142 GEOLOGY OP THE HIGH PLATEAUS. 



rences which take place in waters and in their vicinity. It has long been 

 held that water plays an essential part in volcanic eruptions, and it is quite 

 natural that we should infer from the association that the penetration of 

 water to the internal fires is after all the determinant; but, on the other 

 hand, we cannot leave out of view the fact that there is water on the land 

 as well as in the sea, and that every year from 30 to 50 inches of rain are 

 ordinarily poured over the surface and the underground water-ways and 

 fissures are kept full. An abundant penetration may, therefore, take place 

 on land as well as. under the sea. It does not seem justifiable, therefore, 

 to conclude that the mere presence of water is the sole determinant. There 

 is, however, one class of processes peculiar to bodies of water. It is be- 

 neath their surfaces that sediments are accumulated, often to the thick- 

 ness of thousands of feet, until by their gross weight they subside. It may 

 be that the ultimate cause of volcanism will eventually be traced to the 

 shifting of vast loads of matter from place to place upon the earth's sur- 

 face, but at present this subject has not been investigated from a mechan- 

 ical standpoint with sufficient method and system to admit of safe generali- 

 zation or even of legitimate speculation. 



The assumption that a rise of temperature is the proximate cause of 

 volcanic energy, then, is not a wholly arbitrary postulate, but is consistent 

 with a wide range of facts; brings into order not only the broader but also 

 the subordinate facts of volcanology, and apparently affords a working 

 hypothesis. 



