669 



VOLCANO. 



VOLCANO. 



670 



Profe.sor James D. Forbea, and others agree in ascribing this origin 

 to some of the mountains in central France, and apparently on sufficient 

 evidence. It is, however, not a phenomenon admitting of frequent 

 citation. 



Now, the districts thus classed together are not only related by 

 geographical proximity, but have some real subterranean as well as 

 apparent superficial connection. Humboldt and Darwin speak con- 

 fidently of the great volcanic regions of the Andes as one grand system 

 of subterranean activity ; though the manifestations of this at the 

 surface offer local peculiarities, both as to time and circumstances. 

 Mr. Darwin has been led, by the investigation of the volcanoes and 

 earthquakes of the Cordilleras of the Andes, to regard them all as 

 depending primarily on the disturbance of a vast internal sea of melted 

 rock, spread below a large part of South America. 



Other conclusions, equally on a large scale, which have been drawn 

 by M. de Beaumont from other classes of phenomena, have a direct 

 bearing on this subject. M. de Beaumont has inferred that the principal 

 mountain-ranges throughout the world have their several geological 

 dates determinable by comparing the positions of the disturbed and 

 the undisturbed strata in and around them : that to each great period 

 of fractures in the earth's crust belongs a certain prevalent direction in 

 which those fractures happened ; and though this view may be subject 

 to particular objections and restrictions, there is this great truth in it, 

 that the several systematic fractures which it professes to refer to one 

 certain geological date have each an assignable date. This date being 

 assigned, we find that the earth's cruat has in ancient geological times 

 been broken by lines of fracture or bent into flexures, 10, 60, 100, of 

 several hundred miles long, and this often (there are many examples 

 in the British Isles) with no unusual exhibition of really volcanic rock 

 on the line, and even with little appearance of unerupted granite or 

 sienite. These great fractures traverse nearly all regions, with no 

 special reference to active or extinct volcanoes, and it is clear that they 

 are due to a general cause, which has been in operation through all 

 past geological periods, and which produced effects exactly comparable 

 in kind, if greater in degree, to those now performed by modern earth- 

 quakes. But if we consider the account of the effects of the great 

 Lisbon earthquake in 1755, which extended over Europe, changing 

 momentarily the level of the land, raising waves 60 feet high at Cadiz, 

 and 18 feet at Madeira, and causing sensible disturbance in the West 

 Indies and Loch Fyne ; or the narratives of the Chilian earthquakes in 

 1822 and 1835, the former of which raised the sea-shore for 100 miles, 

 and the latter rent and shattered the entire provinces of Canqueres and 

 Conception in every direction it will remain very doubtful in our 

 minds whether the internal power to which earthquakes owe their 

 force has really decreased, or the violence of the earthquake been 

 moderated and relieved by the intermitting action of volcanoes. Mr. 

 Darwin speaks confidently of earthquakes and volcanic eruptions in 

 South America as parts of the same phenomenon, now one and then 

 the other, or both together, but at different points, relieving the 

 pressure on the " internal sea of molten rock ; * and this view, which is 

 the largest, appears at the same time the simplest and best founded of 

 all the postulates for a general theory of volcanoes. 



This able writer has indeed by a simple inference brought us at 

 once to the basis of this theory. He has inferred that the primary 

 shock of an earthquake ia caused by a violent rending of the strata, 

 which on the coast of Chili and Peru seems generally to occur at the 

 bottom of the neighbouring sea. 



Here then we take our basis of a general theory of volcanic actions. 

 The earth's crust is subject to fractures, and has always been subject 

 to fractures on a great scale : below the surface of the earth is now, 

 and was in ancient geological periods, an internal sea of molten rock ; 

 this sea is agitated and thrown bodily from its place by the rending 

 of the strata : a icare of translation (not an ordinary undulation) is 

 generated in the liquid mass [WAVES AND TIDES], which passes 

 rapidly onwards and moves the land on its crest, in a given direction : 

 this is the earthquake. A portion of the melted rock is forced by the 

 general pressure into cavities of the rocks, or spread out in irregular 

 sheets on the bed of the sea ; these are the dykes and interposed beds 

 of Plutonic rock : to some part of the internal hot fluid, water finds 

 access, and the steam which is generated and confined supports local 

 columns of melted rock, in particular fissures of the earth's crust, till 

 the lava finds vent and flows to the surface, or is driven up in dust and 

 scoriie by the violent extrication of the vapour : this is the local 

 volcanic action. As to the composition of that internal sea of melted 

 rock, we may admit it to contain unoxidised metalloids, if by this 

 means we can better explain the peculiar chemical nature of the pro- 

 ducts which come to the surface ; and thus we find at last only one 

 condition remaining to be satisfied, namely, the condition of a continual 

 and progressive destruction of the equilibrium of the internal masses 

 of the earth, which causes the violent rending of the strata ante- 

 cedent to earthquakes and volcanoes. On this point we need not 

 enlarge. The general progress of geological and physical science has 

 rendered it very probable that the disturbance of the equilibrium 

 of the earths internal masses, which has at so many geological 

 epochs been exalted to an intensity equal to sink and raise hun- 

 dreds of miles square, and to fold into complicated contortions the 

 seemingly solid crust of the globe, is due simply to a slow change and 

 gradual diminution of the earth's internal heat. Great fractures, 



Plutonic rocks, and volcanic accumulations, are of all geological agea ; 

