the following: that the rudimental materials of the globe, to the utmost depth we are able to trace them, existed at its earliest period, in one confused and liquid mass; that they were afterwards separated and arranged by a progressive series of operations, and an uniform system of laws, the more obvious of which appear to be those of gravity and crystallization; and that they have since been convulsed and dislocated by some dreadful commotion and inundation that have extended to every region, and again thrown a great part of the organic and inorganic creation into a promiscuous jumble.

Now the only two causes that can enter into the mind of man as being competent to the fluidity that appears at first to have existed throughout the whole crust of the earth are FIRE, or a peculiar SOLVENT. But, if a solvent, that solvent must have been WATER: for there is no other liquid in nature in sufficient abundance to act the part of a solvent upon a scale so extensive.

And hence our inquiries into this subject become in some degree limited, and are chiefly confined to what have been called the PLUTONIC and the NEPTUNIAN hypotheses; the origin of the world in its present state from igneous fusion, and from aqueous solution. Both these theories are of very early date, and both of them have been agitated in ancient as well as in modern times with a considerable degree of warmth as well as of plausible argument.

Among the ancients, Heraclitus seems to have headed the advocates for the former theory, and Thales, or rather Epicurus, the supporters of the latter. In what may be regarded as modern times, Hooke may, per

haps, be held the reviver of the Plutonic system, which has since, as I have already observed, been supported by the cosmological doctrines of Buffon and Dr. Herschel. Its principal champions, however, in the present day, are Dr. Hutton, Professor Playfair, and Sir James Hall; names, unquestionably of high literary rank, and entitled to the utmost deference, but most powerfully opposed by the distinguished authorities of Werner, whose system I have just glanced at, Saussure, Kirwan, Cuvier, and Jameson, not to mention that the general voice of geologists is very considerably in favour of the latter class of philosophers, and consequently of the Neptunian or aqueous hypothesis. Let us then take a brief view of each of these theories in their order.

According to the former, or the Plutonic conjecture, heat is the great source, not only of the original production, but of the perpetual reproduction of things. This theory supposes a regular alternation of decay and renovation. Of decay induced by the action of light, air, and other gases, rain and other waters, upon the hardest rocks, by which they are worn down, and their particles progressively carried towards the ocean, and ultimately deposited in its bed; and of renovation, by means of an immense subterranean heat, constantly present at different depths of the mineral regions; which operates in the fusion and recombination of the materials thus carried down and contained there, and afterwards in their sublimation and re-exposure to view in new strata of a more compact and perfect character. Hence the existing strata of every period consist, upon this theory, of the wreck of a former world, more or less completely fused and elevated by the agency of violent heat, and reconsolidated by subsequent cooling: of the general nature of which heat, however, we are still left in a considerable degree of ignorance. "It is not fire in the usual sense of the word," observes Mr. Playfair, "but heat, which is required for this purpose; and there is nothing chimerical in supposing that nature has the means of producing heat, even in a very great degree, without the assistance of fuel or of vital air. Friction is a source of heat, unlimited for what we know in its extent; and so, perhaps, are other operations chemical and mechanical; nor are either combustible substances or vital air concerned in the heat thus produced. So, also, the heat of the sun's rays in the form of a burning glass, the most intense that is known, is independent of the substance just mentioned; and though the heat would not calcine a metal, nor even burn a piece of wood, without oxygenous gas, it would doubtless produce as high a temperature in the absence as in the presence of that gas."

This subterranean heat, moreover, is supposed to derive a very considerable accession of power from the vast superincumbent weight that is perpetually pressing upon its materials; in confirmation of which a variety of curious experiments are appealed to, and especially a very ingenious set lately carried into effect and described by Sir James Hall, by which it has been rendered probable, that when the gases of any fusible substance, as the carbonic acid of carbonate of lime, for example, are rendered incapable of flying off, a much less quantity of actual heat is sufficient for the purpose of fusion than when such gases, freed from a heavy compression, can escape with facility. Now, the subterranean heat being supposed to exist at prodigious depths below the surface, the substances on which it operates

Illustrations of the Huttonian Theory of the Earth. Edinb. 1802.
Illustrations of the Huttonian Theory, &c.

must be so enormously compressed, as not only to render them easily fused, but in many instances to prevent their volatilization after the fusion has taken place; and from this circumstance it is possible, we are told, to explain a variety of appearances and qualities in minerals, and to answer a variety of objections which would otherwise weigh heavy against the general theory.

To the principle of an alternate decay and renovation, separated from the means by which they are supposed, upon this theory, to be accomplished, there seems to be no very serious objection. It is as readily allowed by the Neptunian as by the Plutonic geologist, that the strata of the earth are liable to waste, and are indeed perpetually wasting; and that the waste materials are carried forward to the sea. But the appearance of shells in limestone and marbles, in which the sparry structure is as perfect as in primary limestone, and through which are distributed veins of crystallized carbonate of lime, together with a variety of similar facts, fatally militate against the agency of heat as an universal cause; since, in such case, allowing it to have been sufficient to produce the general effect of crystallization, every vestige of the structure of the shells must have been destroyed, and every atom of the carbonic acid totally evaporated.

It is, secondly, useless to argue, that there are other sources of heat than combustion or deflagration; because, admitting the fact to Mr. Playfair's utmost desire, it can be satisfactorily proved that all these sources are as little capable of acting in the interior parts of the globe, to the extent supposed in the theory before us, as combustion itself, which is relinquished by its defenders as incompetent to their purpose. But even allowing the full operation of all, or of any one of these causes, we have no method pointed out to us by which this subterranean heat is duly preserved and regulated-no controlling power that directs it to the proper place at the proper season, without which it must be as likely to prove a cause of havoc and disorder as of renovation and harmony. It is useless, therefore, to pursue this theory any farther. In spite of the magnificence of its structure, the universality of its application, the plausibility of its appearance, and the talents with which it has been supported, it is built upon assumption alone; it lays down principles which it cannot support, and deals in fancy and conjecture rather than in solid facts and firm evidence.

Let us next, then, take a glance at the theory by which this is chiefly opposed, and which, as I have already observed, is denominated the NEP

TUNIAN.

Under this hypothesis the two substances that were first evolved out of the general chaos on the formation of the earth, and chemically united to each other, were hydrogene and oxygene, in such proportion as to produce water, which is a compound of these substances, and in such quantity as to be able to hold every other material in a state of thin paste or solution. Of the materials thus held in solution granite is supposed to have been produced first, and in by far the greatest abundance. It hence, consolidated first, probably forms the foundation of the superficies of the globe, and perhaps the entire nucleus of the globe itself; and, as has been already seen, while it constitutes the basis of every other kind of rock, rises higher than any of them. It consists, as we have already observed, of felspar, quartz, and mica, all which must therefore have concreted by a crystallization nearly simultaneous; and from its containing no organic remains, it is obvious that it must have been formed prior to the existence of the animal and vegetable kingdoms. All the other rocks, upon this hypothesis, began

« AnteriorContinuar »