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and heat to constitute a case of genuine combustion. There must have been a time therefore, as it should seem, when these substances existed uncombined with oxygen even on the surface, and there is no reason to deny, that the process of oxygenation may still be incomplete at those vast depths, where air and water are admitted but slowly or at distant intervals.

Indeed, the late discoveries of Mr. Grove* have rendered it almost demonstrable, that admitting the leading principle for which all the advocates of the mechanical theory contend, namely the existence of a very high temperature pervading at one time all the constituents of our globe,-a temperature sufficient to reduce to a state of fusion at least, if not of volatility, the most stubborn bodies of which we have any cognizance,--a period must have occurred in the history of our planet when those

very

actions were set up of which we assume the continuance at the present day.

For Mr. Grove, by showing, that at a heat attainable by artificial means, even the attraction between oxygen and hydrogen is overcome, and water resolved into its elements, has rendered it more than probable that at the high temperature supposed to have prevailed in the infancy of our system, all the constituents of the globe must have been mutually diffused, and though thus intermixed, would have continued in a state of perfect chemical indifference one to the other. Now if under these circumstances we suppose the temperature to have sunk down to that point at which the elective attraction of certain of the elements for each other prevailed over the repulsive force of heat, we have a right to infer the occurrence of those very phænomena and the formation of those very products, which our theory assumes to be going on at the present day wherever volcanos exist. There is therefore no antecedent absurdity in imagining that volcanic action may consist in a process of oxygenation, caused, in part at least, by the presence of these substances, and all that seems necessary is to ascertain how far the known phænomena accord with such an hypothesis.

The other class of theories, which begins by assuming the high temperature, and then deduces from it the other phæno

* Philosophical Transactions, 1846.

mena, seems at first sight to have an advantage over the preceding one, inasmuch as the existence of internal heat may be thought to be in a manner ascertained, whilst that of the alkaline or earthy metalloids, uncombined with oxygen, is at most only probable; and accordingly many have been induced to prefer this mode of accounting for the phænomena, as less hypothetical, and requiring fewer postulates.

They forget, however, that the existence of an internal heat is assumed alike on either supposition, and that the true point of dispute is, whether it can best be explained by the presence of a melted or ignited mass in the interior of the globe, or by a process of oxygenation going on amongst its constituents.

It is indeed a common fallacy to set down internal and central heat as identical, although a moment's consideration will convince us that the one is a matter of observation, the other purely of inference, and that the only decisive mode of establishing the latter proposition, would be by demonstrating that the nucleus of the globe either is, or at least once was, in a state of fluidity.

Now the only direct argument in favour of the internal fluidity of the globe is deduced from its figure, which has been proved to be that of an oblate spheroid ; a form, it is contended, which could not have been imparted to it unless it had been originally liquid, and from whence the advocates of the above hypothesis conceive themselves at liberty to infer, that it is in this state at present.

Neither of these propositions however can be regarded as demonstrated. Sir J. F. Herschel has shown, in his “Treatise on Astronomy,' that the oblate figure of the globe may only have arisen from its long-continued rotation, this being the point to which, under this condition, it must tend, and which it would ultimately attain, even as its surface is at present constituted.

Professor Playfair, in his Illustrations of the Huttonian Theory' (p. 435), has also contended, that if the surface of the earth has been repeatedly changed from sea to land, the figure of the planet must in that case have been at length brought to coincide with its actual one.

But Mr. Lyell has gone further, and in accordance with the views of Poisson shows, that if portions of the interior of

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the earth had become from time to time fluid, from chemical causes acting locally, these fluid portions would be impelled towards the equatorial regions in obedience to the centrifugal force, and thus produce an upheaval of the surface in the parts adjoining. Hence the oblate spheroidal figure of the earth affords no independent proof of its original, and still less of its present universal fluidity.

