VOLCANO. 



ele«en months before, it ftill retaiiifd a red heat, which was 

 very confpicuous in fomc of the apertures even in the day- 

 time ; and a ftafT being placed upon it, immediately took 

 fire. Ferrara itates, that when tlie current, wiiith flowed 

 from Monte Roifo on Etna in 1 669, was perforated at Ca- 

 tania in 1709, flames broke out, and it continued to fmoke 

 on the furface after rain, at the beginning of the prefent 

 centur)-. Now whatever may be the mafs of a current of 

 lava, the heat could not remain fo great after fuch long in- 

 tervals of time, were it not prodigioufly more powerful 

 when it firil flowed. It is obvious that the heat of the in- 

 ternal fire cannot be lefs than that of the lava which flows 

 from it, which, we have before obferved, is fometimes equal 

 at leall to the heat of the mol powerful rcverberatory fur- 

 nace. Where the lava pofTeffes a much lefs degree of heat, 

 we are not warranted in afTertitig that the niternal fire was 

 lefs intenfe ; for various circumllances may modify and di- 

 minifli the heat of the lava itfelf, fuch as the acccfs of water 

 near the furface, which may mix with it in the crater, and 

 produce a torrent of mud, or may cool it fo much as to in- 

 creafe its tenacity, until it can fcarcely flow when it firft 

 ifFues from the crater. The following circumftance is well 

 deferving attention. On opening fome of the houfcs in 

 Torre del Greco, which were nearly buried in the lava 

 that flowed from the foot of Vefuvius in 1794, various 

 ftriking effefts were obferved, which could only have been 

 produced by the long continued agency of intenfe heat ; 

 effefls which we have at prefent no means of imitating. 

 Among others, even iron utenfils had been partly vola- 

 tilized, andcryftals of fpecular iron-ore were formed on the 

 furface. For a knowledge of this important faft, the 

 public is indebted to the honourable H. G. Bennet, who 

 brought away various fpecimens from the newly opened 

 houfes. 



The quellion refpefting the fituation of volcanic fires may 

 be thus Hated : — Does the fire in volcanic mountains ori- 

 ginate in the mountain itfelf, or is it fituated at a great 

 depth beneath the furface ? It has been an opinion com- 

 monly entertained, that volcanoes originally break out in 

 mountains already formed, and cover them with lava and 

 fcoriae. Hence it is fuppofed by fome philofophers, that 

 there exillcd primitive or fecondary mountains, where we 

 now obferve the Peak of Tenerifle and Etna, or Vefuvius, 

 and that volcanic fire has merely covered the furface with 

 its produiSs, or efFefted a change in the external form of 

 thefe mountains. On the other hand, it is contended that 

 volcanic mountains are either entirely the produfts of fub- 

 terrancan fire, and have been formed by the lava and fcoriae 

 thrown up, as was the cafe with the volcanic range of 

 JoruUo in New Spain, and Monte Nnovo near Naples ; or 

 that they have been raifed by fnbterranean heat, which has 

 foftened and upheaved the regular beds and (Irata that form 

 the cruft of the globe, as was the cafe at Malpays, already 

 defcribed ; and on this upraifed furface a volcanic cone has 

 been formed, when the eruption of the volcano took place. 

 To determine thefe quellions, where hiftory is filent refpcA- 

 ing the formation of volcanoes, we mult examine their ftruc- 

 ture at the bafe and the fummit, and attend to the pheno- 

 mena which accompany the eruptions. Some volcanic hills 

 are fo entirely compofed of fcorirc and lava, that we can 

 have no hefitation in believing that they have been formed 

 by eruptions. Tliis might be aflerted of .loruUo, of Monte 

 Roflb, and Monte Nnovo, if even we had no well authenti- 

 cated accounts of their formation ; and hence we may infer 

 that the fource of the fire is fituated far below the bafe of 

 thefe hills. Other volcanic mountains of larger ii/.c are 

 partly compofed of beds and llrata, to which wc cannot 



10 



afcribe a volcanic origin. According to the obfervations of 

 Mr. Leckie, during his rcfidence in Sicily, calcareous ftrata, 

 with marine organic remains, reft on Ijeds of volcanic tufa, 

 on the eaftern fide of Etna, and dip towards the fea. 

