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51° Fahr., these numbers give a rate of io for every fifty-eight feet. If, then, the increase of heat is only 100° per mile, at a depth of less than ten miles everything must be red-hot, and at thirty or forty in a melted state. It was by all admitted that the rise of temperature with the depth is not at all local, but occurs in whatever part of the earth the observation may be made. The general conclusion thus furnished was reinforced by the evidence of volcanoes, which could no longer be regarded as merely local, depending on restricted areas for the supply of melted material, since they are found all over the land and under the sea, in the interior of continents and by the shores, beneath the equator and in the polar regions. It had been estimated that there are probably two thousand aerial or subaqueous eruptions every century. Some volcanoes, as Ætna, have for thousands of years poured forth their lavas, and still there is an unexhausted supply. Everywhere a common source is indicated by the rudely uniform materials ejected. The fact that the lines of volcanic activity shift pointed to a deep source; the periodic increments and decrements of force bore the same interpretation. They far transcend the range of history. The volcanoes of central France date from the Eocene period; their power increased in the Miocene, and continued through the Pliocene; those of Catalonia belong to the Pliocene, probably. Coupled with volcanoes, earthquakes, with their vertical, horizontal, and rotary vibrations, having a linear velocity of from twenty to thirty miles per minute, indicated a profound focus of action. The great earthquake of Lisbon was felt from Norway to Morocco, from Algiers to the West Indies, from Thuringia to the Canadian lakes. It absolutely lifted the whole bed of the North Atlantic Ocean. Its origin was in no superficial point.
A still more universal proof of a high temperature affect. Proof from ing the whole mass of the interior of the globe was believed density. to be presented in the small mean density of the earth, a density not more than 5•66 times that of water, the mean density of the solid surface being 2.7, and that of the solid and sea-surface together 1.6. But this is not a density answering to that which the earth should have in virtue of the
Arguments for former High Temperature.
attraction of her own parts. It implied some agent capable of rarefying and dilating, and the only such agent is heat. Although the law of the increase of density from the upper surface to the centre is unknown, yet a comparison of the earth's compression with her velocity of rotation demonstrated that there is an increasing density in the strata as we descend. The great fact, however, which stands prominently forth is the interior heat.
Not only were evidences thus offered of the existence of a high temperature, and, therefore, of the lapse of a long time by the present circumstances of the globe; every trace of its former state, duly considered, yielded similar indications, the old evidence corroborating the new. And soon it appeared that this would hold good whether considered in the inorganic
or organic aspect. Inorganic In the organic, what other interpretation could be put on proofs of a former high
the universal occurrence of igneous rocks, some in enormtempera
ous mountain-ranges, some ejected from beneath, forcing their tortuous way through the resisting superincumbent strata; veins of various mineral constitution, and, as their relations with one another showed, veins of very different dates? What other interpretation of layers of lava in succession, one under another, and often with old disintegrated material between ? What of those numerous volcanoes which have never been known to show any signs of activity in the period of history, though they sometimes occur in countries like France, pre-eminently historic? What meaning could be assigned to all those dislocations, subsidences, and elevations which the crust of the earth in every country presents, indications of a loss of heat, of a contraction in diameter, and its necessary consequence, fracture of the exterior consolidated shell along lines of least resistance? And though it was asserted by some that the catastrophes of which these are the evidences were occasioned by forces of unparalleled energy and incessant operation,—unparalleled when compared with such terrestrial forces as we are familiar with,—that did not, in any respect, change the interpretation, for there could have been no abrupt diminution in the intensity of those forces, which, if they had less
Its Consequence a Long Duration.
293 ened in power, must have passed through a long, a gradual These ne
cessarily decline. In that very decline there thus spontaneously came imply long forth evidences of a long lapse of time. The whole course of Nature satisfies us how gradual and deliberate are her proceedings; that there is no abrupt boundary between the past and the present, but that the one insensibly shades off into the other, the present springing gently and imperceptibly out of the past. If volcanic phenomena and all kinds of igneous manifestations,—if dislocations, injections, the intrusion of melted material into strata were at one time more frequent, more violent,-if, in the old times, mundane forces possessed an energy which they have now lost, their present diminished and deteriorated condition, coupled with the fact that for thousands of years, throughout the range of history, they have been invariably such as we find them now, should be to us a proof how long, how very long ago those old times must have been.
