TRUE VOLCANOES. 403 



ries), 198, or nearly seven eighths of the 225 still active vol- 

 canoes of the whole earth. The volcanoes nearest the poles 

 are, so far as our present geographical knowledge goes, in 

 the northern hemisphere the volcano Esk, on the small isl- 

 and of Jan Meyen, in lat. 71 I 7 , and west long. 7 30' 30"; 

 and in the southern hemisphere Mount Erebus, whose red 

 iiames are visible even by day, and which Sir James Boss,* 

 on his great southern voyage of discovery in 1841, found to 

 be 12,400 feet high, or about 240 feet higher than the Peak 

 of Teneriffe, in- lat. 77 33' and long. 166 58' 30" east. 



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 al- 

 ready, in another place (Cosmos, vol. i., p. 243), mentioned 

 the confused theory of Trogus Pompeius under Augustus, 

 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 hy- 

 pothesis of the sea-water penetrating into the volcanic focus 

 seemed to acquire a firmer foundation at the time of the dis- 

 covery of the metals of the earth by Davy, but the great dis- 

 coverer himself poon 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 lo- 

 cally diminished thickness of the inflexible portion of the 

 earth's crust, might, in my opinion, offer a greater appear- 

 ance of probabilty. It is not difficult to imagine that at the 

 margins of the upheaving continents which now form the 

 more or less precipitous littoral boundary visible over the 

 surface of the sea, fissures have been produced by the simul- 

 taneous sinking of the adjoining bottom of the sea, through 

 which the communication with the molten interior is pro- 

 moted. On the ridge of the elevations, far from that area of 

 depression in the oceanic basin, the same occasion for the 

 existence of such rents does not exist. Volcanoes follow the 

 present sea-shore in single, sometimes double, and sometimes 

 even triple parallel rows. These are connected by short 



* Sir James Ross. Voyage to the Antarctic Renions, vol. i., p. 217. 

 220, and 364. 



f Gay-Lussac, Reflexions sur les Volcans in the Annales de Chimie et 

 de Physique, t. xxii., 1823, p. 429 ; see above, p. 163, note * ; Arago, 

 (Euvres completes, t. iii., p. 47. 



