On the Agency of Water in Volcanic Eruptions, 155 



or incoherent sandstones, the strata consist of bedded compact lime- 

 stone or other rocks, as represented in diagram, fig. 8, through which 

 the water passes in open fissures, then the result will be different ; for 

 the rock being divided horizontally by the planes of bedding, and verti- 

 cally by joints and cracks constituting the fissures in which the water 

 is lodged, when the water surrounding the blocks V explodes or flashes 

 into steam they will be forced or blown out with the escaping steam 

 into the lava current L, in which they will be carried to the surface 

 and ejected with the lava-scorise by the explosions of the elastic 

 vapour. The cavities formed by the expulsion of the blocks V are 

 immediately injected with the lava from column L which eventually 

 consolidates there. At the next eruption, when the former conditions 

 of water level have been restored, the renewed explosions at m n will 

 expel not only other portions of the strata, but also portious of these 

 intruded masses of lava. It is thus that in the earlier eruptions of 

 Vesuvius, which form the slopes of Somma, blocks of altered limestone 

 derived from the underlying Tertiary and Cretaceous strata are 

 common ; they are scarcer in the later eruptions. It was while forming 

 the walls of the duct, and in contact with the molten lava and heated 

 water or vapour, and not while imbedded in the lava, that these rock 

 masses probably underwent that metamorphism which, in the case of 

 the Vesuvian lavas, resulted in the formation of the very varied group 

 of minerals met with in the ejected blocks of Somma. Similar meta- 

 morphosed blocks are found in the old lavas of many volcanoes 

 (Santorin and others), whilst the blocks of quartz mica-schist occa- 

 sionally met with have undergone little or no change.* 



Besides these blocks of Sedimentary rocks, there are also found in 

 the more recent lavas of many volcanoes blocks of lava having the 

 character of the early erupted lavas. Fouque mentions that in the 

 eruption of Santorin in 1866, there were enclosed blocks of a lava of 

 which the composition differed considerably from that of the flowing 

 lava, and that they contained the minerals common in the old lavas of 

 that island. f It is difficult on any other hypothesis to account for the 

 presence of such blocks. 



I consider, therefore, that a volcanic duct passing through a certain 

 number of permeable strata charged with water, and through volcanic 

 matter also charged with water below a certain level, is surrounded 

 at these points by, as it were, an explosive material under repression, 

 which only requires a disturbance of the equilibrium, under which it 

 exists when the volcano is in a state of repose, to explode with violence. 

 Further, as the superheated vapour from each point of contact forces 

 its way into the ascending lava, the contiguous portion of highly 

 heated water behind it is driven forward, replacing that which has 



* Fouque, " Santorin," p. 10. 

 f Ibid., p. 12. 



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