On the Agency of Water in Volcanic Eruptions. 137 



have been caused by their absorption under ground, either to restore 

 some water level reduced by a former eruption, or to fill fissures in 

 course of formation preceding the eruptions of 1867 and 1868. 



It seems to me, therefore, to use the words of Professor Phillips, 

 that the observations respecting the effects produced on wells and 

 springs by volcanic eruptions and earthquakes, " have been too often 

 and too carefully made to allow of a serious doubt on the subject;" he 

 asks, " What is the cause of it ? and why is it an indication of coming 

 disaster ?" 



§ 4. The Hydro-geological and Statical Condition of the Underground 

 Waters in and under a Volcanic Mountain. 



The cause is, I believe, not far to seek, when the hydro-geological 

 conditions of the strata composed above of volcanic matter, and below 

 of sedimentary strata, are considered. 



So well known is the absorbent power of a volcanic surface, that 

 the mention of the fact hardly seems necessary, except in corroboration 

 of subsequent statements and for the purpose of independent testi- 

 mony. On ordinary strata it is roughly estimated that about one- 

 third of the rainfall passes under ground, but on volcanic surfaces the 

 whole rainfall soon disappears, a small proportion only being lost 

 by evaporation. Amongst innumerable notices of this fact, it will 

 suffice to mention those of two experienced authorities. Lyell re- 

 marks on the dry and arid surfaces of Etna, and on the rapid absorp- 

 tion of the rainfall, and observes that " the volume of rain-water and 

 melted snow commonly absorbed by a lofty mountain like Etna, is 

 enormous ;* again, Piazzi Smyth, in describing his ascent of Teneriffe, 

 says, "that though so much rain had fallen lately, not a trickling 

 stream, not even a drop of standing water, was anywhere to be seen ; 

 the pumice-stone ashes had swallowed all up." f 



Yolcanic mountains being composed of streams of lava of very vari- 

 able width and length, irregularly alternating with more widely spread 

 layers of scoriae and ashes, the whole mass would be permeable were 

 it not that the decomposition of some and the consolidation of other 

 beds, by atmospheric and aqueous agencies, have formed here and 

 there impermeable beds, which hold up the rain-waters, and furnish 

 local supplies to wells and springs. But where such impermeable 

 beds do not intervene, the rain-water penetrates to greater depths, 

 and is there stored until the line of water-level reaches to such a 

 height that the hydrostatic pressure forces it outwards, and causes it to 

 escape at the points I as springs either temporary or perennial (fig. 1). 



This storage may take place either in the lava or in the beds of 

 scoriae and ashes. Solid lava is impermeable, but water penetrates 



* " Phil Trans.," vol. 148 (1858), p. 763. 

 f " Teneriffe," p. 349. 



