On the Agency of Water in Volcanic Eruptions. 143 



rock, but variably fissured, the rise of the line of water-level varies 

 from 13 to 150 feet in the mile, and in some strata it is even more. 



It will therefore be apparent, that in the case of the irregular and 

 complex beds forming a volcanic mountain, the height of the water- 

 level is subject to too many conditions to be determined accurately, 

 except by experiment, and for this few opportunities present them- 

 selves. There is, however, an available natural datum line, namely, 

 that furnished by the escape of springs, at certain high levels on the 

 mountain slopes. 



As springs issuing from a body of homogeneous strata or strata 

 which intercommunicate, depend for their permanence upon the 

 water stored up in the interior of the mountain, at a level above that 

 of the point of escape, it follows that if there is a point of permanent 

 escape, we may conclude that in the ground behind, all the strata below 

 that level are under the line of permanent saturation, and therefore 

 charged with water ; and further, as just said, this line of permanent 

 saturation must stand the higher the further it goes into the body of 

 mountain. 



Now, on Etna, Wattershausen* describes a spring in the valley of 

 St. Giacomo, near Zafarana, which he says is the only point at so 

 high a level at which a tolerably strong spring constantly issues. It 

 at once forms a small waterfall, and runs some distance until lost in 

 some volcanic sands lower down. The strata, where they issue, are 

 composed of alternating layers of tuff and compact lava. Its escape is 

 due to the circumstance that there is here a ravine which cuts through 

 the bed, and either touching on the line of water-level or intersecting 

 the junction between an impermeable and a permeable bed, and thus 

 taps the subterranean waters. Wattershausendoes not give the height of 

 the ground, but from a section of Abich's, which passes near the spot, 

 I infer it to be about 2000 feet above sea-level. f On the other side of 

 Etna the river Simeto, or one of its tributaries, rises near Bronte, at a 

 height of about 2200 feet above sea-level. This likewise indicates the 

 existence of a perennial spring. I find also that at other points 

 around the mountain at and about this level, streams commence 

 which point to a like line of water-level. 



These figures are only roughly approximate, but they constitute our 

 only available data, and in taking a mean level of 2000 feet, I believe 

 I am rather below than above the mark. If, therefore, we take 

 Wattershausen's section of Etna, supplemented by Abich's, which 

 follows nearly the same line, and gives, moreover, the height of the 

 several points, the following diagram would represent generally the 

 massif of the mountain and the position of the line of saturation. 



From the points of permanent issue which we have noted at Zafa- 



* " Atlas de l'Etna," Part I and PL Y. 



f Vesuvius and Etna, 1837. Sections of Etna. 