 (Bakewell's Introduftion to Geology, 2d edit. p. 316.) 

 Hence we may infer that the primeval eruptions took place 

 under the fea, and that their produfts of tufa were covered 

 with marine depofits, before the mountain emerged from 

 the ocean. In other words, the exiftence of the volcano 

 preceded that of the mountain itfelf, the firft eruptions 

 taking place under the fea, the whole mafs of the bafe hav- 

 ing been upraifed at a fubfequcnt period. Calcareous beds 

 occur in fome of the Canary iflands, which are all volcanic ; 

 and thougli the bafe of the Peak of Tenerifle, according to 

 Humboldt, rifes amidft a feries of bafalts and old lavas, he 

 does not confider thefe as a progreflive accumulation of 

 lavas, but as having been formed under and elevated from 

 the fea. On attending to the circumftances which accom- 

 pany the formation of new iflands, he fays, we find that 

 thefe extraordinary eruptions are generally preceded by a 

 fwelling of the foftened cruft of the globe. Rocks appear 

 above the water, before the flames find their way or lava 

 iflfues from the crater. We muft, therefore, diftinguifh be- 

 tween the nucleus raifed up, and the mafs of fcorise and lava 

 thrown upon it. This, as we h. ve before obferved, is the 

 cafe in fome inftances : there are others, however, in which 

 pumice and fcoris have been thrown up from under the fea ; 

 but both phenomena prove that the fource of volcanic fire is 

 feated at great depths below the furface of the ground. 

 Were the fource of volcanic fire feated in the mountain from 

 which the eruption takes place, it is impofllble to conceive 

 that it could continue burning for fome thoufand years, 

 without tiie mountain talhng in ; and when the lire was once 

 extinft, it does not appear probable that it (hould ever burft 

 forth again in the fame place. M. Werner and his fol- 

 lowers have placed the feat of volcanic fire in beds of coal ; 

 but as thefe occupy the upper ftratL' of the globe, being 

 fituated above the primary and lower fecondary beds, they 

 can have no great comparative depth, and the objeAions 

 juft ftated apply to this theory in full force. For if beds 

 of coal wera once burned out, or extinguifhed in one place, 

 we can aflign no conceivable reafon vviiy volcanic fires 

 fliould breiik out in the fame place again, after a ceflation 

 of feven hundred years, and (hould continue to burn for 

 many hundred years afterwards, as was the cafe with Ve- 

 fuvius. Indeed, the opinion of volcanic fire being derived 

 from the ignition of coal-beds, appears to us a fuppofition 

 altogether inadequate to explain their origin, and the extent 

 of iheir operations. 



Mr. Whitehurft, in his " Inquiry into the original State 

 and Formation of liie Earth," 410. 1778, apprcliends, that 

 fubterraneous fire inuft at different times have exifted uni- 

 verfally in the bowels of the eartli, and that in union with 

 water, or by the expanfive power of fteam, it has produced 

 the immenfe continents, aa well as the moinitains of our 

 globe, and alfo the univerfal deluge. When thefe fires were 

 firft kindled, by what fort of fuel they are ftill maintained, 

 at what depths below the furface of the earth they are 

 placed, whether they have a mutual comnuinication, of 

 what dimenlions they confift, and how long they may con- 

 tinue, arc qiieftions which do not admit an eafy decifion. 

 Some, with M. Buffon, have placed the feat of the fire of 

 volcanoes towards the centre, or near the fnmniit of the 

 mountains, which thev fn|)pofe to fnniini the matter emitted. 

 But if this were the cafe, that part of the mountain which 

 is fituated above the fuppofed (eat of the fin-, muft be de- 

 ft roycd or diflipatcd in a fliort time ; whereas an eruption 



uuially 



