On the Agency of Water in Volcanic Eruptions, 



139 



of the size of oar largest railway tunnels, at its end dipping towards 

 the centre of the mountain. 



Cavities originating in these ways must have been formed at all 

 times and in many lava streams, and although a certain number of 

 them, especially those due to the upward escape of elastic vapours, 

 may have been filled up by succeeding lava streams, this would not 

 be the case with tunnel-caverns opening downwards. Nor would 

 these streams always fill up even open fissures, as they push before 

 them a mass of solidified debris, which forms a pavement protecting 

 the underlying mass. 



The lava throughout a volcanic mountain may therefore contain a 

 greater or lesser number of caverns, which serve, whenever they 

 happen to He below the normal line of water-level, as so many reser- 

 voirs. The mass of the lava is further riddled with fissures of all 

 dimensions, which act as water- channels and channels of intercom- 

 munication. 



Again, the beds of scoriae, ashes, and tufaceous deposits serving to 

 build up volcanic mountains, and which overlap the lava streams, and 

 extend to considerable depths, are often water-bearing. Some contain 

 powerful springs, like stratum No. 4, which was met with in the Palace 

 well at a depth of 368 feet beneath the surface at Naples (p. 141). The 

 shallow surface wells of the district are commonly in beds of this 

 character. 



Even the more impermeable tufaceous beds contain cavities which 

 when under the line of water-level, must serve as reservoirs. These 

 cavities, which attain a size of 2 feet or more in height, and are 

 lengthened out in a vertical direction, like the flues of chimneys, have 

 been formed by the disengagement of elastic vapours during the con- 

 solidation of the beds, that consist, in the Naples district, of volcanic 

 tuff with trachytic and other rock pebbles.* These beds have a wider 

 extension than the lava masses, which further decrease in importance 

 as they trend from the central area of eruption. f 



The dykes running in vertical lines through volcanic mountains 

 form another structural feature having an important bearing upon the 

 question under consideration, for they traverse radially the beds of 

 ashes, scoriae, tufa, and lava wrapping round the central duct, with 

 which they serve to place them in communication. Besides these 

 great radial dykes, which are often extremely numerous, there is a 

 network of small fissures or dykes branching olf from them in all 

 directions. 



During the eruption of Etna in 1865, a rent was formed at the 



* Dufrenoy, " Ann. des Mines," 3rd Ser., vol. xi, pp. 113, 120 (1837). 



f Some "volcanic mountains are, however, composed almost entirely of ash and 

 scorise beds, and others of lava streams ; the line of water-level will be modified in 

 accordance with these conditions. 



