256 



Prof. J. Prestwich. On the 



[Apr. 16, 



cavities, and so also are especially most of the great sheets of lava 

 (basalt), which welled out through fissures in late geological times. 

 These vast fissure eruptions, which in India and America cover 

 thousands of square miles, and are several thousand feet thick, seem 

 conclusive against water agency, for they have welled out evidently in 

 a state of great fluidity, with extremely little explosive accompani- 

 ments, and often without a trace of scoriae mounds. The general 

 presence of non-hydrated rocks and minerals is also incompatible 

 with the permeation of water which the assumption involves. 



It has been suggested by some writers that large subterranean 

 cavities may exist at depths in which the vapour of water is stored 

 under high pressure, but the author shows that such natural cavities 

 are highly improbable in any rocks, and impossible in calcareous 

 strata. 



The author proceeds to account for the presence of the enormous 

 quantity of the vapour of water, so constantly present in eruptions, 

 and which, in one eruption of Etna, was estimated by Fouque to be 

 equal to about 5,000,000 gallons in the twenty-four hours. He refers 

 it to the surface-waters gaining access during the eruptions to the 

 volcanic ducts either in the volcanic mountain itself, or at com- 

 paratively moderate depths beneath. He describes how the springs 

 and wells are influenced by volcanic outbursts. By some observers, 

 these effects have been referred to the influence of dry and wet seasons, 

 but there are so many recorded instances by competent witnesses, 

 as to leave little doubt of the fact. This was also the decision of 

 the inquiry by the late Professor Phillips, who asks, why is the 

 drying up of the wells and springs an indication of coming disaster ? 



The author then considers the liydro-geological condition of the 

 underground waters. He points to the well-known fact, that on the 

 surface of volcanoes the whole of the rainfall disappears at once, and 

 shows that when the mountain is at rest, the underground water 

 must behave as in ordinary sedimentary strata. Therefore, the water 

 will remain stored in the body of the mountain, in the interstices of 

 the rocks and scoriae, and in the many empty lava- tunnels and cavities. 

 The level of this water will rise with the height of the mountain, and 

 he estimates that it has at times reached in Etna a height of 5,000 to 

 '6,000 feet, while the permanent level of the springs at the base of the 

 mountain seems to be at about 2,000 feet. The water does not, 

 however, form one common reservoir, but is divided into a number 

 •of independent levels by the irregular distribution of the scoriae, 

 Java, &c. These beds are traversed by vertical dykes running radially 

 from the crater, so that, as they generally admit of the passage of 

 water, the dykes serve as conduits to carry the water to the central 

 vduct. 



Little is known of the sedimentary strata on which volcanoes stand. 



