612 REPORT— 1881. 



palaeozoic and crystalline rocks. Under ordinary circumstances, all the permeable 

 strata and all fissured rocks become charjjed with water up to the level of the 

 lowest point of escape on the surface, or if there should be an escape in the sea- 

 bed, then to the sea-level, })lus a difference caused by friction. 



The extreme porosity of lavas is well known. All the water falling on the 

 surface of Etna and Vesuvius (except where the rocks are decomposed and a 

 surface soil formed), disappears at once, passing into the fissures and cavities formed 

 by the contraction of the lava in cooling. Not only are these fissures tilled, but the 

 water lodges in the main duct itself, and occasionally rises to a height to fill the 

 crater. Beneath the mass of fragmentary and cavernous volcanic materials form- 

 ing the volcano, lies the original compact mass of sedimentary strata, &c. Owing 

 to the fortunate circumstance of an Artesian well having been sunk at Naples, 

 we know that under 735 feet of volcanic beds there are 787 feet of sedimentary 

 strata consisting of alternating strata of marl, sands, and sandstones, some water- 

 bearing, others impermeable. This boring passed through three water-bearing 

 beds— one in the volcanic ashes, the second in tlie Sub-Apennine beds, and the 

 third in the Cretaceous strata at the bottom. The water from the lowest spring 

 rose 8 feet above the surface, or 81 feet above the sea-level. 



It is well known that where the strata crop out in the sea-bed, the pres- 

 sure of tlie column of inland water forces the fresh water outwards, so as 

 to form freshwater springs in the sea, as at Spezzia and elsewhere on the 

 Mediterranean coast. It is this fundamental hvdrostatic principle which keeps 

 wells in islands, and in shores adjacent to the sea, free from salt water, as in the 

 Isle of Thanet. But though the head of inland water is sufficient to force back 

 the sea-water under ordinary conditions, if the normal conditions are disturbed by 

 pumping to an extent that lowers the line of water-level to below that of the sea- 

 level, then the sea-water will flow inwards until an equilibrium is established. 



When undisturbed, the underground fissures and cavities of the volcanic 

 materials forming a volcano soon become filled bv the infiltration of rain- 

 water from the surface, while the strata on which they rest are charged, or not, 

 with water, according as they are permeable or impermeable— following the usual 

 laws affecting underground waters. No eruption of lava can then take place 

 without coming in contact with these underground waters. The first to be 

 affected will be the water in the cavities of the mountain and the crater. As 

 the pressure of the ascending column of lava splits the crust formed in the vent 

 subsequently to the preceding eruption, the water finds its way to the heated 

 rock, and leads to explosions more or less violent. Further, as the fluid lava breaks 

 more completely through the crust, and the mountain becomes fissured bv the force 

 and pressure of the ascending column, all the water stored in the mountain suc- 

 cessively flows in upon the hot lava, and flashes off into steam. Thence those 

 more violent detonations and explosions— those deluges of rain arisins from the 

 condensed steam— with which the great eruptions usually commence. As the 

 more superficial waters lodged in the superincumbent lavas and ashes are ex- 

 hausted, the springs in the deeper underlying sedimentarv strata, cut into by the 

 fissures through which the main ducts pass, come into play, and discharge their 

 contents more or less rapidly into those ducts, where, when the water reaches the 

 pomt, where the pressure permits, it flashes into steam and rises in vast bubbles of 

 vapour to the surface of the lava. Of the quantity of this underground water 

 some notion may be formed by the fact that the deepest of the three springs under 

 Naples discharged, when first tapped, two cubic metres (440 gallons) per minute. 

 The water may pass in bodily in consequence of the powerful shocks and vibrations 

 shattering the strata, and so causing masses of rock to fall in from the sides of 

 the main duct, together with the water lodged in the beds; or it may pass in by 

 capillarity, for it is well known that this state exercises a remarkable influence 

 on the conditions of equilibrium on the two sides of a porous body, and M. Baubree 

 has shown that water will pass through sandstone against a pressure of steam 

 greater than that of the column of water. The experiments were only carried to 

 theextent of a steam-pressure of two atmospheres, but it was evident that the 

 limits of the power were not reached. They further also showed that heat mate- 



