134 DYNAMICAL QEOLOGY. 



tion all the time. But most volcanic eruptions are peri- 

 odic. The period of intermission may be ten, or twenty, 

 or fifty, or one hundred years. 



Number, Size, and Distribution. Humbol(lt enu- 

 merates 225 volcanoes as known to have erupted in the 

 past century. The number now known is doubtless much 

 greater. In size they vary from little mounds (monticles) 

 to Mount Etna, 11,000 feet ; Mauna Loa, 14,000 feet ; 

 and Aconcagua, 23,000 feet. In this last case the whole 

 height is not due to volcanic eruptions, for the cone 

 stands on a mountain plateau many thousand feet high ; 

 but the others are wholly built up by eruption. The 

 laws of distribution may be briefly stated as follows : 1. 

 Volcanoes are mostly on islands in the midst of the sea, 

 or on the margins of continents bordering the sea. 

 Only a very few have been found at a distance from the 

 sea. The Pacific Ocean is the greatest theatre of volcanic 

 activity. Its surface is dotted over with volcanic islands, 

 and its margin is belted about with a fiery girdle of 

 volcanic vents. 2. Volcanoes occur usually in lines, as 

 if connected with a crust fissure, or else in groups, as if 

 connected with a subterranean lake of fused matter. 

 The most remarkable linear series of volcanoes is that 

 which, commencing in the volcanoes of Fuegia, con- 

 tinues, as a chain of active vents, along the Andes 

 and Mexican Cordilleras ; then along the Sierra and 

 Cascade, as the recently extinct volcanoes of these 

 chains ; then along the Aleutian Isles and Kamschatka ; 

 then through the Kurile Isles to Japan and the Philip- 

 pines ; then with more uncertainty to New Guinea, New 

 Zealand, the Antarctic Continent, Deception Island, and 

 back again to Fuegia, after completing the circle of the 

 globe. The most remarkable groups are the Javanese 

 group, the Hawaiian group, the Icelandic and the Medi- 

 terranean groups. 3. Volcanoes are found mostly in 

 strata of comparatively recent date, and the retiring of 



