1906. 



SEE-THE CAUSE OF EARTHQUAKES. 369 



bottom can take place by natural process. So long as there was no 

 means of explaining the subsidence of the bottom as a part of a 

 general process in nature, the acceptance of such a violent hypothesis 

 presented great difficulty. 



To make entirely clear how collapse of the bottom may occur 

 after the expulsion of the steam-saturated lava from under the bed 

 of the Andean trough, we may observe that the release of the in- 

 tense pent-up pressure must tend to produce a sudden and somewhat 

 violent cooling in the stratum from which the lava is expelled. Its 

 support of the overlying bed of the ocean trough is thus largely 

 withdrawn, and sinking may easily follow. If this does not happen 

 in every case (and we have no reason to think it so frequent an 

 occurrence), it would probably follow at certain intervals, when the 

 successive expulsions of material have reduced the underlying 

 stratum of lava to a state of small density, in which the medium is 

 filled very largely with bubbles of steam and therefore rapidly cooled 

 when the pressure is released by an ejection of lava. This gives, I 

 think, a simple conception of a self-adjusting system, such as is so 

 often found in nature, by which the continuous process of expulsion 

 of lava may go on, and the level of the sea bottom be adjusted auto- 

 matically. But whether this is the exact process or some improve- 

 ment may be suggested when our knowledge is more extended, it is 

 clear that some such automatic mechanism is at work, and that it has 

 operated in similar troughs all over the world throughout geological 

 history. 



As the water did not withdraw and later return as a great wave 

 during the recent San Francisco and Valparaiso earthquakes, we 

 know that the sea bottom did not sink in the case of either of these 

 great disturbances. The expulsion of the lava, however, must leave 

 the sea bottom less stable and increase the probability of its sinking 

 when the next severe earthquakes occur at these places. As the 

 San Francisco earthquake was much less severe than that at Val- 

 paraiso, the probability of the sea bottom sinking off the Californian 

 coast is much less than off the coast of Chili ; yet such a subsidence 

 with the accompanying seismic sea wave is sure to come sooner or 

 later in all places subject to heavy earthquakes. In 1812 the whole 

 of southern California was severely shaken by earthquakes; on De- 



