VOLCANO. 



VuI.CASo. 



of iU surface, by the ample laying on of material, whether abraded 

 from the land, or chemically abstracted from the sea iteelf. That 

 matter u in process of abrasion and transportation from the Und into 

 the ocean nt every instant, and along every coart line, we know ai a 

 matter of fact. We know, too, that all existing strata, however enor- 

 mous their thickness, Aaw been formed at the bottom of the tea, and 

 it if. therefore, no hypothesis, but a perfectly legitimate assumption, 

 that the awe prooeai ii itUl in progress, no matter how slowly, from 

 thii cause, at least in the vicinity of ooaat lines ; and when we look at 

 the vast amount of organised exuviaj which constitute so large a 

 portion of many of the secondary and tertiary beds the secretions of 

 molluscs, infusoria, and Eoophytoa and bearing in mind the large 

 proportion of continental substance which has been so formed, look to 

 the evidence afforded by deep-sea soundings, and by coral forma- 

 tions, that the same process ia going forward in open sea, far out 

 of the reach of coast washing and river deposit (the material being 

 talcr* up chr*,icuUy by the river and coast waters, and chemically 

 t.rtrarted from (Ann, when diffused by currents, by the processes of 

 organic life) we shall at once perceive that any amount of pressure on 

 the one hand and relief on the other, which the geologist can possibly 

 require to work out his problem, and any law of distribution of that 

 relief and that pressure, is available without calling in the aid of 

 unknown causes." 



In apposition with these views of the physical geographer, taken, as 

 it were, from an eminence based on all science (and to which we shall 

 return), we may appropriately place tbe most recent view of the rela- 

 tions of earthquakes and volcanoes, regarded on a cosmical scale, that 

 taken by Mr. Mallet in his fourth report, the following portions of 

 which embrace the principal points of the subject, as resulting from his 

 own seismological researches and those of M. Perrey. 



" Should it ultimately prove a fact, as rendered probable from the 

 beautiful investigations of Darwin, that there are great areas of gradual 

 subsidence now in motion beneath the Pacific, it may still happen (though 

 it ia not probable) that seismic, or even volcanic bands may traverse 

 such areas of subsidence, without materially affecting their general 

 downward movement Although many portions of the earth's surface 

 now show evidences of vertical instability, either slowly or per taltum, 

 occasionally rising or sinking, these effect* are all comparatively insig- 

 nificant in 'extent The great formative forces, whatever they were, 

 upon which the elevated land of the great continents and tbe depres- 

 sion of the ocean-beds depended, have ceased sensibly to act The 

 function of the volcano and the earthquake in the existing cosmos is 

 not creative, but simply preservative ; and vast as they appear to the 

 eye and sense, their effects are very small in relation to the totality 

 of the great terrestrial machine. If, however, such large areas of 

 oceanic subsidence as have been supposed really exist, they will most 

 probably be found situated almost centrally within the oceanic sub- 

 basins, and hence surrounded, but not traversed, by seismic bands [or 

 lengthened are subject to earthquakes]. 



" There is one fact, which is shown by the relative positions of the 

 greatest volcanic areas upon our globe (and these the most active) and 

 of the areas of probable subsidence, that ia worthy of fixing our 

 attention. 



" It will be observed that tbe bands of probable subsidence are 

 tolerably adjacent to tbe greatest seats of volcanic activity, and that 

 tbe Utter generally have subsiding areas at more than one side. Thus, 

 in the Pacific, the band of subsidence is along the great volcanic girdle 

 from Celebes to New Zealand, and thence stretches between (and at 

 one point may cut through) the line of sub-oceanic volcanic girdles, 

 from the New Hebrides to tbe Marquesas. 



" Again, the great volcanic horseshoe girdle of Sumbawa ia between 

 the subsiding area in tbe China Sea, north of Borneo, and the blue 

 coral bands north of Australia, which whole continent, or at least iU 

 western and northern part*, may probably be subsiding also. Lastly, 

 in the north we have Iceland and its volcanic system, between the 

 (inking coasts of Greenland and those of the Baltic. 



