페이지 이미지
PDF
ePub

ing cataracts, where you perceive only that the tender amethyst of the sky has taken a deeper tinge. That undulating line of the crystalline hills tells of broad, dreary moors, dark, sullen streams, sparse fields of stunted com. That sweeping, melting, waving line of the tertiaries tells of stately forest and gardened plain, of lordly mansions and bustling villages. The Mosaic record, as interpreted by the advocates of the Age theory, gives the horizon lines of successive geological eras. Its descriptions, they maintain, are correct, viewed as horizon lines. They convey the largest amount of knowledge concerning the several periods which could possibly be conveyed under the given conditions. Such is the method or logic of the Age theory of Mosaic geology; and it is manifest that, whatever may be its scientific value, it is no more to be refuted by the mention of geological facts which the Mosaic record does not specify, than the accuracy of a map, constructed on the scale of half an inch to the hundred miles, would be impugned by proving that it omitted a particular wood, rock, hill, or village.

It is indispensable to the establishment of this theory, that the geological changes which the earth has undergone shall admit of being arranged in certain divisions. The lines of demarcation between these may be drawn within wide limits of variation ; but should it become an unquestioned truth of geologic science that absolute uniformity of phenomena has reigned in our world so long as the geologist traces its history, the Age theory would be untenable. The theory does not require that the " solutions of continuity" should be abrupt or catastrophic. On the contrary, the "morning" and "evening" of the Mosaic record suggest gradation ; and the pause of night, with its silence, its slumber, its gathering up of force for new outgoings of the creative energy, by no means suggests cataclysm or revolution. But the days or periods, though they may melt into each other with the tender modulation of broad billows on a calming sea, must possess a true differentiation, and cannot be accepted by those who believe in absolute geological uniformitarianism. We are not sure, however, that any geologists profess this creed, and the views propounded by very eminent geologists on the nature of the changes which have taken place on

N«w Seiibs.— Vol. XIV., No. 5.

the earth appear to us to satisfy the requirements of the Age theory, in respect of division and succession. In the sixth edition of his "Elements of Geology" Sir Charles Lyell writes thus :—" Geology, although it cannot prove that other planets are peopled with appropriate races of living beings, has demonstrated the truth of conclusions scarcely less wonderful—the existence on our planet of so many habitable surfaces, or worlds, as they have been called, each distinct in time, and peopled with its peculiar races of aquatic and terrestrial beings." He proceeds to state that living nature, with its " inexhaustible variety," displaying "infinite wisdom and power," is "but the last of a great series of pre-existing creations." Mr. Darwin, in the fourth edition of his "Origin of Species," makes the weighty remark that "scarcely any palaeontological discovery is more striking than the fact that the forms of life change almost simultaneously throughout the world." Qualifying his words by the statement that they apply chiefly to marine forms of life, and that the simultaneity referred to does not necessarily fall within "the same thousandth or hundred-thousandth year," he writes as follows :—

"The fact of the forms of life changing simultaneously, in the above large sense, at distant parts of the world, has greatly struck those admirable observers, MM. de Verneuil and d'Archiac. After referring to the parallelism of the palaeozoic forms of life in various parts of Europe, they add, 1 If, struck by this strange sequence, we turn our attention to North America, and there discover a series of analogous phenomena, it will appear certain that all these modifications of species, their extinction, and the introduction of new ones, cannot be owing to mere changes in marine currents, or other causes more or less local and temporary, but depend on

feneral laws which govern the whole animal ingdom.' M. Barrande has made forcible remarks to precisely the same effect. It is indeed quite futile to look to changes of currents, climate, or other physical conditions, as the cause of these great mutations in the forms oMife throughout the world, under the most different climates."

Mr. Darwin holds that " looking to a remotely future epoch," the later tertiaries, namely, "the upper pliocene, the pleistocene and strictly modern beds of Europe, North and South America, and Australia, from containing fossil remains, in some degree allied, and from not in35

eluding those forms which are only found in the older underlying deposits, would be correctly ranked as simultaneous, in a geological sense."

These statements afford, we think, a sufficient basis for the general scheme of Mosaic geology which we are considering; and it may be remarked that the latest of the geological epochs of simultaneity, as defined by Mr. Uarwin, would agree indifferently well with the last of the Mosaic days or periods, as defined by Hugh Miller.

