VOLCANO. 



VOLCANO. 



611 



and rh+mV-* 1 elements a tnily liquid Ut-, independently of the agency 

 o/ water. 



The view s of this consummate volcanic geologut, however, are now 

 probably to be regarded as forming only a first approximation to a 

 true theory of the nature of lava. The researches which have been 

 subsequently pursued in the chemistry of geology, and in the artificial 

 synthetic production of minerals, as well as in the geology of the 

 metamorphic rocks, and respecting the action of the igneous rocks 

 upon the sedimentary strata, tend to modify the original conclusions 

 of Mr. Scrope, but still only in the manner which has characterised 

 the progress of all great discoveries in science. The necessary agency 

 of water, together with intense heat, in the production of the crystalline 

 rocks, has been urged by Scheerer and E. de Beaumont. Its operation 

 through a great range of temperatures, both in producing and in alter- 

 ing rocks, has been demonstrated or illustrated by Bunsen and Delesse, 

 and more recently by a remarkable series of experiments by Professor 

 Daubree of Strasburg. He has shown " that the molecular state of 

 the water in lavas, 'be it what it may,' has had a great effect in the 

 formation of silicate*, even when anhydrous. It causes them to 

 separate, and to crystallise at a temperature much below their point 

 of fusion ; it enables them to crystallise in an order of succession 

 different from that of their fusibility ; thus, for example, leiicite, on 

 infusible silicate of alumina and potash, occurs in lavas in well-formed 

 crystals, often of large sixe. To this, Ludwig, in his German trans- 

 lation of Daubree' a essay, adds that the crystals of leucite often con- 

 tain fragments of lava, and even small crystals of the very fusible 

 mineral augite." These results are most apposite to, and beautifully 

 elucidatory of, the true nature of lava as originally observed by Mr. 

 Scrope. His views, perhaps, would have sooner been entertained by 

 other geologists, had he more explicitly distinguished between the 

 vitreous and the stony lavas when flowing. The former when in the 

 purest and most characteristic condition present us with the state of 

 igneous fusion in its most perfect form ; solidifying into glass when 

 rapidly, and crystallising when very slowly, cooled. Taking volcanic 

 phenomena as a whole, there doubtless exists every gradation between 

 this and the hydroplastic condition of lava such as that of Vesuvius 

 and Etna, which, however rapidly cooled, does not solidify into glass, 

 but into an aggregate rock or stone, identical with that resulting 

 from its slow cooling in nature. It is important to observe that the 

 glassy lavas are converted by slow cooling not into stony lava, but 

 merely into the crystalline state of the combination of silicates of 

 which they consist. The stony lavas, however, when truly fused, 

 whether naturally or artificially, become the glassy lavas if quickly 

 cooled, or the corresponding vitrite or crystalline substance if allowed 

 to cool slowly. Such also is evidently the origin of the vitreous lavas 

 in volcanoes, their observed transitions from the glassy to the crystalline 

 or stony form depending partly on their mineral constitution and their 

 condition when in the flowing state, and partly on the circumstances 

 and rate of their refrigeration. Of all this we now possess ample 

 evidence, both geological and experimental. For some of the former 

 we may refer to the facts detailed by Mr. Darwin in his 'Geological 

 Observations on the Volcanic Islands,' &c. ; and as an example of the 

 latter, to the results of fusing basalt, which is an ancient lava, and 

 when melted and rapidly cooled becomes a glass scarcely distinguishable 

 from obsidian, but when gradually cooled does not return to its original 

 condition of a stony lava, an aggregate rock consisting of several mineral 

 substances, but becomes a kind of pearlstone the peculiar crystalline 

 condition of the glass. The importance of these latter facts in reference 

 to the acceptance and right understanding of Mr. Scrope's views has 

 already been urged by Mr. Brayley [WATT, GREGORY, in Bioo. Div.], and 

 been frequently adverted to by him in lectures on igneous geology 



Water undoubtedly is second only in quantity among the products 

 or educts of volcanic action, to the solid earthy matter, oxides, like it, 

 some of non-metallic combustible, some of metallic bases. Its func- 

 tions in the origination of Plutonic and volcanic action were first sug- 

 gested, as we shall find, by the discoverer of the chemical nature ol 

 that earthy matter, Sir H. Davy. Those which it possesses in volcanic 

 . themselves, partly in the condition of vapour, and |irtly 



is probable in a peculiar intermediate condition not yet under- 

 stood, were first truly discovered by Mr. Scrope, and established in 

 his ' Considerations on Volcanoes.' The liquid products of voloanoo 

 also contain, though rarely, the sulphuric and muriatic acids ; am 

 among the substances of most interest in aiding to complete the theory 

 of nh/TTDJ'1'ft 1 actions, are sublimations of common salt, and muriate o 

 n.m..i The origin of these where the volcanoes are situated by 

 the sea-side cannot lie doubtful. Boracic acid is another product o 

 this kind occurring in the crater of volcanoes (Daubeny) ; but this we 

 now know is an abundant element of the earth's crust, widely dissemi 

 uated, though but rarely in great quantities in a single locality. Com 

 pounds of boron, however, do not appear to play an important part in 

 the history of volcanoes. 



