354 G. P. Scrope on Craters, and the Liquidity of Lavas. 
of 1753, persons who ventured to the summit of the cone ob- 
served 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 appearance within the great crater of Etna 
in 1788. In the volcano of the Isle of Bourbon, Bory de St. 
Vincent describes a source of very liquid and glassy lava cease- 
lessly and somewhat tranquilly boiling over in concentric waves 
rie the summit of a dome-shaped hillock composed of its over- 
owings. 
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 aériform explosions, it follows that 
each is in fact simply the external orifice of a more or less cylin- 
bore drilled through the preéxistent rocks by re 
¥ 
toa bubble of air or gas rising through water. Indeed the a. 
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, 28 12 
the Vesuvian one of 1822, the eructations, for such they are, 
succeed each other so rapidly as to produce an almost continuous 
Rome steam-boilers. And 
a black column of stones, scoria, and ashes may be seen to § 
As 8 to a vast height, generally attended with copious discharges 
electricity generated by the friction of the ejected fragments, 
and forming a singular contrast to the jet of aériform matters. 
_ In some rare eases it is possible to witness the actual rise an 
cee of these great bubbles of vapor. zani on bis 
sit to Stromboli in 1780 saw the liquid surface of lava at 
heat within the orifice of the voleano surge alternately 
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