There is yet another proposition which must be established if the Age theory of Mosaic geology is to be maintained. The scheme depends essentially on the theory of central heat. We saw that Miller undertakes to account for each of the six Mosaic days or periods. As a geologist, indeed, he felt himself to be under a special obligation to explain the creative operations of the third,, fifth, and the sixth days, that is to say, the day on which vegetable life was created, and the successive days on which different orders of vertebrate animals were introduced into the world; but he gives delineations of the prophetic vision of the first two days, and he assigns the occurrences of the fourth day, namely, the appearance of the sun and moon, to the Permian and Triassic periods. In one word, he accepted the responsibility of adapting his scheme of reconciliation to all the dayperiods of Genesis, and he was perfectly aware that the hypothesis would require to be rejected if the theory of central heat were invalidated. His geological explanation of the first four days depends explicitly upon the opinion that, at the time when the earth entered upon those changes which are chronicled by geological science, it was under the influence of intense heat, and gradually cooling and solidifying. In the first day thick darkness lay upon the surface of the earth, owing to the canopy of steam, impermeable by light, under which it lay shrouded. During the second day the light began to penetrate the vapory veil, and dim curtains of cloud raised themselves from the sea. On the third day the forests, which ■were heaped up for us into treasuries of coal, came into existence, and Miller accounts for their luxuriance by supposing that the heated and humid state of the atmosphere of the planet, still depen

dent upon the central fires, favored their growth. It was not until the fourth day that the blanket of the ancient night was rent asunder, that sun, moon, and stars beamed out, and that a state of the atmosphere and a succession of summer and winter, day and night, identical with those we now witness, began. Possibly enough, had Miller found himself ultimately forced to abandon the theory of central heat, he would have entrenched himself, as in a second line of defence, in the three specially geological day-periods. But he never contemplated an abandonment of the doctrine of central heat. He held that the earth was once a molten mass, and that the series of changes through which it has passed arose naturally out of this fact. The crust of granite he believed to have been enveloped, in the process of cooling, by a heated ocean whose waters held in solution the ingredients of gneiss, mica-schist, hornblende-schist, and clay-slate. The planet gradually matured "from ages in which its surface was a thin earthquake-shaken crust, subject to continual sinkings, and to fiery outbursts of the Plutonic matter, to ages in which it is the very nature of its noblest inhabitant to calculate on its stability as the surest and most certain of all things." In short, he maintained that "there existed long periods in the history of the earth, in which there obtained conditions of things entirely different from any which obtain now—periods during which life, either animal or vegetable, could not have existed on our planet; and further, that the sedimentary rocks of this early age may have derived, even in the forming, a constitution and texture which, in present circumstances, sedimentary rocks cannot receive."

Sir Chailes I.yell rejects absolutely the theory of central heat as a mode of accounting for those changes on the terrestrial surface which are classified by geologists. He declares that no kind of rocks known to us can be proved to belong to "a nascent state of the planet" Disclaiming the opinion "that there never was a beginning to the present order of things," he nevertheless holds that geologists have found "no decided evidence of a commencement." Granite, gneiss, hornblende-schist, and the rest of the crystalline rocks, "belong not to an order of things which has passed away; they are not the monuments of the primeval period, bearing inscribed upon them in obsolete characters the words and phrases of a dead language; but they teach us that part of the living language of nature which we cannot learn by our daily intercourse with what passes on the habitable surface."

From the phenomena of precession and nutation, Mr. Hopkins, reasoning mathematically, inferred that the minimum present thickness of the crust of the earth is from 800 to 1,000 miles. This conclusion is the basis of Sir Charles Lyell's opinion respecting the Plutonic agencies which take part, or have taken part, in the formation of rocks. He shows by diagram that, if even 200 miles are allowed for the thickness of the crust, seas or oceans of lava five miles deep and 5,000 miles long might be represented by lines which, in relation to the mass of the earth, would be extremely unimportant. "The expansion, melting, solidification, and shrinking of such subterranean seas of lava at various depths, might," he contends," suffice to cause great movements or earthquakes at the surface, and even great rents in the earth's crust several thousand miles long, such as may be implied by the linearlyarranged cones of the Andes, or mountain-chains like the Alps." To invoke the igneous fusion of the whole planet to account for phenomena like these is, therefore, he concludes, to have recourse to a machinery "utterly disproportionate to the. effects which it is required to explain."

