350 SEE— TPIE CAUSE OF EARTHQUAKES. [October 19, 



tries. Not all volcanoes thus upheaved would be sufficiently large 

 and energetic in their action, to be widely or distinctly felt, because 

 some would be small and slow in their action and hence produce 

 no sensible sea waves. Yet when we consider how much of the sea 

 passes unobserved, and how many sea waves actually are reported, 

 an average of ten submarine volcanoes per year does not seem too 

 high an estimate. Those which are -small and in deep water will 

 escape notice entirely; and in midocean none except the powerful 

 eruptions could make themselves felt, because there is no means of 

 detecting a disturbance of sea level, unless it happens to be large 

 enough to disturb a passing ship. 



In his well-known article on " Geology " in the Encyclopedia 

 Britannica, ninth edition, Sir Archibald Geikie says : 



" At the Hawaii Islands, on 25th February, 1877, masses of pumice, 

 during a submarine volcanic explosion, were ejected to the surface, one of 

 which struck the bottom of a boat with considerable violence and then floated. 

 At the same time when we reflect to what a considerable extent the bottom 

 of the great ocean basins is dotted over with volcanic cones, rising often 

 solitary from profound depths, we can understand how large a proportion of 

 the actual eruptions may take place under the sea. The foundations of these 

 volcanic islands doubtless consist of submarine lavas and fragmentary ma- 

 terials,, which, in each case, continued to accumulate to a height of two or 

 three miles, until the pile reached the surface of the water and the phenomena 

 became subaerial. The immense abundance and wide diffusion of volcanic 

 detritus over the bottom of the Pacific and Atlantic Oceans, even at dis- 

 tances remote from land, as has been made known by the voyage of the 

 ' Challenger,' may indicate the prevalence and persistence of submarine 

 volcanic action, though at the same time, it must be admitted that an ex- 

 tensive diffusion of volcanic debris from the islands is effected by winds 

 and ocean-currents." 



The evidence therefore that the sea bottom is very leaky and in 

 a constant state of eruption in many places seems conclusive. In 

 other places very few eruptions occur, because the underlying rocks 

 are less leaky and the sea is too shallow to exert much pressure. 

 The number of earthquakes under the sea must be much greater 

 than those on land, area for area ; but here again few are observed, 

 and still fewer traced to their centers, because the data are in- 

 sufficient. 



It is fully recognized, however, that many submarine earth- 

 quakes occur off the east coast of Japan and the Philippines, in the 



