On the Agency of Water in Volcanic Eruptions. 149* 



a depression in the line of water-level in the form of an inverted cone, 

 large in proportion to the amount of friction, and this continues 

 until, on the cessation of pumping, the water gradually recovers its 

 normal level by influx from the surrounding beds. The discharge of 

 water from the crater k an analogous operation, and must be attended 

 with the same consequences.^ 



This flow of water towards the volcanic centre is greatly facilitated 

 by the dykes radiating from that centre, and which, intersecting at 

 right angles the several water-bearing masses, serve as conduits to 

 carry the water into the main duct of the volcano. Nevertheless, 

 there may be some areas so isolated as to* escape. If, however, a large 

 fissure were formed through the head of water ab or cb, and happened 

 to open out on the slope of the mountain at a level lower than the 

 water-level in the interior of the mountain, then the escape of water 

 would be outwards, and its volume would be in proportion to the 

 mass of the mountain, above that level, or to the height of the water- 

 level at b- Suck a* fissure, probably during an exceptionally high 

 water-level, might account, as before mentioned, for the flood on the 

 slopes of Etna in 1665, and it is possible that the same cause may 

 have produced some of the great water and mud discharges recorded 

 of other volcanoes. 



The progress of the eruption by which the level of b is gradually 

 lowered finally determines the whole of the available drainage of v 

 into the central cavity or duct : and if any portion of v be below the 

 sea-level, and that portion contain any permeable beds, or if it be 

 traversed by any of those radial dykes, which, at the high tide of b, 

 carried its waters into the central duct, then whenever the level of b 

 descends below the point of the escape of the fresh inland waters into 

 the sea, the same permeable strata or the same dykes will serve to 

 carry the salt water from the sea to the central area.f So also, should 

 any fissures, under these circumstances, open at the sea-level after the 

 higher inland head of water is drained, then the water from the sea 

 will flow into that fissure and be carried inwards into the mountain. 

 Professor Moseley mentions % a submarine eruption, that in 1877 

 took place off the Hawaiian coast, in a depth of from 150 to 400 feet 

 of water, 50 miles from Mauna Loa, during which a fissure, in all 



* The vapour of water constitutes by far the largest part of the elastic fluids 

 given off during eruptions, probably x %^o or even ~riuo °f tne whole. M. Fouque 

 estimated that the quantity of vapour projected from Etna in the eruption of 1865 

 amounted to the large quantity of 22,000 cubic metres, or about five million gallons 

 daily. 



t Just as wells adjacent to the coast and deeper than the sea-level are subject to 

 an influx of sea water if the pumping is carried too far, or the level of the springs 

 too much lowered. 



% " Notes by a Naturalist on the ( Challenger,' " p. 503. 