Sir Charles Lyell derives an argument against the theory of central heat from the consideration that it would, in his opinion, involve the existence of tides in the internal fire-ocean, which tides would register themselves in the swellings and subsidences of volcanoes. "May we not ask," he says, "whether, in every volcano during an eruption, the lava which is supposed to communicate with a great central ocean, would not rise and fall sensibly; or whether, in a crater like Stromboli, where there is always melted matter in a state of ebullition, the ebbing and flowing of the liquid would not be constant?" We venture to remark that this argument does not seem unanswerable. No one denies that the crust is at present consolidated to the depth of at least from thirty to eighty miles. The capacity of known chemical forces to produce intense heat

in this region is not disputed. The eruptions of now active volcanoes might arise, therefore, from processes going on in a part of the crust separated by solidified strata from the internal reservoir of liquid fire, and not accessible to its tides. We might ask also, in turn, whether observations have been made upon volcanoes in a state of eruption exact enough to determine whether they are or are not influenced by internal tides?

It is affirmed by Mr. David Forbes, in a recent number of Nature, that Professor Palmieri stated, as the result of observations made by him during the last erupti6n of Vesuvius, "that the moon's attraction occasioned tides in the central zone of molten lava, in quite a similar manner as it causes them in the ocean." Mr. Forbes adds that " a further corroboration of this view is seen in the results of an examination of the records of some 7,000 earthquake Shocks which occurred during the first half of this century, compiled by Perry, and which, according to him, demonstrate that earthquakes are much more frequent in the conjunction and opposition of the moon than at other times, more so when the moon is near the eartb than when it is distant, and also more frequent in the hour of its passage through the meridian." If these statements are correct—and we have no reason to call them in question—the supposed fact, which Sir Charles presumed to tell in his favor, has been converted into an ascertained fact' which tells most forcibly against him.

In the latest edition of his " Principles of Geology," Sir Charles Lyell seems, in at least one passage, to assume that this controversy is at an end.

"It must not be forgotten" (these are his words) "that the geological speculations still in vogue respecting the original fluidity of the planet, and the gradual consolidation of its external shell, belong to a period when theoretical ideas were entertained as to the relative age of the crystalline foundations of that shell wholly at variance with the present state of our knowledge. It was formerly imagined that all granite was of very high antiquity, and that rocks, such as gneiss, mica-schist, and clay-slate, were also anterior in date to the existence of organic beings on a habitable surface. It was, moreover, supposed that these primitive formations, as they are called, implied a continual thickening of the crust at the expense of the original fluid nucleus. These notions have been universally abandoned. It is now ascertained that the granites of different regions are by no means all of the same antiquity, and it is hardly possible to prove any one of them to be as old as the oldest known fossil organic remains. It is likewise now admitted, that gneiss and other crystalline strata are sedimentary deposits which have undergone metamorphic action, and they can almost all be demonstrated to be newer than the lately discovered fossil called Eozoon Canadense.

With all deference to one whom we acknowledge to be among the very ablest living geologists, we must say that this language strikes us as more emphatic than the state of the discussion warrants. We do not undertake absolutely to maintain the theory of central heat as explaining the formation of the granitic and metamorphic rocks, but we cannot admit, what Sir Charles seems to imply, that the time has arrived when investigation and experiment on the subject may be relinquished, and the tone of dogmatic confidence assumed. The reasonableness of permitting a certain degree of suspense of judgment regarding it becomes the more evident when we observe that Sir Charles is not prepared to maintain against astronomers that the planet was not originally fluid. "The astronomer," he says, "may find good reasons for ascribing the earth's form to the original fluidity of the mass in times long antecedent to the first introduction of living beings into the planet; but the geologist must be content to regard the earliest monuments which it is his task to interpret as belonging to a period when the crust had already acquired great solidity and thickness, probably as great as it now .possesses, and when volcanic rocks, not essentially differing from those now produced, were formed from time to time, the intensity of volcanic heat being neither greater nor less than it is now."

There can be no doubt that astronomers have been startled into something like general protest against the rigid uniformitarianism of Sir Charles Lyell. Differing as they do very widely in their conceptions of the probable manner in which planets are formed, they seem to agree that those bodies have their beginning in heat and in fusion. The phenomena of variable stars, taken in connection with the revelations of spectrum analysis, demonstrate that the combustion and the cooling of starry masses are occurrences

not unknown in the economy of the universe. If Sir Charles declines to contest the astronomical position of the original fluidity of the planet, considerable plausibility will continue to attach to that geological doctrine which connects the crystalline rocks with the fluidity in question. Those rocks, from the most ancient granites to the most recent clay-slates, occupy a large proportion of the earth's surface. Their great general antiquity is indisputable. The theory that they furnish the link between the past and the present of the earth's crust—that they furnish the point where the lights of geological and of astronomical science meet —strongly commends itself to the mind.

