,9o6.] SEE— THE CAUSE OF EARTHQUAKES. 291 



M. Daubree conceives that if the la3'er of rock were of great thickness, 

 and a very high temperature maintained in the cavity, a correspondingly high 

 steam pressure would result, which would be sufficient to raise lava in the 

 vent of a volcano, and to produce earthquakes ; while the force so obtained 

 might after expenditure be again and again renewed. 



"This theory requires the occurrence of cavities at great depth (' sup- 

 posons une cavite separee des eaux de la surface') communicating with the 

 volcanic vents. But the only argument in favor of cavities existing seems 

 to be that the requisite mechanical force is obtainable by means of them ; 

 but it seems a priori impossible that there should be such cavities." 



These passages are of interest in connection with Part VII I. 

 of ^this paper, where it is shown that such cavities or partial cavities 

 develop from the expulsion of lava from under the bed of the 

 sea, and the resulting subsidence of the bottom causes the great 

 sea waves which so frequently follow violent earthquakes. 



§ 7. Historical development of the theory of the penetration of 

 sea zvater. 



Although these passages were found too late to have influenced 

 the theory developed in this paper, they are cited here for conveni- 

 ence, and to show some of the historical aspects of the problem of 

 the penetration of sea water. It was much discussed also in Hum- 

 boldt's time, as we learn from his remarks in the Cosmos: 



" The geographical distribution of the volcanoes which have been in a 

 state of activity during historical time, the great number of insular and lit- 

 toral volcanic mountains, and the occasional, although ephemeral, eruptions 

 in the bottom of the sea, early led to the belief that volcanic activity was 

 connected with the neighborhood of the sea, and was dependent upon it for 

 its continuance." 



" For many hundred years," says Justinian, or rather Trogus Pompeius, 

 whom we follow, " Etna and the Eolian islands have been burning, and 

 how could this have continued so long, if the fire had not been fed by 

 the neighboring sea?" hi order to explain the necessity of the vicinity of 

 the sea, recourse has been had even in modern times, to the hypothesis 

 of the penetration of sea-water "into the foci of volcanic agency, that is to 

 say, into deep-seated terrestrial strata. When I collect together all the facts 

 that may be derived from my own observations and the laborious researches 

 of others, it appears to me that everything in this involved investigation 

 depends upon the questions whether the great quantity of aqueous vapours, 

 which are unquestionably exhaled from volcanoes even when in a state of 

 rest, be derived from sea-water impregnated with salt, or rather, perhaps, 

 with fresh meteoric water; or whether the expansive vapours (which at a 

 depth of nearly 94,000 feet is equal to 2,800 atmospheres) would be able at 



