Unacquainted with Mr. Brayley's views, Professor John Phillips, at about the same time (1838), in his Treatise on Geology,' in the Cabinet Cyclopmdia; remarked,—" There is not, we believe, any attempt on record to deduce all the chemical phenomena of volcanoes from the hypothesis of general heat below the surface of the earth ; we must therefore, at present, suppose this is difficult, except upon the admission of that powerful absorption of oxygen, from water, which the chemical hypothesis provides." Proceeding to inquire whether the of the latter " involve the rejection of the hypothesis of a per vading high temperature below the surface of our planet," and replying in the negative, he continues : "It appears to us very clear, that the union of the two speculations here brought into comparison is not only practicable, but reasonable, and even neceosary." We have seen, how ever, that an attempt had been made to deduce all the chemical phe noinena of volcanoes from the hypothesis of subterranean heat, by the intervention of the thermotic theory, certain chemical consequences being assigned to the latter. Several years after, Prof. Phillips penned the view of the subject taken in the preceding section of the present article ; and his most recent ideas respecting it are stated in the following terms in his first Presidential Address to the Geological Society, 1859.
" That the earth is still fluid within, under the regions of volcanic action, and ever ready to pour out its melted constituents under the pressure of elastic vapour, is evident by all the phenomena of volcanic excitement. Is this fluidity due to the residual heat of the globe, still effective in these regions, or maintained if not excited here by the chemical process of oxidation, by the decomposition of water, and the reunion of one of its elements with the uneombined bases of the earths, alkalies, and metals The answer, if taken from volcanic phenomena alone, appears ambiguous. The chemical products of volcanoes, indeed, require the admission of water to the roots of the fiery action, and the decomposition of it there ; but this seems not decisive of the question, whether the bases of the alkalies and earths and metals exist uncom. blued with oxygen in these situations, chemists of eminence taking different views of the matter." " If we take earthquakes for our guide, these tremors appear to follow laws which apply to elastic solids, not undulating fluids, and yet they presuppose a shock or displacement [see col. 669], such as a fluid support for a solid crust might well account for." All the preceding inductions and speculations, however, will be affected by the conclusions at which we may arrive on a subject of a cosmical nature, and relating to the structure of the planet. This is the probable thickness of the solid crust of the globe, assuming it to consist of a fluid nucleus of high temperature inclosed in a solid shell.
The phenomena of the increase of temperature with the depth, and their consequences with regard to the heat to which the bodies com posing the crust must be subject at comparatively small depths, as briefly noticed above, have led some to conclude that the crust, or external solid shell, is not of thickness than sixty or seventy miles, and others have considered it even less : Mr. Darwin,
for example, from his researches on the South American volcanoes, infers a probable thickness of twenty miles only. Mr. W. Hopkins, and Archdeacon Pratt, of Cambridge, Professors Hennessey and Haughton, of Dublin, all mathematicians and physicists, and all having a well-earned right to independent judgment iu matters of physical geology, have severally investigated this problem, and the latter two inquirers differ greatly from the former, if not in some degree from each other, in the thickness they respectively assign to the crust. But we are disposed to with Archdeacon Pratt, that the result Mr.
Hopkins has obtained agrees best with all our knowledge. The nume rical result of his refined investigation (1839-1841) is, that the least admissible thickness of the crust must be about one-fifth of the earth s radius ; a result which after many years' devotion to physical geology he has recently (1859) confirmed, remarking that, " without assigning any great importance to an exact numerical result," he retained full confidence in the investigation, "as showing that the thickness of the crust could not be so small as 200 or 300 miles, and consequently that no geological theory can be admitted which rests on the hypothesis of the crust being nearly as thin as it has been frequently assumed to be." This conclusion will necessarily affect all our views relative to the causation of plutonic and volcanic phenomena, on whatever foundation they may rest. The source of volcanoes must be in cavities contained in the solid crust at depths probably not greater than those at which the solidity of the crust had been supposed to terminate, by those geologists who reasoned only from the known increase of heat from the surface downwards. Mr. Hopkins, himself, proposes to explain their phenomena " on the supposition that a portion of matter more fusible than the general mass of the globe, exists in a state of fusion in subterranean reservoir», forming so many subterranean lakes of deter minate extent ; in some cases originally distinct ; in others, com municating with adjoining lakes by more or less obstructed channels." The' view, however, of plutonic) and volcanic action at which we have now arrived, combining the thermotie with the chemical theory, and including also the consideration of the thickness of the earth's crust, will itself require modification, in 'consequence of Mr. Grove's remarkable observation of the decomposition of water by heat alone, independently of chemical action.* From this it would appear to follow irresistibly, first, that in the very interior of the globe the elements composing water must exist in a state of separation from each other, as it were, rigidly separated by heat ; but still, from the immense pressure to which they must neces sarily be subject, in a very dense state, so that, upon redtiction of temperature—lowering of the heat—they would immediately enter into combination.