Lavis — On the Structure of Rocks. 129 



will diminish in a progressive manner as saturation increases. The 

 amount that may be taken up is demonstrated by the enormous 

 volumes that escape during an eruption. Were it possible to cal- 

 culate the amount of vapour expelled during any great eruption, 

 and to estimate the solid ejectamenta — also a difficult matter — these 

 added together would give us the composition of the paste imme- 

 diately before its expulsion, unless we have the level of the magma 

 surface below that of the drainage-line of the country, in which 

 case the vapour would be increased by the inpour from the porous 

 walls of the chimney, and a pulverization of such water similar to 

 the action of a spray apparatus, when the amount of vapour 

 expelled might be enormously augmented. Such calculations 

 have rarely been attempted. Oavalleri 1 estimated that from 

 Vesuvius, in 1856, during a period of eighteen months, pending 

 which the strombolian state of activity had persisted, that no 

 less than 407 millions of cubic meters of water had been ejected 

 in the form of vapour. We may form an idea of the mass by 

 imagining a lake 6j kilometers square, and 10 meters deep. I 

 cannot form a just conception of the amount of vapour issuing from 

 Vesuvius on the above occasion, but from a long and intimate 

 acquaintance with this volcano during the last six years, it seems 

 to me that the above calculation is greatly in excess of the truth, 

 such a result being quite comprehensible when we take into con- 

 sideration the almost insurmountable difficulties of finding suitable 

 data to go upon. If we form a conception of 516,500 kilogrammes 

 of water escaping every minute in the form of vapour from an 

 aperture of four or five meters in diameter at the most, it certainly 

 seems the feeble state of strombolian action would be out of the 

 question. 



It was also calculated 2 that 22,000 c. meters of water were daily 

 dispersed in the form of vapour by the lateral openings of Etna in 

 the eruption of 1865, that is, equal to 2,000,000 c. meters, for 109 

 days that that eruption lasted. This estimation, which, I believe, 

 is that of Fouque, certainly appears more reasonable than the 

 former. 



1 Considerazioni sul vapore e conseguente calore, &c. Memoir read in the Accad. 

 Fisio-medieo-statistica di Milano, December 27th, 1856. 



2 Quoted by M. Ch.Velain. Les volcans ce qu'ils sont et ce qu'ils nous apprenent. 

 Paris, 1884, p. 45. 



SC1BN. PROC., R.D.S. — VOL. V. PT. III. I) 