The gaseous products of volcanoes are important in the investigation 

 of the chemical theory of the igneous action. Besides the clouds o 

 vapour of wafer (10 abundant in eruptions, and so often productive o 

 local rains), chlorine, azote, sulphuretted hydrogen, sulphurous acid 



Ucn.rd Ilonur, K.R.S., Annlrmsrv Addms to Geological Society o 

 Ucdoa, ISfcl. Ituirt. Joan." lol. nil. pp. xlvtU xllx. 



and carbonic acid, are the most common. The evolution ..f snlphu- 

 retted hydrogen (depositing sulphur) continues under various ciruum- 

 tances after other signs of activity have ceased in particular volcanic 

 regions ; and even after the craters have fallen in and become full of 

 water, mineral springs and springs rich in carbonic acid flow with little 

 variation for centuries (many such have been flowing from before the 

 commencement of history to the present day), while azotised waters, 

 rising to the surface along the lines of fissures more ancient than any 

 .nown volcanic systems, concur with them in demonstrating the almost 

 ntorminably slow process by which subterranean heat rises to the 

 surface of the earth. 



Chemical Hypothetit of Voleanic Action. The nature of these var 

 roducts, and the order in which they successively make their appear, 

 ance, have been the bases for speculations as to the chemical processes 

 oing on in the interior of volcanic regions. Sir H. l>.i\ \ 

 f the metallic bases of the earths and alkalies, and f tin* extraordinary 

 ppetency for oxygen of several of these bases (potassium, so< linn 

 uggested to that great chemical philosopher a new and ingenious 

 lypothesis of volcanic action. Water admitted to some of the metallic 

 bodies alluded to is instantly decomposed, and its oxygen abnorlwd, 

 with an immediate and very remarkable evolution of heat and light, 

 while the metals become earths or alkalies. The substances most abun- 

 dant in volcanic products contain these earths, and these alkalies, 

 namely, potash, soda, lime, silica, alumina, &c., in various combina- 

 ions., evidently the result of successive crystallisations from a fluid 

 mas*. In this hypothesis it is assumed that the interior portions of 

 he earth consist in part of the metallic bases of the earths and alk 

 hat water is from time to time admitted to these; that violent i.mi- 

 lustion and great heat follow ; that the oxides generated are melted 

 together, constituting lava, while the hydrogen, and some of the water 

 mdecompoeed. go off to form new combinations with sulphur, rhlo- 

 line, carbonic acid, tc., which are liberated from pn-viotu ststes I 

 leat and the various chemical agencies set in activity. The jxiwer 

 which raises the lava, and throws out the clouds of ashes and scoriae, is 

 he undecnmposed and confined steam. 



Whoever looks carefully at this hypothesis will find in it much that 

 s admirable, and little that U open to strong objection, if it be regarded 

 merely as a theory of the eruption of rolcanoa, not as a theory of the 

 changes in the condition of the interior parts of the globe, of which 

 volcanic action is one of the visible exponents. 



It is some recommendation of this view that it seems to unite itself 

 with a general and not improbable speculation regarding the origin of 

 ,he more ancient Plutonic rocks, which certainly must be supposed to 

 lave passed through a very similar series of changes to those which 

 ava has undergone. Those rocks have the same bases as lava ; it is the 

 natural result of chemical reasoning, that the elements which are now 

 combined in them existed at some earlier time in a separate state ; the 

 oxidated and melted granite crust of the earth is formed by the union 

 of these elements, and, according to the hypothesis of Davy,* the new 

 rocks which volcanoes yield are produced by a somewhat similar process 

 of oxidation and fusion. 



But this hypothesis was nevertheless neglected by its author for 

 reasons which do not appear to have been fully stated by himself. 

 It was taken up by Dr. Daubeny, and has been maintained by him 

 with much perseverance and ingenuity of research as a sufficient 

 ' Chemical Theory of Volcanoes.' We may call it the ' Hypothesis of 

 Subterranean Oxidation,' and develop it, according to Dr. Daubeny, as 

 follows : 



Below the surface, at a depth of a few miles, the interior of the 

 earth is assumed to contain the earthy and alkaline metalloids, iron 

 and other metals, sulphur and sulphuretted salts. Slow combustion 

 happening amongst them, even under the continents, by slight additions 

 of moisture and air, generates particular gases (nitrogen, carbonic acid, 

 sulphuretted hydrogen, &c.) ; these rise and combine with .-; 

 which issue along lines of natural fissures, or are discovered in artificial 

 wells, often giving to them a temperature higher than that of the 

 country where they occur. Under the sea or large bodies of water, 

 and especially along lines of sea-coast (where fissures may be supposed 

 more numerous than elsewhere), water may be admitted to the interior 

 more easily and in greater quantity, and may occasion phenomena of 

 the same order, accompanied by other effects more powerful, rapid, and 

 characteristic, until the process ceases for awhile by the choking of the 

 passages which admitted the water. 



The water, decomposed by contact with the metalloids, yields its 

 oxygen to them ; the hydrogen is liberated, but not allowed to escape 

 in great quantity alone, for it readily, under the influence of heat, 

 combines with sulphur into sulphuretted hydrogen, or, with the oxygen 

 of atmospheric air (if any be present), reconstitutes water. Nitrogen 

 is thus liberated, and may be conceived to pass off partly free, partly 

 combined with hydrogen, so as to constitute ammonia, which again 

 unites with chlorine (derived from the sea water), and constitutes sal- 

 ammoniac. While oxygen (derived from atmospheric air) is plentiful 

 in the volcanic channels, the hydrogen will not unite with sulphur, 

 which accordingly combines with oxygon into sulphurous acid. Wlirn 

 the oxygen is consumed, sulphuretted hydrogen is formed in abuin 



rhll. Trans.' 1858. 



