60 VOLCANOES 



(4) The intermittency of volcanoes and their mode of distribu- 

 tion add to the difficulty of the whole subject, but any complete 

 theory must explain them. The views which bring volcanic action 

 into relation with the mechanical changes in the crust, are those 

 which seem most consonant with the known facts of the past and 

 present distribution of the vents. 



Here, for lack of space, we must leave the subject. Enough 

 has been said to show how far we still are from understanding the 

 mystery of volcanoes. 



