673 



VOLCANO. 



VOLCANO. 



nrt 



seen, were altogether in ita 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 Berzelius 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, 

 as 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. Scrope's " views of the origin 

 and constitution of lava, &c.," then and recently considered so 

 anomalous, "as other probable truths involved in the new [that is, 

 the thermotic] 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. 



Unacquainted with Mr. Brayley's views, Professor John Phillips, at 

 about the same time (1838), in his 'Treatise on Geology,' in the 

 ' Cabinet Cyclopaedia,' 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 

 results 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 necessary." We have seen, how- 

 ever, that an attempt had been made to deduce all the chemical phe- 

 nomena 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 uucombined 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 uncorn- 

 binwl 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. 60y], 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 



AUT9 AXI> SCI. DIV. VOL. VIII. 



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 geologists to conclude that the 

 crust, or external solid shell, is not of greater 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 in 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 agree 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 reservoirs, 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 thermotic 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 

 heal ; but still, from the immense pressure to which they must neces- 

 sarily be subject, in a very dense state, so that, upon reduction of 

 temperature lowering of the heat they would immediately enter 

 into combination. 



Secondly, that even in the superficwl lakes of molten or flowing 

 matter, to which, after Mr. Hopkins, as we have seen, we must attri- 

 bute volcanoes, universally distributed below the mere surface of the 

 earth, as shown by Mr. Darwin, the result of the deposition on the 

 bed of the ocean of new deposits, causing the rise of the isothermal 

 surfaces, as demonstrated by Babbage and Herschel in the more 

 heated parts of those lakes water will exist, not as steam or as 

 incandescent or ignited water, but as mixed but uncombined oxygen 

 and hydrogen gases. At the same time the mere effect of heat will bo 

 modified by pressure, and the chemical action of the bodies present, so 

 that we have here another great result perfectly in harmony with the 

 thermotic theory on the one hand and the chemical theory on the 

 01 her. But we are not to expect any evidence of the existence of free 

 oxygen and hydrogen below to arrive at the surface, for as the upper 

 parts of the molten flood will be of inferior temperature in them, or 

 in then- ignited solid roof, the gases will reunite into water, and 

 eventually ascend to the surface, as steam, or mingled with the mineral 

 elements of. lava, in some state intermediate between those of liquid 

 r.ii'l vaporous water, to be disengaged as steam in the volcano, and from 

 the surface of currents of lava. 



Thus, all the great powers and forces which govern the material 

 elements of the globe mutually act and react upon each other. Pres- 

 sure and condensation caused by gravity, combination resulting from 

 chemical affinity or attraction, heat, producing alternately rarefaction 

 or condensation, combination or separation, accordingly as it is related 

 to the other forces ; the whole being so held in equilibrium between 

 themselves and the antagonistic action of the sun upon the surface of 



* It has been dou 

 experiment is really d 



ibtcd whether the decomposition of water in Mr. Grove'a 

 due to the action of heat ; but there is no room for doubt 



