f 40 R. MALLET ON AN HITHERTO UNNOTICED CIRCUMSTANCE 



40. On an hitherto unnoticed Circumstance affecting the Piling-up 

 of Volcanic Cones. By Robert Mallet, Esq., F.R.S., F.G.S. 

 (Read June 20, 1877.) 



Besides the two great groups of conditions which mainly determine 

 the general form and shaping-out of volcanic cones, many secondary 

 forces concur in greater or less degree in the production of the varied 

 and often complicated phenomena which constantly or occasionally 

 are observable in or around these cones. Some of these forces are 

 mechanical, others physical or molecular, acting separately or, more 

 often, in combination. It is not my intention here to enter upon 

 any systematic discussion of the play of these varied forces, which, 

 it may be remarked, have never yet been submitted to a careful and 

 systematic analysis by the mechanical and physical philosopher, and 

 which remain amply to reward competent labour, when bestowed 

 upon them, but which can only be adequately treated at a length and 

 in such detail, descriptive and analytic, as must extend to a volume, 

 or at least to a succession of memoirs. 



My present purpose is merely to notice prominently one cir- 

 cumstance, very commonly belonging to the piling-up of volcanic 

 cones, which, so far as my knowledge extends, has entirely escaped 

 hitherto the observation of vulcanists, but which, nevertheless, must 

 often exercise important influences upon the exterior form and 

 internal structure of volcanic cones, especially at and about their 

 lower portions, and may even occasionally affect surrounding strata, 

 entirely outside the circuit of the base of the cone. Volcanic cones 

 as found upon our globe, when largely viewed are little else than 

 " cinder-tips," aggregated of the loose material blown out in a 

 heated but discontinuous state, and of fused matter belched or 

 poured forth. They are therefore evidences of that explosive action 

 which characterizes fitfully, or constantly, all existing volcanic action, 

 and which appears to me, in common with many able American 

 geologists, to distinguish it from those quiet and unexplosive over- 

 flows of melted matter, which often take place upon a scale much 

 vaster than anything which is presented to us by existing volcanic 

 action. Such outflows appear referable to a geological epoch more 

 or less anterior to those which belong to the existing volcanic regime 

 of our globe. We cannot as yet fix with any exactness the boun- 

 daries in geological time when the earlier system, roughly called 

 that of " fissure-eruption " gradually ended, and the existing 

 " system of ejection " at explosive foci, and the heaping-up of cones 

 of ejeeted'material, began. 



I have ventured to express my belief that our existing volcanic 

 system does not go back much beyond the Tertiary period. l T pon 

 this much difference of opinion may exist, and is justifiable in 

 our existing ignorance of the actual circumstances of almost all the 

 volcanos of our globe ; and years of the patient labour of many future 