Neither, if we grant the earth to have been originally fluid, is there any direct proof that it would have continued so up to the present time; for the progressive augmentation of heat observed at the slight depths below the surface to which man has penetrated, only proves that the temperature of the crust is higher than that of its superficies, not that it is considerable enough to retain the substances of which the interior is made up in a state of fusion.

There is something indeed very striking in the law of the increasing temperature of the globe, and in the practical illustration of its truth afforded by the fulfilment of M. Arago's prediction as to the thermal condition of the water which would be obtained from the Artesian well at Grenelle, but persons who build upon this and other similar facts are not always sufficiently alive to the extremely small distance beneath the surface to which man has penetrated, compared to the diameter of the globe itself.

As well might a Lilliputian, who with his utmost efforts had only bored into the white of the orange which he had extracted from Gulliver's pocket, conclude that this same tough material pervaded the entire mass, in ignorance of the pulp which it enveloped, as the philosopher who finds warm water at the bottom of his deepest well or mine pronounce, that the temperature goes on progressively to increase from thence to the centre.

The following wood-cut, taken from a larger engraving executed some years ago in illustration of the comparative depth and elevation of remarkable mines in Great Britain and other countries, may render this fact more familiar, by impressing upon our minds the very limited extent within which our observations upon the internal temperature of the globe are at present circumscribed.

D. Dolcoath Mine, Cornwall, 1110 ft. A. Wheal Abraham, Cornwall, 1452 ft. C. Consoliated Mines, Cornwall, 1350 ft. (Woolf's Shaft).-P. Ditto (Pearce's Shaft), 1464 ft. B. Wheal Betsy, Devon, 720 ft. F. Wheal Friendship, Devon, 1100 ft. S. Sampson Mine, in the Hartz, 2230 ft. K. Kitzpuhl Mine, in the Tyrol, 2764 ft. V. Valenciana Mine, Mexico, 1770 ft.

a. Mining districts of about 300 ft. b. Cornwall. c. Highgate and Hampstead Hills, 400 ft. d. Shooter's Hill, 446 ft. e. Dartmoor, 1800 ft. f. Mining district of Andreasberg, about 2000 ft. g. Ingleborough, 2361 ft. h. Brocken Mountain, 3486 ft. i. Snowdon, 3571 ft. k. Mountain of Falkenstein, about 5000 ft. 1. Ben Nevis, 4380 ft. m. Table land of Mexico, 7000 ft. n. Mountains near Guanuxuata, 7000 to 8000 ft.

Moreover, according to Mr. Hopkins's calculations, the thickness of the solid crust of the globe must at the least approach to 400 miles, and probably does not fall short of from 800 to 1000,-a sufficient proof that the dogma which assumes a regularly increasing rate of temperature from the circumference downwards, can hold good only down to comparatively small depths; for we know of no substance, or combination of substances, which would not enter into fusion at a point far below that which must be reached by the earth at even a few miles' depth according to that assumption.

Indeed, according to the calculations of M. Cordier, we should arrive at the melting-point of iron at a depth not exceeding twenty-four miles, and this too according to the extravagantly high estimate of its fusing-point founded upon observations with Wedgwood's pyrometer.

Thus the present fluidity of the interior of the earth is still more a matter of surmise than its original state of liquefaction.

After all, the proper mode of grappling with the question before us seems to be, not to lose ourselves in conjectures, as to what may by possibility be the condition of the globe at inaccessible depths, but to pass in review the actual phænomena of volcanos, and see whether we can best deduce them from the mere effects of the protrusion of a melted mass of matter, or from a process of combustion, originating in materials which may still exist in an unoxidized state below. In order however to determine this, it will be necessary to consider at some length, first, the geographical situation of volcanos; secondly, the character of the substances evolved by them in a gaseous state, and of the products resulting; and thirdly, that of the lavas and other matters ejected in a solid or liquid condition : from whence we shall be led on to examine, fourthly, the depth at which volcanic action is seated; and lastly, the mode in which mountain masses and extensive tracts of country are built up of the materials that have resulted from the operation of internal fires.