On the Agency of Water in Volcanic Eruptions. 



147 



mountain. Its distribution must be irregular from the circumstance 

 that volcanoes consist of alternating beds of permeable scoriaceous 

 and of solid or decomposed impermeable materials, and as each of 

 these is of limited extent, there may be a number of independent 

 water-levels. It consequently follows that one may be affected by the 

 disturbances accompanying an eruption, while others adjacent do not 

 suffer, or suffer but little. It is only when these several levels are 

 traversed by the same set of rents that intercommunication is esta- 

 blished. The following section shows how numerous are the beds on 

 the flanks of a volcano, and how they are traversed by dykes. 



Fig. 5. — Section on the slopes of the old Volcano of Santorin (Fouque). 

 a. Lara flows ; b. Permeable scoriae and ashes ; c. Dykes. 



The hydro-geological conditions of a volcanic mountain during the 

 period of repose are of the same character as those ordinarily obtaining 

 in Sedimentary strata, but whenever an eruption takes place these 

 conditions are disturbed, and a special class of phenomena ensue. 

 (See Plate 1.) 



After a period of rest, the chilled superficial plug of lava in the 

 volcanic duct will be of a thickness proportionate to the duration of 

 that period, and when the upward pressure of the molten lava 

 beneath begins to exert itself, the resistance will be in a ratio to the 

 thickness of the plug. If thin, it will soon yield ; small cracks and 

 fissures will be gradually formed, into which the water lodged in the 

 beds around, or may be in the crater, will find its way by slow degrees, 

 causing an increase in the discharge of vapour, and giving rise to 

 detonations, small at first, but increasing gradually as the heated lava 

 breaks through the crust. Should, on the other hand, the plug be 

 thick, it will not yield so readily to the pressure of the ascending 

 column of lava ; but when the tension has reached a certain point it 

 will rupture more or less suddenly, forming deep fissures through 

 which the water will be precipitated into the molten lava beneath, in 

 quantity sufficient to produce those powerful and paroxysmal ex- 

 plosions with which the eruptions usually commence after lengthened 

 periods of rest. 



As a consequence of these detonations and explosions the fabric of 



L 2 



