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



siderable accuracy. If the depth is not uniform, but the topography 

 of the ocean basin known, the observer would still be able to locate 

 the hypothetical submarine volcano with considerable precision.^ 



Within historical times several submarine volcanoes have been 

 observed to rise above the sea about South America and elsewhere, 

 in places where the depth was small. From the circumstances that 

 the sea is generally very deep off the Chilian and Peruvian coast, 

 where the most violent earthquakes occur, one would expect but 

 very few of these volcanoes to reach the surface. Yet the large 

 number of sea waves following violent earthquakes may afford us 

 some idea of the activity of submarine earthquakes and perhaps 

 volcanoes in that part of the world. It is probable that not less 

 than one hundred such earthquakes with sea waves occur along the 

 South American coast in a century, and of these not less than ten 

 have done great damage. 



If the supposed eruption is some distance out at sea, the effect 

 on shore would be small, because the level is not so much changed, 

 owing to the great body of the intervening water. Also, when a 

 violent earthquake occurs and but slight recession of the water is 

 noted, followed by a wave after considerable interval, the indications 

 would point to a great eruption at considerable distance. On the other 

 hand, if the recession of the water is quick and the wave returns 

 after a short interval, the eruption should be comparatively near the 

 shore. Thus by a study of the waves observed the place and char- 

 acter of the eruption may be approximately determined. 



In some cases the sea is said to be bodily upheaved, and rises 

 with the utmost suddenness. In such cases the volcano may be 

 very near shore, or the sea bottom may be upheaved in the form of 

 a ridge or cone without submarine eruption. Since the earthquakes 

 under the sea are very numerous, there is a great probability that 

 all these movements of the sea should be observed occasionally. 



Major Button adopts the view that the sea bottom sinks when 

 the sea withdraws from the shore, and I also consider this the 

 most probable cause in the great majority of cases. It is the more 

 logical to accept this view since we now see how a sinking of the 



^ If such an eruption occurred the surface of the sea would be likely to 

 show evidence of it, by ejected pumice, ashes, and other volcanic debris. 