" If we admit then, as certain, that these vast tracts are subsiding, we 

 can scarcely withhold our belief that the subsidences are due to. and 

 are the equivalent in bulk of, the solid ejecta and exhalations of these 

 various great volcanic areas respectively. The assumed area and extent 

 of subsidence of those supposed subsiding tracts are, however, I 

 apprehend, greatly overrated 



" The seimnic intensity in any part of the world, so far as originating 

 impulse U concerned, does not seem to be connected with the super- 

 ficial character, to the greatest known depth, of the geologic forma- 

 tions, beyond what connection is necessarily inferential from the seismic 

 bands where they exiirt), following, on the whole, the lines of moun- 

 tains and ridges that separate the surface-basins of the earth, whether 

 volcanic or not. While, therefore, the seismic waves diverge from 

 axial lines that are generally of tbe older rock formations, and often 

 of crystalline igneous rocks, or actively volcanic, they penetrate thence 

 formations of every age and sort, even to plains of tbe most recent 

 pot-plristoccne cUy, sands, and gravels; and occasionally, by tbe 

 secondary efforts of great shocks, these loose materials are shaken or 

 cau rd to slip and gather up into new forms (as in the I'lluh Bund nt 

 the iniiiitliH df the Indus, &), and so the earthquake has come to be 

 mistakenly viewed as a direct agent of elevation. Its true cosmical 

 function is the very opposite : it ia part of the dislocating, degrading, 



and levelling machinery of the surface of our globe, while the part of 

 the volcano is restoration and renewal. Both are, however, not creative, 

 but conservative (strange as it may sound), and suited to the period of 

 man's appearance and possession of the earth." 



The philosophic investigator of the volcanic system of the Canaries 

 (Von Buoh) has arranged the group* of volcanoes, which have thus 

 been briefly sketched, into two systems. 1. Central Volcanic Systems, 

 where the vente are grouped round some principal cone, as ,-Ktna, or 

 arranged in an expanded area, as Iceland. 2. Linear Volcanic Systems, 

 as the grand chain of Asiatic tales and the lofty range of the Andes ; 

 and this view is perhaps of the more importance, because it is applic- 

 able to the ancient Plutonic rocks, which, (nun other consider 

 we have inferred to be of the nature of unerupted lava 1 ) 

 Sienitic fiw of the Malvern Ililla may be contrasted with the sr, 

 ffrau/ii of traps about Charnwood Forest and the country north of the 

 Cheviot Hills. 



In the one cose the crust of the earth has yielded to pressure, and 

 has been broken in many places near a certain point; in th> 

 it has yielded along a certain line of weakness in the rocks. V.*n 

 Buch imagined that the Central volcanic systems, like those of the Mont 

 d'Or and the Plomb du Cantal in France, had been originally formed 

 by an uplifting of the ground in a rudely dome-shaped elevation 

 (Erhebungs-cratere) ; while along the linear volcanoes a great fault had 

 occurred Exactly similar suppositions have been employed for the 

 more ancient examples of unerupted Plutonic rocks ; but in each cane 

 there is a part certain, namely, the fracture along the line, and a part 

 disputable, namely, the upheaval in a dome. Sir C. Lyell is indisposed to 

 admit in any case the origin of a volcanic vent by upheaval in a dome- 

 like figure ; he even dissents from the opinion or narration of Hum- 

 boldt respecting the elevation of Jorullo by inflation, and from the 

 conclusions of De Beaumont and Dufresnoy regarding the Mont < i > 

 the Plomb du Cantal. This is a point which would be of little con- 

 sequence, but for the interest justly attached to any inference con- 

 cerning the origin of volcanic phenomena. That there bare been some 

 rudely dome-shaped elevations in the older strata, in connection with 

 disturbances of the interior of the globe, is evident to any one who has 



Monte Nuovo, 440 feet. 



Pyke.. 



Vesuvius. 



\Yoolhopc elevation, 



1, Line of the Aymeitrr limeitone ; 2, line of the \Vrnlock limettono; 9, line 

 of the Woolhope limestone ; 4, line of the Ctradoc *aiiiltone [now i 

 the Upper Lland nrrv llock.) 



studied the strata in the vicinity of Woolhope, described by Sir H. I. 

 Murchison, of which the subjoined cut gives a cross-section That the 

 structure of such stratified domes of elevation is entirely differem 

 that of a volcanic cone of eniption is evident by contrasting with the 

 former figure the section across Vesuvius and the profile of the < 

 form Monte Nuovo. Some further information on this head appears 

 under the article STRATIFICATION, in NAT. HIST. Div., and upon tin; 

 whole it is certain that in respect of a volcanic mountain or i 

 whose internal structure is sufficiently exposed, there arc char.ictci itic 

 marks by which the existence of ' craters of elevation " can be affirma- 

 tively proved, if any such craters exist Von Buch, De Beaumont 



