94 
NEWER PLIOCENE PERIOD. 
[Ch. VII. 
floods do occasionally sweep down the flanks of Etna when 
eruptions take place in winter, and when the snows are melted 
by lava. 
Many of the angular fragments may have been thrown out 
by volcanic explosions, which, falling on the hardened sur- 
face of moving lava-currents, may have been carried to a con- 
siderable distance. It may also happen, that when lava ad- 
vances very slowly, in the manner of the flow of 1819, described 
in the first volume*, the angular masses resulting from the 
frequent breaking of the mass as it rolls over upon itself, may 
produce these breccias. It is at least certain, that the upper 
portion of the lava-currents of 1811 and 1819, now consist of 
angular masses to the depth of many yards. 
D'Aubuisson has compared the surface of one of the ancient 
lavas of Auvergne to that of a river suddenly frozen over by 
the stoppage of immense fragments of drift-ice, a description 
perfectly applicable to these modern Etnean flows. 
* Chap. xxi. 
