334 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 23, 



air with a cloud of vapour and a fierce roar ; while steam seemed to 

 be at intervals blowing off from another neighbouring vent. Hoffman, 

 who visited the same volcano a few years later, describes in minute 

 detail precisely the same phsenomena. 



The vast size of some craters, already noticed, may afford a notion 

 of the enormous volumes of gasiform matter that must have been 

 discharged through them at the time of their formation by continu- 

 ous explosions lasting for weeks and even months ; since each indi- 

 vidual bubble of vapour must have been of a magnitude to fill the 

 entire horizontal section of the crater ; and even for some time to aid 

 in enlarging the area of this aperture by violent pressure against its 

 rocky sides. The prodigious force with which they ascend, and 

 therefore the great depth at which they are generated, may be judged 

 from the vast vertical height, measured in miles, to which they have 

 been seen to shoot up a continuous columnar fountain of ejections, 

 consisting not merely of scoriae and ashes, but often of rocky frag- 

 ments of great size. 



These, by their mutual friction, as they alternately fall back and 

 are thrown up again, become, as already has been said, greatly com- 

 minuted ; and the source of the explosive vapours having sooner or 

 later exhausted its energies, the accumulation of these ashes in the 

 vent at length appears to stifle their further development, and quies- 

 cence for a time ensues. [I am speaking here, of course, of the case 

 of such a paroxysmal eruption as I had the advantage of witnessing 

 in 1822.] 



I have said that every crater is more or less circular in figure ; 

 but, since the orifice of discharge will almost necessarily be opened on 

 the least resisting point of some fissure broken through the solid pre- 

 existing rocks, we might expect its section to be often lengthened in 

 the direction of this fissure, and consequently to be rather oval 

 than strictly circular. And this expectation is justified by obser- 

 vation. Sometimes two orifices have been opened upon the same 

 fissure so near together that their craters or cones intersect each 

 other. In the range of Puys of Auvergne and the Velay such 

 examples are frequent. And in the eruption of 1850 of Vesuvius 

 two craters were formed on the summit of the cone divided only by 

 a narrow ridge ; their common horizontal axis coinciding with the 

 line of the great fissure, which in the preceding year had been visibly 

 broken through the side of the cone towards the north-east. Some- 

 times aeriform explosions take place from openings upon lateral fis- 

 sures, and produce those minor, or (as they are often called) parasitic 

 cones, of which several examples occur on the flanks both of Vesuvius 

 and Etna. At other times, the explosions are confined to the central 

 vent of the volcano, the lava alone welling out, perhaps, at some 

 lateral orifice. This, indeed, is the normal character of these phaeno- 

 mena. And it is this habitual predilection (as it may be called) of 

 volcanic eruptions for the same identical vent, that occasions in so 

 many instances the heaping up of some vast mountain mass above 

 and around it, subject to the occasional blowing up of the central 

 portion, to be re-formed again and again by subsequent eruptions. 



