1906. J 



SEE— THE CAUSE OF EARTHQUAKES. 371 



IX. Concluded Theory of Volcanoes. 



§ 42. Other theories of volcanic action. 



The four fundamental facts mentioned in § 10 have been fully 

 considered, and we have found that the hypothesis of the penetra- 

 tion of sea water into the crust of the earth affords a natural and 

 satisfactory explanation of all volcanic phenomena. Such hypotheses 

 as the following: (i) Lava flowing out of a molten interior, is 

 contradicted by the rise and fall of the columns of lava in volcanoes, 

 as if forced up by the elastic pressure of steam, which also escapes 

 in eruptions; (2) molten reservoirs, contradicted by the same phe- 

 nomena; and moreover neither (i) nor (2) enables us to account 

 for the observed distribution of volcanoes; (3) melting by relief of 

 pressure, and (4) melting by crushing, encounter the same diffi- 

 culties, and others besides. None of these four hypotheses can be 

 seriously considered. 



There remains Major Button's recent suggestion that radium 

 is the exciting cause. But the researches of the Hon. R. J. Strutt 

 have shown that all the principal rocks of the earth's crust, especi- 

 ally granite, contain large quantities of radium, and since these 

 rocks underlie all the continents, we should expect abundant active 

 volcanoes everywhere inland if radium were the exciting cause, 

 whereas in fact they appear in the depths of the sea or along the 

 shores of the oceans. The cause of volcanic action is thus narrowed 

 down to the penetration of water into the heated rocks of the earth's 

 crust, and all other hypotheses may be unhesitatingly rejected. 



We shall now adduce some further considerations bearing on 

 the aqueo-igneous theory, with a view of throwing additional light 

 upon particular phenomena. 



§ 43. Certain objections to the theory of the penetration of sea 

 water. 



The beginning of this theory may be traced back to Lucretius, 

 and perhaps to Aristotle,^ and hence we shall first answer two ob- 

 jections which have been urged against it. 



First, it is held that the temperatures of the lavas are too high, 

 2,000° to 3,000°, whereas one would expect the temperature to be 



^ In more recent times it has been treated by Sir J. Prestwich, in a paper 

 " On the agency of water in vokanic eruptions," Proc. Roy Soc, April 16, 1885. 