Thus, therefore, was perceived the necessity of co-ordinating Support the scale of time with the scale of space, and such views of nomical the physical history of the earth were extended to celestial bodies which were considered as having passed through a similar course. In one, at least, this assertion was no mere matter of speculation, but of actual observation. The broken surface of the moon, its volcanic cones and craters, its mountains, with their lava-clad sides and ejected blocks glistening in the sun, proved a succession of events like those of the earth, and demonstrated that there is a planetary as well as a terrestrial geology, and that in our satellite there is evidence of a primitive high temperature, of a gradual decline, and, therefore, of a long process of time. Perhaps, also, considering the rate of heat-exchange in Venus by reason of her proximity to the sun, the pale light which it is said has been observed on her non-illuminated part is the declining trace of her own intrinsic temperature, her heat lasting until now.
If astronomers sought in systematic causes an explanation Astronomi. of these facts,-if, for instance, they were disposed to examine imply slow how far changes in the obliquity of the ecliptic were connected changes. therewith,-it was necessary at the outset to concede that the
The Length of Time scale of time on which the event proceeds is of prodigious duration, this secular variation observing a slow process of only 45":7 in a century; and hence, since the time of Hipparchus, two thousand years ago, the plane of the ecliptic has approached that of the equator by only a quarter of a degree. Or if, again, they look to a diminishing of the eccentricity of the earth's orbit, they were compelled to admit the same postulate, and deal with thousands of centuries. Under whatever aspect, then, the theory was regarded, if once a former high temperature was admitted, and the fact coupled therewith that there has been no sensible decline within the observation of man, whether the explanation was purely geological or purely astronomical, the motion of heat in the mass of the earth is so slow, yet the change that has taken place is so great, the variations of the contemplated relations of the solar system so gradual—under whatever aspect and in whatever way the fact was dealt with,—there arose the indispensable concession of countless centuries.
To the astronomer such a concession was nothing extraordinary. It is not because of the time required that he entertains any doubt that the sun and his system accomplish a revolution round a distant centre of gravity in nineteen millions of years, or that the year of e Lyræ is half a million of ours. He looks forward to that distant day when Sirius will disappear from our skies, and the Southern Cross be visible, and Vega the polar star. He looks back to the time when y Draconis occupied that conspicuous position, and the builders of the great pyramid, B.c. 3970, gave to its subterranean passage an inclination of 26° 15', corresponding to the inferior culmination of that star. He tells us that the Southern Cross began to be invisible in 52° 30' N., 2900 years before our era, and that it had previously attained an altitude of more than 10°. When it disappeared from the horizon of the countries on the Baltic, the pyramid of Cheops had been erected more
than a thousand years. Proofs of
We must pass by a copious mass of evidence furnished by aqueous ef- aqueous causes of change operating on the earth's surface, fects,
though these add very weighty proof to the doctrine of a long
period. The filling up of lakes, the formation of deltas, the cutting power of running water, the deposit of travertines, the denudation of immense tracts of country, the carrying of their detritus into the sea, the changes of shores by tides and waves, the formation of strata hundreds of miles in length, and the embedding therein of fossil remains in numbers almost beyond belief, furnished many interesting and important facts. Of these not a few presented means of computation. It would not be difficult to assign a date for such geographical events as the production of the Caspian and Dead Seas from an examination of the sum of saline material contained in their waters or deposited in their bed, with the annual amount brought into them by their supplying rivers. Such computations were executed as respects the growth of Lower Egypt and the backward cutting of Niagara Falls, and, though they might be individually open to criticism, their mutual accordance and tendency furnished an evidence that could not be gainsaid. The continual accumulation of such evidence ought not to be without its weight on those who are still disposed to treat slightingly the
power of geological facts in developing truth. To such facts were added all those, with which volumes And from might be filled, proving the universality of the movements of ments of the solid crust of the earth,-strata once necessarily horizon
crust, tal, now inclined at all angles, strata unconformable to one another,-a body of evidence most copious and most satisfactory, yet demonstrating from the immensity of the results how slowly the work had gone on.
How was it possible to conceive that beds many hundred feet in thickness should have been precipitated suddenly from water? The mechanical condition implied slow disintegration and denudation in other localities to furnish material; their contents showed no trace of violence; they rather proved the deposition to have occurred in a tranquil and quiet way. What interpretation could be put upon facts continually increasing in number like those observed in the south-east of England, where freshwater beds a thousand feet thick are covered by other beds a thousand feet thick, but of marine origin? What upon those in the north of England, where masses once up