i9o6.] SEE— THE CAUSE OF EARTHQUAKES. 349 



Another island eruption on Thera occurred forty-five years 

 later, and the ancients were perhaps familiar with additional sub- 

 marine outbreaks on the Italian coast ; otherwise probably few if any 

 more submarine volcanoes were noted till modern times. ^ 



A few submarine volcanoes, either new or of recent origin, were 

 noted by the early navigators, but the phenomenon of volcanic 

 upheavals at sea did not attract much attention till the time of 

 Humboldt, who made a careful study of all such outbursts. In the 

 Cosmos Humboldt mentions five submarine volcanoes which ap- 

 peared in the first half of the nineteenth century. Probably it 

 would be difficult to give a complete list of all -the submarine vol- 

 canoes which have ever appeared, as the records are widely scat- 

 tered, and to be of value would have to be critically examined in 

 each case. It is also clear that more such volcanoes would now be 

 noticed than in former centuries, because the travel and explora- 

 tion of the sea is more extensive than in former ages ; yet even now 

 great areas of the ocean are quite seldom visited by ships, so that 

 the world knows little or nothing of what is going on in a large part 

 of the globe. If a submarine volcano of short duration should be 

 upheaved, it might again disappear and leave no record of its ex- 

 istence. Numerous islands are also raised in the sea without visible 

 volcanic outlets. The average number of submarine volcanoes 

 upheaved in a century, if careful watch could be kept on the oceans 

 throughout the world, might prove to be something like one every 

 two years, or 50 in a century. This, however, is merely the num- 

 ber of peaks which would raise their heads above the water. As 

 most of the oceans are much too deep for them to show above the 

 sea level, it is clear that the number which remain covered by the 

 sea is very much greater than those which rise into view, in a ratio 

 of perhaps something like 20 to i. Thus in the whole earth there 

 may be, and probably are, 1,000 submarine volcanoes erupted in a 

 century, or an average of ten in a year. In this way we may ex- 

 plain some of the great sea waves so often encountered by navi- 

 gators, and frequently noted by the tide gauges in civilized coun- 



eminence has been solidified by time into a naked rock** (Humboldt's, 

 Cosmos, Vol. I, p. 239). 



^The list island chances in the Mediterranean recorded by Pliny in his 

 Natural History is quite impressive (cf. Lib. ii.). 



