ON ITS EXTERIOR. VOLCANOES. 325 



titles. We may cite as such the influence of enormous 

 pressure on the fusibility, the very different heat-con- 

 ducting power of different rocks, the remarkable dimi- 

 nution of the conducting power with great increase of 

 temperature treated of by Forbes, the unequal depth of 

 the bed of the ocean, and all the local accidents in the 

 connection and character of the fissures which lead down 

 to the fluid interior ! If the greater frequency of vol- 

 canoes and the more frequent and active intercourse 

 between the interior and the atmosphere in some regions 

 of the earth, is to be explained by the greater proximity 

 of the upper limit of the fluid interior to the surface, 

 this proximity may itself depend either on the relative 

 mean difference between the heights of the bed of the 

 sea and the continent, or on the unequal vertical depth 

 at which in different latitudes and longitudes the surface 

 of the molten fluid mass is situated. But where does 

 such a surface commence ? are there not intermediate 

 degrees between perfect rigidity and perfect fluidity? 

 Transitional states, which have often come into discussion 

 in controversies respecting the plasticity of some plu- 

 tonic and volcanic rocks which have been raised to the 

 surface, and respecting the movements of glaciers ? Such 

 intermediate states escape from mathematical considera- 

 tion as much as does the state of the so-called fluid 

 interior under an enormous pressure. If it be in itself 

 not altogether probable that heat should everywhere 

 continue to increase in arithmetical progression with 

 increasing depth, so also there may occur local inter- 

 vening disturbing causes, e. g. by subterranean basins, 

 or hollows, in the solid mass, which may be from time 

 to time partially filled from below, with fluid lava and 



