670 EEPOET— 1888. 



Beds are now exposed •wliicb, at the time of tbeir folding-, lay fully eight miles 

 below the surface, and must, therefore, have been far below this neutral zone, which 

 was then (at the end of the Palteozoie Age) less than five miles deep. 



Similar facts might be adduced from other parts of the world, and it is there- 

 fore difficult to avoid the conclusion that the laj-er in question has been placed too 

 near the surface, though of the actual existence of such a zone, after a careful study 

 of these iovestigations, scarcely a doubt can be entertained. 



It should further be urged, as another difficulty lying in the way of the im- 

 plicit reception of the numerical conclusions of the mathematicians, that if the 

 layer of 'no strain ' lie at the depth stated, all seismic foci must be situated at less 

 depth. Many of them doubtless are so, but that of the great Neapolitan earth- 

 quake of 1857 was placed, by Mr. Mallett at the depth of seven miles, and that of 

 the Charleston earthquake of 1886 at the depth of twelve miles. AVithout pledg- 

 ing ourselves to exactness in these figures, it is somewhat difficult to reconcile 

 them with those of the mathematician. 



Little doubt can be entertained that an actual and existing state of things has 

 teen revealed by the mathematicians, needing, however, the joint action of them- 

 selves and the physicists in order to adjust its details. 



8. On the Causes of Volcanic Action. By J. Logax Loblet, F.G.S. 



After citing recent opinion as to the absence of an adequate explanation of the 

 causes of volcanic action, the author showed that the accumulation of knowledge 

 of the controlling facts of volcanic phenomena and the amount of attention which 

 had been given to the question placed the subject on firmer grounds, and made the 

 finding of a satisfactory solution of the problem now more probable. 



The difficulty of the question lay in the great number of facts and the apparent 

 conflict of many of them. 



It was, in the first place, necessary that the leading and controlling facts should 

 be recognised and kept in view. "With tliis object a compendium or concise state- 

 ment of forty-five such facts was given, followed by a brief review of the various 

 theories that had been advanced from Lemery's in 1700 to Prestwich's in 1886, 

 with in each case mimerical references to the facts in the compendium which were 

 in the author's opinion at variance with the respective hypotheses. 



The author's own conclusions were then submitted, which are briefly as 

 follows : — 



A. That the primary cause of the formation of lava is the internal heat of the 

 globe inducing chemical action in subterranean regions where the materials and 

 conditions are both favourable. 



That since the fusion-point of temperature of solids is raised by extreme 

 pressure, conditions for chemical action may be changed from unfavourable to 

 favourable by the removal or relief of vertical pressure by lateral or tangential 

 pressure. 



That certam substances are fusible at low or moderate temperatures, and that 

 thus at very moderate depths chemical action may be locally commenced that will 

 extend until sufficient heat is produced to effect rock-fusion. 



B. That the cause of the ejection of lava from its source, and its rise in the 

 volcanic tube is the increase of bulk consequent upon the change from the solid to 

 the fluid state, aided by the formation of potentially gaseous compomids by chemical 

 reactions among the original materials of the magma. 



That the ascent of the lava in the volcanic tube may be affected by the weight 

 of the atmosphere and by lunar attractive influence, and that therefore a volcanic 

 vent is a thermometer, and, secondarily, a barometer and helkusometer ' combined. 



C. That the explosive effects of volcanic eruptions are altogether secondary-, 

 and are due to the access of sea and land water, by percolation through cool rocks, 

 to fissures up which lava is ascending. 



That this water, when converted into steam, opens, by its expansive power, 



' From eKKvcts — (us = attraction. 



