The removal of matter from above to below the sea, in the pro duction of sedimentary strata, produces a subversion of the equilibrium of pressure, and, as we have seen, and which is the most important effect, a subversion of the equilibrium of temperature. But the pro ceas, as described, by which this is effected, must be excessiedy slow, and it will depend, "let, on the depth of matter deposited [as already explained]; 2ndly, on the quantity of water retained by it under the great squeeze it has got ; 3rdly, on the tenacity of the incumbent mass, —whether the influx of caloric from below, wine!' MUST TAKE MACY, acting on that water, shall either heave up the whole mass as a conti nent, or shall crack it, and [the results of the action of the heat upon the sedimentary matter and the water] escape as a submarine volcano [or a linear series of such volcanoes, afterwards to become subabrial and insular], or shall be suppressed until the mere weight of the continually accurnulting mass breaks its lateral supports at or near the coast-linen, and opens there a chain of volcanoes." For a further account of these and other consequences of the rise of the isothermal surfaces, the reader must consult the original mere of the authors of the induction, or of what, for the sake of convenience, we have termed the " thermotic theory of Plutonic and volcanic action." But we may now refer to Sir J. Herschel's account of the facts of the local die tribution and systematic arrangement of volcanoes, cited iu a previous column (666), as evincing their entire agreement with his theoretical views here given.
Such being the position of the subject, an effort was now made to deduce from the thermotic theory, in the most general, but also in the most explicit, manner, the chemical theory of Plutonic and volcanic action, and to show that the latter, as originated by Davy, adopted by Gay-Lussac, and explicitly advocated by Daubeny (though Davy's speculation had been rejected by one of the authors of the thematic theory), was in reality a simple and necessary consequence of the theory of the secular variation of the isothermal surfaces, explained by Babbage and Herschel, and applied by them to account for the same phenomena. This was done by Mr. Brayley, in a Friday evening die course at the Royal Institution (where, thirty years before, Sir H. Davy had announced his theory), deliver) May 11th, 1838, and published in the ' Philosophical Magazine,' series 3, voL ail., pp. 533-536.
Viewing the subject in the most general approximate manner, agree ably to the amount of our actual knowledge at the time, and unavoid ably disregarding a multitude of modifying considerations which must enter into the discussion of the problem, in order to obtain an exact solution of it, Mr. Brayley first pointed out, on the one hand, how great a thickness of deposited matter would be required for the original surface to attain even a moderate temperature above that due to its geographical position ; but, on the other hand, at how insignificant a thickness (or depth), compared to the earth's radius, adequate tem peratures for Plutonic action would occur,----all these inferences being founded on the observed law of tho increase in temperature in descend ing of one degree of temperature (1° Fahr.) for every fifty (moro accurately sixty) feet of depth. The depths at which the temperatures of boiling water and of ignition respectively would be found, are stated above. At the depth of 26 miles, less than part of the earth's radius, cast-iron would melt, or the temperature of 2786' be attained ; at 50 miles depth, a temperature of 5000'; and at 100 miles, only of the earth's radius, one of nearly 11,000' ; either of which, from all analogy, would be more than adequate to the effects required ; for it cannot be doubted that at such temperatures even the most infusible and fixed bodies known to form the earth's crust would be not only liquified but volatilised.
Davy, after repeatedly advocating his own theory, had finally relin quished it, for reasons which it is remarkable that he should not have seen, were altogether in its support, when the theory was duly followed out—in favour of that of an ignited nucleus of the earth, but admitting at the same time that it was adequate to the explanation of all the phenomena it sought to account for.
After drawing attention to these points, Mr. Brayley proceeded to argue that if the theory of volcanoes dependent on that of the secular variation of the isothermal surfaces were true, then the chemical theory must also be true, as being necessarily involved in the wider generalisation of the former. The grounds of the argument were the following, which, as nearly a quarter of a century has elapsed, we now state in the words of Mr. Brayley's original enunciation of his deduction.
" The new deposits formed at the bottom of the sea by detrital matter must. inevitably contain much carbonaceous and other combus tible materials derived from organised beings, and these would become distributed sometimes in a finely-divided state, intimately mingled with earthy bodies,—that is, with the oxides of the earthy, alkaline, and common metals. At the exalted temperatures implied in the theory, many of these oxides, including those of the earthy and alkaline bases, would become reduced to the metallic state ; the ignited water with which the whole would of necessity be saturated, would be decom posed ; its oxygen re-oxidising the bases, and its hydrogen being evolved in an uncombined state. Now one of the most abundant elements in all the detrital matter would necessarily be oxide of iron, which would thus be presented, in a state of minute division, to incan descent, but enormously compressed free hydrogen, by which, agree ably to known results of experiment, it would be reduced to the metallic form, water being re-composed. A new affinity would now come into action : finely divided metallic iron being in intimate contact with the earthy and alkaline oxides, they would be reduced, as in the ordinary method of obtaining potassium and the process by which Davy and Berzeliue first succeeded in deoxidising the combustible bases of silica and alumina, and would eventually react upon the water still present By this constant circulation of affinities, exerted simulta neously in different portions of the heated mass, according to their respective temperatures and to the local distribution within it of the various substances evolved (dependent on their respective properties, OA modified by the enormous pressures to which they would be subject), chemical equilibrium would alternately be established and subverted ; and all the phenomena and effects of plutonic and volcanic action would ensue." Mr. Brayley also briefly alluded to Mr. Scropo's " views of the origin and constitution of lava, &e.,"—then and recently considered so anomalous,—"as other probable truths involved in the new [that is, the thermotie] theory,' from which indeed they were considered in this discourse to be as necessarily deducible, as, in the present article, they have been advocated as confirmable by known facts.