34 VOLCANOES 



never exceeded a superficial layer of fifty miles in thickness. The 

 next step was the formation of a solid crust at the surface by cool- 

 ing, leaving beneath it a layer which is not under sufficient pressure 

 to be consolidated, and, being protected by the crust, has not yet 

 cooled to the point of solidification. Hence, the transition from 

 one layer to another is gradual and not abrupt. Such an hypothe- 

 sis is believed to avoid, on the one hand, the astronomical objec- 

 tions to a substantially fluid earth, and, on the other, certain 

 geological difficulties in the way of accepting the belief that the 

 earth is practically solid throughout. But, unfortunately, we are 

 yet far too ignorant to decide between these different views, and 

 it is merely a question of greater or less probability, according to 

 the available evidence. 



The subterranean or igneous agencies may be conveniently 

 classified under four main heads: (i) volcanoes, (2) thermal 

 springs, (3) earthquakes, (4) changes of level between land and 

 sea. Of these, the second, including thermal springs, geysers, 

 and the like, will be considered in a later chapter, because they 

 are mixed agencies, and cannot be well understood until we have 

 learned something of ordinary springs and their operations. 



I. Volcanoes 



A volcano is usually a conical hill or mountain, having an open- 

 ing, through which various molten or solid materials are cast up. 

 The essential part of the volcano is the opening or vent, which 

 establishes a connection with the highly heated interior of the 

 globe. The mountain, when present, is secondary, and is formed 

 by the materials which the volcano itself has piled up ; it is thus 

 the effect and in no sense the cause of the phenomena. 



Present Distribution of Volcanoes. — The geographical distribu- 

 tion of volcanic vents has greatly changed at different periods of 

 the earth's history. There are few large land areas which do not 

 display traces of former volcanic activity, though such action may 

 have died out ages ago, never to be renewed, and no active vent 

 be found for great distances around the now extinct centres. 



