THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE V. VOL. VI. 



No. X. — OCTOBER, 1909. 



OE,IG}-I3Sr.A.3L -A^I^TICLES. 



I. — The Mechanism of Volcanic Action, being the Opening 

 Address to Section III (Vulcanologt) of the International 

 Geographical Congress. 



By H. J. Johnston-Lavis, M.D., F.G.S., etc., Professor of Vulcanology in 

 the Royal University of Naples. 



(PLATES XXIV AND XXV, AND TEXT-PLATE.) 



IN a discussion of this kind it is advisable to be as concise as 

 possible, eliminating minor details, so as to give prominence to 

 the main outlines of any theory one holds. This communication, 

 which the Council of the Ninth International Congress of Geography 

 have honoured me by asking me to address to you, I propose to put 

 into the form of a 'credo'. To this I shall add a few fundamental 

 facts upon which my reasoning was based, leaving minor ones for 

 discussion at greater leisure elsewhere. For convenience I propose 

 to divide my theory into two sections. In the first I shall review 

 what may be conveniently called deep volcanic action, and in the 

 second that group of phenomena that occur when igneous matter 

 nearly reaches the surface or actually finds an exit thereon. 

 Unfortunately, in the first case I am obliged to rely on hypotheses 

 and deductions, whereas in the second section, that of superficial 

 volcanic action, there are a number of fundamental facts and observa- 

 tions upon which to base speculation, and to which I propose to 

 draw your attention. 



Of one fact we are certain, and that is our globe is surrounded by 

 a solid crust, which wherever it can be examined shows unmistakable 

 and almost universal evidence of compression, wrinkling, and dis- 

 location. This crumpling and crushing are equally inexplicable, unless 

 we admit that since the initial solidification of the earth's crust its 

 lower, still cooling part or support has undergone contraction so as to 

 crowd together the already cooled burden of the upper part that this 

 contracting mass carries. 



No one has yet attempted to even suggest that the part of our globe 

 subjacent to the solid crust has shrunk from other causes than a loss 

 of heat. "We may therefore look upon the idea of contraction as due 

 to cooling to be a universally accepted fact. 



DECADE T. VOL. VI. — NO. X. 28 



