On the Agency of Water in Volcanic Eruptions. 145 



This intersection of the line of water-level in Etna was probably 

 due to the peculiar shape of the mountain, and the rapidity of the 

 slopes above the Yal del Bove. In volcanic mountains generally, 

 this level is too depressed to be within reach of surface irregularities 

 and fissures. 



With respect to the condition of the sedimentary strata under a 

 volcanic mountain, very few observations have been recorded, M, 

 Constant Prevost, after visiting most of the volcanic districts in 

 Europe, concluded that they were not in general much disturbed — 

 that the volcanoes were on lines of fissures but not on lines showing 

 much lateral compression, or on anticlinals. If we may take 

 Vesuvius as a type, we should conclude that the sedimentary strata 

 pass under the mountain and crop out in the adjacent sea-bed with 

 little interference from faults. For the tertiary strata under Naples, 

 come to the surface on the hill ranges further inland, and dip 

 continuously seaward ; and the fact of the overflow of water from 

 the water-bearing beds — water of course passing down from the 

 outcrop on the surface — to the height it actually attains, shows that 

 the continuity of the beds cannot be materially interrupted. 



The well-sections at Naples determine the existence of at least 

 four water-bearing strata, of which the deeper one (1524 feet) dis- 

 charged about 600,000 gallons daily, rising 102 feet above the sea-level. 

 So great a pressure would show that this stratum had its outcrop 

 inland at a considerable elevation above Naples, and the volume of 

 the spring would prove that the head of underground water above the 

 line of sea-level was large. Consequently, as the plane of this water- 

 bearing bed must be traversed under Vesuvius by the volcanic duct, 

 the sides of that duct or fissure will at that point be subject to the 

 amount of hydrostatic pressure indicated by these conditions, or to a 

 pressure, apart from friction, equal to that of about 53 atmospheres. 



With respect to the ordinary mode of escape of the underground 

 waters, that portion held in the more superficial volcanic beds will 

 escape as springs on the slopes or at the foot of the mountain ; but 

 with respect to the underlying sedimentary strata — when they are 

 adjacent to the sea and crop out in the bed of the sea — the surplus 

 waters (or those annually added to the underground stores by the 

 rainfall) will escape, 1st, in springs flowing on the surface ; 2nd, by 

 springs issuing in the sea- bed — and the size of these springs will 

 depend on the hydrostatic pressure, and on the resistance and friction 

 of the conducting strata. When the water-passages are contracted, 

 as in sand and sandstone, the submarine springs will be small and 

 slow : but when large and more open, as in limestones, the discharge 

 in the sea-bed will, as on the coast of Spezzia and elsewhere on the 

 Mediterranean, form large and powerful springs of fresh water rising 

 through the waters of the sea. 



VOL. XLI. L 



