372 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



no higher than from 700° to 1,000°. A sufficient answer to this 

 objection is the pressure within the earth's crust, which removes the 

 depth of fusion to a lower layer where the temperature is higher; 

 and, moreover, in all eruptions the temperature is no doubt greatly 

 raised by the violent churning the lava receives before reaching the 

 orifice of the volcano, and by superheated steam escaping through 

 it. The lava would naturally flow from that depth at which motion 

 under the earth's crust is easiest, and the temperature observed is 

 therefore about what should be expected. 



Second, it is held that the fractured portion of the earth's crust 

 which would permit a ready penetration of water is confined to a 

 layer not more than five or six miles deep, the great pressure lower 

 down operating to close all crevices ; and it is therefore claimed that 

 water going down fifteen or twenty miles would penetrate the re- 

 maining ten to fourteen miles of unbroken rock with great difficulty. 

 In answer to this objection it may be said that the depth to which 

 rocks are fractured is not certainly known ; but whatever it may be, 

 Daubree's experiments show that the force of capillarity may cause 

 the water and steam to keep on descending till the vapor reaches a 

 temperature where it is rapidly absorbed and diffused among the 

 rock, just as gases are in hot steel ; and when the vapor becomes 

 superheated its explosive violence is greatly increased, and hence 

 this also w^ould tend to raise the observed temperature of the lava, 

 because it is chiefly the hotter lava, still further heated in ejection, 

 which would be forced out of volcanoes. 



These objections, therefore, present no serious difficulty to the 

 theory that volcanic action depends on nothing but the penetration 

 of sea water. 



It is sometimes said that earthquakes accompanying volcanic 

 eruptions are shallow, and it has therefore been inferred that the 

 lava comes from no great depth. Perhaps a more correct view 

 would be to hold that the throat of the volcano does not be- 

 come closed to a great depth, by partial cooling of rock since the 

 last eruption, and the shocks naturally proceed from this point of 

 resistance rather than from the source of the steam and lava rising 

 beneath the volcano, which may be much deeper, and yet give no 

 sensible indication of their movement till the resistance becomes con- 



