404 REACTION OF THE INTERIOR OF THE EARTH 



with or without spontaneous ignition, are produced by 

 permanently increased temperature imparted to the 

 waters of the springs and to gaseous exhalations, and 

 by chemical diversity of composition. The highest, 

 and, in its manifestations, the most complicated degree 

 of activity is presented in volcanoes, as these elicit the 

 great and diversified processes of cystalline rock-forma- 

 tion by the dry method, and are therefore not mere 

 agents of dissolution and destruction, but are also 

 formative, and bring substances into new combinations. 

 A considerable portion of very recent, if not of the 

 most recent, rocks are the work of volcanic activity; 

 whether, as is still the case at many points of the earth's 

 surface, by pouring forth molten masses from their own 

 conical or dome-shaped frameworks ; or whether, in the 

 youth of our planet, basaltic and trachytic rocks may 

 have flowed forth directly in a fluid state from a net- 

 work of open fissures, in proximity to sedimentary 

 strata. 



I have endeavoured, in the preceding pages, to deter- 

 mine in the most careful manner the locality of the points 

 at which communication between the fluid interior of the 

 earth and the external atmosphere has long remained 

 open. I have now to sum up the number of these 

 points, to distinguish the volcanoes which still continue 

 active from among the much greater abundance of such 

 as have been so within remote historic periods, and to 

 consider them in their distribution as continental and as 

 insular volcanoes. If the entire number which I think 

 I may assume as the lowest (" nombre limite," " limite 

 inferieur ") were simultaneously in a state of activity, 