 but as our existing land is, in respect of a very large part of its surface, 

 of very recent date, and volcanic cones of loose materials canuot with- 

 stand the wasting action of the sea, it is no wonder that the antiquity 

 of volcanoes if judged only by the relation of volcanic products visible 

 on the land to the stratified crust of the earth, appears much inferior 

 to that of the Plutonic rocks, which were formed among the strata of 

 every age, under circumstances which admitted of their being preserved 

 But if we more cloaely study this matter, and compare marine volcanic 

 sediments, such as have been spread by the waves round the base of 

 Sciacca, or Sabrina, with the "trappean" sandstones described by Sir 

 R. I. Murchison interposed amongst the Silurian strata, we shall perceive 

 that local volcanic excitement consequent on general changes in the 

 internal condition of the earth is a phenomenon of all geological periods. 



The subject of the production of volcanic cones, whether by eruption 

 or by elevation, or partly by both, has been adverted to in the pre- 

 ceding considerations, and the remarkable example of Jorullo, re- 

 peatedly mentioned in the descriptive part of this article, cited from 

 Humboldt as one of elevation. Since their original publication, much 

 research, in which have been observed numerous facts explicatory of 

 the subject, and much reasoning and discussion relating to it, has 

 been instituted and made public, especially by Mr. Scrope, in continua- 

 tion, or rather resumption of his arguments in support of the reality 

 of the production of volcanic cones exclusively by eruption, by Sir C. 

 Lyell, and by M. Abich, all on the same side. 



The most recent view of the subject, in relation to these researches 

 and discussions, taken by Professor John Phillips, F.R.S. (' Anniv. 

 Address to Geol. Soc.' 1859), which it is just to him to quote in this 

 place, is as follows : " In general, it appears probable that cones of 

 elevation are at least of rare occurrence, while cones of eruption are 

 numerous ; but as vertical movement of the ground is an essential 

 condition for volcanic excitement at the outset, we must be prepared 

 to admit the possibility of its occurrence as a part of volcanic history : 

 and the only questions which remain for calm and serious study in 

 reference to a given volcano are How much ? and at what epoch ? 

 Questions not to be answered hastily." On this point, however, we 

 submit that, however theoretically probable it may be, that vertical 

 movement of the ground is essential to the beginning of volcanic 

 excitement, no proof that it is so has yet been adduced, beyond that 

 required to produce fissures, and ceasing with their production ; while 

 induction from the late observations of earthquake-phenomena already 

 noticed, is opposed to it, and some of the results obtained by Mr. W. 

 Hopkins, in his researches in physical geology, appear to indicate that 

 the theory is itself defective, if not erroneous. 



Mr. Scrope's recapitulation on this question (' Quart. Journ. of 

 Geol. Soc.' vol. xv. p. 645), states its present aspect in these terms : 

 " My argument then is, that the ' elevation-crater ' or ' upheaval ' 

 theory, as applied to volcanic action by MM. de Humboldt, De.Buch, 

 de Beaumont, and Dufresnoy, and to some extent by Dr. Daubeny and 

 Professor James Forbes, as well as in several popular geological com- 

 pilations, is an assumption irreconcilable with the appearances it 

 professes to account for, and wholly hypothetical such a process never 

 having been witnessed ; while there is nothing in the form, structure, 

 or composition of any of the cones or craters to which it is applied 

 by its advocates inconsistent with the supposition that they owe 

 their origin to the simple, ordinary, normal, and perfectly intelligible 

 phenomena of volcanic eruptions, as witnessed repeatedly by competent 

 observers as well in the present day as through all past historical 

 times." 



Subsequently to the promulgation of these arguments, evidence of 

 the most precise nature, proving that the volcano of Jorullo is truly a 

 cone of eruption, as contended long ago, even on the evidence of 

 Humboldt himself, by Mr. Scrope and Sir C. Lyell, and that it was 

 formed in no degree by upheaval of which process it had been 

 brought forward by Humboldt as almost a crucial instance has been 

 given by M. H. de Saussure, a geologist of Geneva, the third in 

 descent, we presume, of a name dear to science. He commuuicated 

 this evidence to the ' Socie'te' Vaudoise des Sciences Naturelles,' in 1 859, 

 exactly a century after the surface of the fertile valley of Jorullo had 

 been transformed into a barren sheet of lava, by a catastrophe, having 

 "for one of its results a perfectly characterised volcanic mountain, 

 which suddenly sprung up on the surface of the globe, with un- 

 exampled rapidity and grandeur of proportions." Described about 

 half a century after its formation by the great traveller under the 

 influence of an imposing hypothesis, it became one of the subjects, for 

 another half-century, of a controversy among the most eminent 

 investigators of the globe's physical structure. This controversy, we 

 think, as respects Jorullo, must now cease. M. de Saussure states that 

 the sheets of lava surrounding the mountain called Malpais. which 

 Humboldt regarded to be the result of a softening of the pre-existing 

 surface soil by gases, and its inflation by them from beneath like a 

 bladder, are nothing else than vast outflows of incandescent matter, 

 which have lined the whole valley, filling its cavities and forming pro- 

 montories, just aa a mass of molten lead would spread when formed on 

 an uneven surface. The edges of the Malpais are not a section or 

 broken edge of an elevated tract, but only the lateral or terminal 

 borders of currents of lava. The cone which forms the mountain of 



