IGNEOUS CAUSES OF CHANGE. 243 







CHAPTER VII. 



Volcanic Eruptions. 



" But, even then, the ground 



Heaved 'neath their tread the giant turrets rocked, 

 And fell : and instantly black night rushed down, 

 And from its bosom burst a thunderous crash 

 Stunning and terrible." Wm. Howitt. 



THE number of active volcanoes, and solfataresor vents, from 

 whieh sulphureous and acid vapors and gases are given off, is 

 about 305; of these, 196 are in islands, and the other 109, are on 

 continents. It is however, a remarkable fact that a majority of 

 them are located near the ocean, or large bodies of water; and 

 even submarine volcanoes are not of unfrequent occurrence. Be- 

 sides the volcanoes now in action, there are many undisputable 

 extinct volcanoes, L e., volcanoes which at some period of the 

 earth's existence, but before the historic era, have been in the 

 state of active eruption. In no country is there better evidence 

 of this than in France. There are in the districts of Auvergne, 

 Vivarais, and Cervennes, more than a hundred conical moun- 

 tains, composed of lava, scorise, and volcanic ashes heaped up, 

 many of them still retaining their ancient craters, and in some 

 cases currents of lava may be traced to great distances. The 

 evidences of -volcanic action in the Rocky Mountains we have 

 already alluded to. 



How long a period of repose may be necessary to constitute an 

 extinct volcano, is of course undetermined. We include as such, 

 those which show indubitable evidence of former activity, but 

 which have not had eruptions within the historic era. Tt is by no 

 means necessary that volcanoes, to be considered active, should 

 incessantly emit flames, they may remain for ages choked up, 



