1856.] SCROPE CRATERS AND LAVAS. 333 



persons who ventured to the summit of the cone observed jets of 

 liquid lava thrown up from the surface of a mass which occupied 

 the bottom of the crater, and conducted itself exactly in the manner 

 of a liquid in ebullition. Spallanzani remarked a similar appear- 

 ance within the great crater of Etna in 1788. In the volcano of 

 the Isle of Bourbon, JBory de St. Vincent describes a source of very 

 liquid and glassy lava ceaselessly and somewhat tranquilly boiHng 

 over in concentric waves from the summit of a dome-shaped hillock 

 composed of its overflowings. 



Circular form of Craters. — A consideration which has not, per- 

 haps, been sufficiently adverted to by geologists speculating on the 

 origin of volcanic craters, is the cause of their invariably circular or 

 nearly circular figure. If I am right in attributing their formation 

 exclusively to aeriform explosions, it follows that each is in fact 

 simply the external orifice of a more or less cylindrical bore drilled 

 through the pre-existent rocks by repeated discharges of highly ex- 

 pansive aeriform fluids (probably for the most part steam) forcing 

 their way upwards at some weak point ; and that it is to the equal 

 pressure in all directions of the expanding fluid that the circular 

 form of the section of this orifice is due, — the same cause, in fact, 

 which gives a spherical form to a bubble of air or gas rising through 

 water. Indeed the eruptive explosions must be considered as occa- 

 sioned by the rise of a succession of enormous bubbles from a great 

 depth in the fluid lava below. Each single explosion attests the 

 bursting of such a bubble from the surface of the liquid mass of lava 

 in the vent. In moderately tranquil eruptions these succeed each 

 other at considerable intervals. In the case of Stromboli, I noted 

 that about five minutes usually occurred between every two explo- 

 sions. When the eruption assumes a violent character, as in the 

 Vesuvian one of 1822, the eructations, for such they are, succeed 

 each other so rapidly as to produce an almost continuous roar, like the 

 bio wing-off of a thousand steam-boilers, xlnd each explosion gives 

 birth to one of those great globular volumes of white vapour, which, 

 rolling over and over each other as they rise in the air in a vast 

 column, occasion one of the most remarkable and magnificent appear- 

 ances of a paroxysmal volcanic eruption. In the midst of these 

 clouds of snowy vapour, a black column of stones, scoriae, and ashes 

 may be seen to shoot up to a vast height, generally attended with 

 copious discharges of electricity generated by the friction of the 

 ejected fragments, and forming a singular contrast to the jet of aeri- 

 form matters. 



In some rare cases it is possible to witness the actual rise and 

 bursting of these great bubbles of vapour. Spallanzani on his visit 

 to Stromboli in 1 780 saw the liquid surface of lava at a white heat 

 vdthin the orifice of the volcano surge alternately upwards, and after 

 bursting like a great bubble, fall back again out of sight. In 1819 I 

 was myself able to witness the same interesting phsenomenon probably 

 from the same position, a high point of the external crater- rim which 

 overlooks the vent. At each belch, a shower of tattered fragments 

 of lava, torn from the surface of the bubble as it broke, rose into the 



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