148 



Prof. J. Prestwich. 



the ^mountain is -shaken — the underground waters suffer disturbance, 

 old water-channels are sanded up, new channels are formed — and the 

 waters are dislodged and driven to seek fresh beds and fresh lodg- 

 ments. These effects are necessarily most strongly felt in the rocks 

 around the centre of eruption, and it is there that they undergo the 

 maximum of displacement and Assuring ; and as the water lodged in 

 that part of the mountain flows into the volcanic duct, it flashes 

 into steam, and is driven out in continuous explosions. Thus, the 

 water stores immediately surrounding the central duct and crater, 

 become gradually exhausted, and their level is more or less lowered. 

 As this loss proceeds, the water lodged in more distant parts of the 

 volcanic mountain flows in to supply the void, and the explosions 

 will be violent and prolonged, according to the available volume of 

 water present in the mass of the mountain. As these central water 

 stores are driven off and become exhausted, and the level of the 

 underground water is gradually lowered, there must necessarily be 

 an influx from the circumference of the mountain to replace the 

 loss caused by the central explosions, and this will continue so long 

 as the central body of water is kept by the eruption lower thau in 

 the strata and fissures at a distance. These different hydraulic con- 

 ditions are represented by the diagrams — sections 1, 2, 3, Plate I. 



In these diagrams v represents the mass of volcanic materials ; S the 

 sedimentary strata ; p the permeable strata.; m the impermeable strata ; 

 a, b, c the normal level of the underground waters in the volcanic 

 mountain ; and I their level in the sedimentary strata. The arrows 

 show the direction in which the waters flow under the normal and 

 also under the altered conditions caused by .the eruption. In section 

 No. 1 the points a and c are fixed levels, while b fluctuates according 

 to the amount of rainfall and to the length of the intervals between 

 the eruptions. Its height above a and c depends upon the distance 

 from those points and on the resistance of the materials through 

 which the water percolates. This of course is apart from any excep- 

 tional interfering causes. The flow of water in v, section 1, will, under 

 normal conditions of repose, be from the centre towards the circum- 

 ference, and it will escape at the lowest levels ; and if there are no 

 lower levels inland, the whole direction will be seaward. In this 

 section, however, the depression at c allows of an escape of some of 

 the water on the inland side of the mountain. 



As the level of b is lowered by the discharge of water into the duct 

 of the crater, the water first ceases to escape at a and c, and then the 

 adjacent body of water below that level sets in with an inward flow 

 towards the central duct, as shown in section 2 ; for it is a well-known 

 fact that in masses of partially resisting strata, such as the Chalk or 

 the New Red Sandstone, excessive pumping (or the removal of water 

 faster than it can be replaced from the surrounding strata) causes 



