1885.] Agency of Water in Volcanic Eruptions. 



251 



In Naples, however, an artesian well found them under the volcanic 

 materials in usual succession, and with several water-bearing beds, 

 from one of which, at a depth of 1,524 feet, a spring of water rose to 

 the surface with a discharge of 440 gallons per minute. When in a 

 state of rest, the surplus underground waters escape in the ordinary 

 way by springs on the surface, or when the strata crop out in the 

 sea, they then form submarine springs. 



During an eruption, these conditions are completely changed. The 

 ascending lava, as it crashes through the solid plug formed during a 

 lengthened period of repose, comes in contact with the water lodged 

 around or may be in the duct, which is at once flashed into steam, and 

 gives rise to explosions more or less violent. These explosions rend 

 the mountain, and fresh fissures are formed which further serve to carry 

 the water to the duct from which they proceed ; or they may serve as 

 channels for the sea- water to flood the crater, when, as in the case of 

 Coseguina and Krakatoa, the volcano is near the sea-level. As the 

 eruption continues, the water stores immediately around the duct 

 become exhausted, and then the water lodged in the more distant 

 parts of the mountain rushes in to supply the void, and the explo- 

 sions are violent and prolonged according to the available volume of 

 water in the volcanic beds. When this store is exhausted, the same 

 process will go on with the underlying water-bearing sedimentary 

 strata traversed by the volcanic duct. 



The author gives diagrams showing the position of the water-levels 

 before, during, and after eruption ; and describes the manner in 

 which, if the strata surrounding the duct and below the sea- 

 level become exhausted, the efflux of the fresh water which passed 

 out to sea through the permeable beds, when the inland waters 

 stood at their normal height above the sea-level, these same 

 beds will in their turn serve as channels for the sea water to 

 restore the lowered water-level inland. Thus, the excurrent 

 channels which carried the land waters into the sea-bed, and 

 there formed, as they often do off the coasts of the Mediterranean, 

 powerful fresh-water springs, now serve as channels for an in- 

 current stream of sea water, which like the fresh waters it replaces, 

 passes into the volcanic duct. This agrees with the fact that diatoma- 

 ceous fresh -water remains are common in many eruptions, and marine 

 remains in others ; also, that the products of decomposition of sea water 

 are so abundant during and at the close of eruptions. With the fall 

 of the water-levels, the available supply of water becomes exhausted, 

 or the channels of communication impeded, and this continues until, 

 with the ceasing of the extravasation of the lava, the eruption comes 

 to an end. 



The author then explains the way in which the water may gain 

 access to the lava in the duct, notwithstanding heat and pressure. 



