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 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 palæontological 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 palæozoic forms of life in various parts of Europe, they add, "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 general laws which govern the whole animal kingdom." M. Barrande has made forcible remarks to precisely the same effect. It is indeed quite futile to look to changes of currents, climate, or physical conditions, as the cause of these great mutations in the forms of life 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, from not including those forms which are only found in the older under-lying 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. Darwin, 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 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 day-periods 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 vapoury veil, and dim curtains of clouds 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 dependent upon the central fires, favoured 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 Charles Lyell rejects absolutely the theory of central heat as a mode of accounting for these 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 linearly-arranged 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 eruption 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 earth 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 favour, 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.