210 J. LOGAN LOBLEY, F.G.S., F.E.G.S.^ ON VOLCANIC ACTION 



since all were based on chemical action. Chemical combination, 

 as the cause of volcanic phenomena, was also supported by 

 Daubeny, but both he and Davy advanced on Lemery and 

 Breislak in regarding water as the source of the supply of the 

 essential element, oxygen. 



Cordier was the advocate of the hypothesis that has been 

 favoured for the longest time and by the greatest number. 

 This is based on the popular assumption that the earth is a 

 great mass of fused matter enclosed in a thin shell or crust, 

 through which by fissures the molten matter or lava issues 

 when the interior mass is pressed upon by the adjustment of the 

 exterior crust required by the shrinkage consequent upon the 

 secular cooling of the whole globe. 



The difficulty of accepting a thin crust led Sir Charles Lyell 

 and Mr. Hopkins to the conclusion that there were probably 

 portions of the crust in a fused condition, these subterranean 

 reservoirs of lava existing where relief of vertical by lateral 

 pressure allowed of the interior heat exerting its melting power, 

 and that the cause of volcanic activity was supplied by the 

 access of water from the sea. 



Dr. Sterry Hunt and my old friend the Eev. Osmond Fisher, 

 contended for a thin exterior crust and a solid central mass 

 with an intermediate ocean of fused matter ; and the great 

 seismologist of the last century, Mr. Mallet, attributed volcanic 

 heat to tangential pressure from secular cooling ; while, still later. 

 Prof Prestwich advocated the importance of land surface water 

 as a factor in the production of eruptions, while accepting a 

 central fluid mass as the source of volcanic lava. 



The impossibility of reconciling any one of these hypotheses 

 with all volcanic phenomena and ascertained scientific facts, 

 leaves the question of volcanic action still undetermined and an 

 unsolved problem. It therefore affords a most interesting subject 

 for consideration and discussion. 



Volcanic Phenomena. 



In estimating the value of any hypothesis it is in the 

 first place necessary that we be acquainted with the 

 phenomena to be accounted for; and each and all of these 

 phenomena must be kept clearly in view. 



Volcanic action may be said to be that which ejects material 

 on to the exterior of the globe from below the surface. A 

 volcano is therefore essentially a communication between the 

 interior of the earth and the exterior, and consequently it is 