These observations derive additional force from the circumstance that Sir Charles Lyell's doctrine of the modem and chemical origin of all crystalline rocks is dependent upon considerations which must be allowed to possess not a little of a hypothetical and precarious character. The phenomena of metamorphism, as arising from heat, from thermal springs, and so on, are well-known and important; but there is nothing like adequate evidence that they are capable of giving the crystalline rocks that structure and aspect under which we behold them. The chemical substances in the crust which Sir Charles presumes to be capable of forming seas of molten matter, five miles deep and 5,000 miles long, have .never placed before human eyes a lake of fire three miles across; is there not a trace of arbitrary hypothesis in supposing that, during hundreds of millions of years, those chemical agencies have been providing, beneath the surface of the world, cauldrons of fire to melt the granites of all known ages, from the Laurentian to the Tertiary, to produce the twistings, undulations, contortions of the metamorphic strata throughout hundreds qf thousands of cubic miles of rock, and to feed every volcano that ever flamed on the planet? Not even to that proposition which is avowedly at the basis of Sir Charles's theory, namely, that the solidified shell of the earth is at least from 800 to 1,000 miles thick, can absolute certainty be said to belong. We are willing to admit the distinguished ability of Mr. Hopkins; but it is a fatal mistake to impute to solutions of problems in mixed mathematics that character of certainty which belongs to the results of purely mathematical reasoning. Into every problem of mixed mathematics one element at least enters which depends for its correctness upon observation. In many cases this correctness depends on the perfect accuracy of instruments, and upon consummate skill in using them. A minute error in the original observation may produce comprehensive error in the conclusion. It is still fresh in the public memory that new and more accurate observation corrected by millions of miles a calculation comparatively so simple as the distance between the earth and the sun. The problem by the solution of which Mr. Hopkins determined that the minimum thickness of the crust is from 800 to 1,000 miles depends for its reliability on certain obscure phenomena connected with precession and nutation. Sir Charles Lyell admits that the problem is a " delicate " one. Mr. Charles MacLaren remarked, and Miller quotes the remark with approval, that Mr. Hopkins's inference "is somewhat like an estimate of the distance of the stars deduced from a difference of one or two seconds in their apparent position, a difference scarcely distinguishable from errors of observation." Add to this that opinions might be quoted from mathematicians of name as decidedly in favor of the theory that the geological changes which have taken place in the earth's crust are due to central heat, as the deduction of Mr. Hopkins is opposed to it. In the ninth edition of his " Principles," i.e., in the edition immediately preceding that now current, Sir Charles informs us that

"Baron Fourier, after making a curious series of experiments on the cooling of incandescent bodies, considers it to be proved mathematically, 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 afterwards been constantly cooled,"

Sir Charles replied to this in the same edition that, if the earth were a fluid mass, a circulation would exist between centre and circumference, and solidification of the latter could not commence until the whole had been reduced to about the temperature of incipient fusion. We fail to see that this is an answer to Baron Fourier. What necessity is there for supposing that

the solidification of the crust commenced before the matter of the globe had been reduced throughout to about the temperature of incipient fusion? The water in a pond must be reduced to about the temperature of incipient freezing before ice can form on the surface, but this does not prevent the formation of a sheet of ice on the top.

In the article in Nature, from which we have already quoted, Mr. David Forbes mentions that M. De Launay, Director of the Observatory at Paris, "an authority equally eminent as a mathematician and an astronomer," having carefully considered Mr. Hbpkins's problem, decided that its data were incorrect, and that it could shed no light whatever on the question whether the globe is liquid or solid. There is some doubt, however, as to the import of M. De Launay"s statement.

We may be the more disposed to wonder at the decision with which Sir Charles Lyell pronounces upon this subject in his latest edition, by the fact that, since the publication of the previous edition, he has modified, to a very serious extent, his conception of the evidence on which the theory which he adopts is based. In the ninth edition of the " Principles" he laid so much stress on Sir Humphry Davy's hypothesis of an unoxidized metallic nucleus of the globe, liable to be oxidized at any point of its periphery by the percolation of water, and thus to evolve heat sufficient to melt the adjacent rocks, that Hugh Miller, in contending against Sir Charles, selected this as an essential part of the argument. In his tenth edition, Sir Charles does not even mention Sir Humphrey Davy's theory. The star under the influence of which the tenth edition was prepared was that of Mr. Darwin. No brighter star may be above the geological horizon, and Sir Charles may have done well to own its influence, but we submit that opinions which undergo important modification within a few years ought hardly to be promulgated as marking the limit between the era of darkness and the era of light in geological discovery.

After all, however, the crucial question is, whether the theory of central heat has any positive evidence to support it. Here we meet, in the first place, with the undisputed fact that heat increases as we descend from the surface of the earth.

« 이전계속 »