292 SEE— THE CAUSE OF EARTHQUAKES. [October i8, 



different depths to counterbalance the hydrostatic pressure of the sea, and 

 thus afford them under certain conditions a free access to the focus." ^ 



Again :- 



" The great number of volcanoes on the islands and on the shores of 

 continents must have early led to the investigation by geologists of the 

 causes of this phenomenon. I have already, in another place (Cosmos, Vol. 

 I, p. 242), mentioned the confused theory of Trogus Pompeius under Augus- 

 tus, who supposed that the sea-water excited the volcanic fire. Chemical 

 and mechanical reasons for this supposed effect of the sea have been adduced 

 to the latest times. The old hypothesis of the sea-water penetrating into 

 the volcanic focus seemed to acquire a firmer foundation at the time of 

 the discovery of the metals of the earth by Davy, but the great discoverer 

 himself soon abandoned the theory to which even Gay-Lussac inclined, in 

 spite of the rare occurrence or total absence of hydrogen gas. Mechanical, 

 or rather dynamical causes, whether sought for in the contraction of the 

 upper crust of the earth and the rising of continents, or in the locally dimin- 

 ished thickness of the inflexible portion of the earth's crust, might, in my 

 opinion, offer a greater appearance of probability. It is not difficult to 

 imagine that at the margins of the up-heaving continents which now form 

 the more or less precipitous littoral boundary visible over the surface of the 

 sea, fissures have been produced by the simultaneous sinking of the adjoin- 

 ing bottom of the sea, through which the communication with the molten 

 interior is promoted. On the ridge of the elevations, far from that area of 

 depression in the oceanic basin, the same occasion for the existence of such 

 vents does not exist. Volcanoes follow the present sea-shores in single, 

 sometimes double, and sometimes even triple parallel rows. These are con- 

 nected by short chains of mountains, raised on transverse fissures, and form- 

 ing mountain-nodes. The range nearest to the shore is frequently (but by 

 no means always) the most active, while the more distant, those more in 

 the interior of the country, appear to be extinct or approaching extinction. 

 It is sometimes thought that, in a particular direction in one and the same 

 range of volcanoes, an increase or diminution in the frequency of the erup- 

 tions may be perceived, but the phenomena of renewed activity after long 

 intervals of rest render this perception very uncertain." 



§ 8. Views of Liicretms on the penetration of sea zvater into 

 ^tna. 



We have quote the above passage because of Humboldt's saga- 

 cious remarks, some of which deal with the theory of the penetra- 

 tion of sea water as held by the ancients. He mentions Trogus 

 Pompeius under Augustus as the author of the theory, but it is 

 remarkable that the same views were held by the poet Lucretius 

 more than half a century before. 



^ Cosmos, Vol. I, p. 242. Bohn's translation. 



^ Cosmos, Vol. V, pp. 431-2. All the citations of Humboldt's works are 

 from the Bohn translations. 



