. W. Hilgard on Mailet’'s Theory of Vuleanicity. 539 
times of spring and neap tides. Mallet does not, however, 
allude to this point; whether from a distrust of Perrey’s data 
and method, or theoretical scruples on the score of “ rigidity.” 
The objection that, according to Mallet’s theory, earthquakes — 
that steam, not static pressure, is the vis-a-trgo. It is, of 
course, very probable that the access of water to the volcanic 
focus* is generally caused, or facilitated, by such crust move- 
perature that can be rendered efficient toward the fusing of 
rocks by the crushing process, under the most favorable cir- 
cumstances, viz: upon the supposition that it takes place in- 
rock has not been materially diminished by the downward in- 
crease of hypogeal temperature. At the most moderate depths 
at which Voloatie phenomena can be supposed to originate, the 
last mentioned factor must exert a very considerable influence, 
* Hutton (loc. cit.) avers that “ to cause a volcano the heat must go to the water 
—the sore pat go to the heat;” but omits any explanation of this singular 
Mi the physical nature of this 
likely to result from its pro- 
does not go into the consideration of 
other di ces 
greater than those employed by him. 
+ Mallet 
“ powder,” and of the thermal and 
duction under pressures enormously 
