224 J. LOGAN LOBLEY^ F.G.S.^ F.R.G.S.^ ON VOLCANIC ACTION 



internal planetary heat of the globe as that which fuses rocks 

 and gives volcanic lava, for the internal heat at less depths 

 than 25 miles is altogether inadequate for the melting of rocks 

 even under the small atmospheric pressure of surface 

 conditions. 



But the rock-fusing temperature tliat gives fluid lava has to 

 be accounted for. 



This at the moderate depth which will alone allow of com- 

 munication by a conduit with the surface, it seems to me, can 

 only be explained by chemical action being brought into play. 

 This, however, does not exclude whatever ehect the planetary 

 heat existing at tliat depth is capable of exerting. At a depth 

 of five miles there is doubtless, in accordance with the lieport 

 of the British Association Committee on Underground Tem- 

 peratures, a temperature of about 500° F. Heat favours 

 chemical action and will cause it to arise where under 

 cooler conditions no chemical action would take place. 

 But chemical action may be prevented or checked by 

 pressure, and the normal pressure at five miles depth is 

 enormous. Then this vertical pressure may be greatly relieved 

 by lateral pressure and other causes, and when so relieved, 

 chemical reactions that had been prevented at a favouring 

 temperature by greater pressure would commence. This 

 chemical action will give an accession of lieat that may 

 give rise to further and more intense chemical action that 

 will still further raise the temperature. By this action and 

 reaction heat may he augmented until a rock-fusing tem- 

 perature is reached. Such action, of course, would only take 

 place where the contents or composition of the rocks gave 

 suitable elements for chemical reactions, and for only so long 

 as those conditions continued. Thus volcanic action in 

 definite and limited areas, as well as the local extinction of 

 volcanic action where geographical conditions are unaltered, 

 may be readily explained. 



Although lessened, the pressure of the exterior rocks may 

 yet be great, and this together with the increase of volume by 

 fusion and the expansion of adjacent rocks by the neighbouring 

 great heat, will force the lava upwards through any conduit 

 available. Sucli a rise of lava may bring it into contact with 

 the water of the exterior rocks, when hydrothermal conditions, 

 or the sudden production of steam, will cause explosive effects 

 and give the earth-tremors and thunderings of incipient 

 eruptions, and may also produce rendings of the surface rocks, 

 and so form passages for great and sudden influxes of sea or 



