Dr. Johmton-Lavis — Mechanism of Volcanic Action. 437 



This exactly fits in with what is frequently found in the distribution 

 of volcanoes along the edges of areas of marked compression or 

 mountain regions. It will explain also the presence of volcanoes 

 having a linear arrangement between closely situated mountain 

 chains or areas, as in South America. Great rifts, such as those 

 of Central Africa and some canon districts, are probably of such 

 origin. In earthquakes of tectonic origin it has been pointed out^ 

 that the piers of damaged bridges have usually been found to have 

 approached each other. This would evidently take place in the areas 

 of positive compression (Fig. 1, C, C). On the other hand, in 

 exceptional cases the piers have been found to have been separated. 

 This might well occur in the area of negative compression (R), or 

 what might well be termed the areas of retraction. The much larger 

 proportion of t?ie former effect on the bridge piers would no doubt be 

 in the much greater ratio of compressional areas to refractional areas 

 on the earth's surface. 



May not ocean basins be in part due to blocks or islands of the 

 contracting zones exerting that diminution of volume in a vertical 

 more than in a horizontal direction, as we have so far been considering 

 it to be ? The peculiar abysmal ocean troughs often at the edge of 

 ocean basins and parallel to chains of volcanoes or interrupted by them 

 could well be explained by the same circumstances. I do not claim 

 that ocean basins are alone due to this cause, but to a combination 

 of these conditions with perhaps the slipping, shearing, and corrugating 

 of the primitive crust over a fluid envelope, and even the tetrahedral 

 collapse of a cooling globe. I lay down here but a general principle 

 to which there may be many exceptions due to the vicissitudes of 

 cooling and the variation in the materials concerned in any particular 

 region, not to speak of the changing position of the earth's axis, 

 the crustal inertia of Professor G. H. Darwin, etc. 



Liquid rock having thus reached a considerable way to the surface, 

 either as simple dykes (Fig. 1, I, I), laccolytes, or sills (Fig. 1, L, L), 

 and so forth, is now in a situation suitable for the second series of 

 phenomena, constituting what I call surface volcanic action, to come 

 into play. 



Surface Volcanic Phenomena. — Two schools of vulcanologists have 

 held opposed views as to the origin of the volatile constituents con- 

 tained in fl.uid igneous rock. One class of writers maintain that the 

 gaseous contents are primordial, and have been contained in the igneous 

 paste from the time that our globe condensed from the nebulous state. 

 Others attribute all the volatile matter still retained in cooled igneous 

 rock or evolved at volcanic mouths and fumaroles to the volatilization 

 of water met by the igneous rock in its journey towards the surface. 

 Probably both are right, but I propose to bring before you a series 

 of my observations that point incontrovertibly to the fact that by far 

 the major part of the volatile constituents of a magma are acquired 

 by it on its journey towards the surface. 



As condensation took place in our planet from a nebulous state, as 

 each layer or shell of rock materials passed from the gaseous to the 



' Professor Hobbs, Niath lateraational Congress of Geography, 1908. 



