32 Notices of Memoirs — Causes of Volcanic Action. 



Spezzia and elsewhere on the Mediterranean coast. It is this 

 fundamental hydrostatic principle which keeps wells in islands, 

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

 Isle of Thanet. Where, however, the head of inland waters is 

 small or impeded, sea-water will enter the permeable strata, and 

 spoil the springs, as in the case of the Lower Tertiary Sands 

 at Ostend, and the Lower Greensand at Calais and in the Somme, 

 in which latter department the underground spring was found 

 affected to a distance of about 1 mile from the sea, but pure at a 

 distance of 9 miles. Further, if where the head of inland water 

 is sufficient to force back the sea-water under ordinary conditions, 

 these ordinary conditions are disturbed by pumping to an extent that 

 lowers the line of water-level to below that of the sea-level, then tlie 

 sea-water will flow inwards until an equilibrium is established. 

 The flow of water under a volcanic mountain may be also influenced 

 by the quaquaversal dip, which there is some evidence that the un- 

 derlying strata there take, owing probably to the removal of matter 

 from below, and the weight of the mountain. If we are to assume 

 that the volcanic ashes and tufas below Naples are subaerial, the 

 original land-surface has sunk not less than 665 feet, and a dip of 

 the underlying strata from the seaward, as well as from inland, has 

 in all probability been caused. This artesian well was cai'ried to 

 the depth of 1524 feet, and passed through three water-bearing beds 

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

 and the third in the Cretaceous strata at the bottom. 



When undisturbed, the underground fissures and cavities of the 

 volcanic materials forming a volcano must soon become filled by 

 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 in and around the crater. As the pressure of the ascend- 

 ing column of lava splits the crust formed subsequently to the 

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

 and leads to exjDlosions more or less violent. When the fluid 

 lava breaks more completely through the old crust, and the 

 mountain is fissured by the force and pressure of the ascending 

 column, the whole body of water stored in the mountain suc- 

 cessively flows in upon the heated lava, and is at once flashed 

 off into steam. Then take place those more violent detonations 

 and explosions — those deluges of rain arising from the condensed 

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

 more superficial waters of the superincumbent lavas and ashes are 

 exhausted, the springs in the deeper underlying strata, cut through 

 by the fissures in which the main ducts are situated, come into play, 

 and varying with the pressure consequent on the rise and fall of the 

 column of lava, discharge their contents more or less rapidly into 

 those ducts, where, when they reach the point, when pressure 



