ANNIVERSARY ADDRESS OF THE PRESIDENT. CXI 



from the rocks through which they pass or from the air, the various 

 products which M. Deville enumerates have been formed. Each 

 order of emanation has therefore its peculiar products, which are, in 

 M. Deville's opinion, merely modifications, under the influence of 

 variable physical and chemical causes, of the same compounds, derived 

 from the incandescent materials of the volcanic eruption. 



The difficulty is greater when the inquiry is directed, not merely 

 to the gaseous vents of the lava-stream, but to the vents of the vol- 

 canic cone itself, assumed, as they are, to communicate by a transverse 

 fissure with the internal focus, as the variations then observed depend 

 on causes similar to those which have produced in the interior of a 

 vein the successive deposition of different substances. Generalizing 

 the phenomena, an analogy is recognized between the gaseous emana- 

 tions which succeed each other in a volcano during the course of a 

 single eruption, and those which, in the series of ages of our globe, 

 have predominated at each successive epoch. As, for example, when 

 at the commencement of an eruption chlorine and fluorine are the 

 gaseous emanations which proceed from the orifices of the lava, 

 whilst phosphate of lime and oxidulated iron are fixed in the rock, 

 can we not perceive an analogy between the workings of existing 

 volcanic forces and those phenomena of emanation which under the 

 influence of the same agents, chlorine and fluorine, have enriched 

 the most ancient consolidated rocks with tourmaline, phosphate of 

 lime, oxide of tin, or, in a word, with that galaxy of bodies, intimately 

 connected one with another, which has been figuratively named 

 the penumbra of granite ? 



M. Deville then endeavours to explain how the gaseous vents are 

 distributed over the volcanic mass, and to prove that their positions 

 can be referred to the great stratigraphical accidents of the country. 



The effect of an eruption is to determine, on the cone, fissures 

 the direction of which prolonged would sensibly pass through the 

 centre of the crater ; and in a volcano in a state of eruption, two 

 kinds of operating agencies may be recognized ; the one eccentric in 

 connexion with the fissures produced by the eruption, or with the 

 orifices of gaseous vents ; the other central, at the summit of the 

 volcano, or common centre to which all the fissures converge, — the 

 first generally acting only during the duration of the eruption, whilst 

 the latter acts variably but constantly. The object of M. Deville is 

 to define precisely the functions performed by each, either in a period 

 of tranquillity, or during the height of an eruption, or at the moment 

 when the intensity of volcanic action retires from the fissures to the 

 normal focus, and thus, by establishing the mutual connexion of one 

 with the other, to obtain the means of predicting the effects to be ex- 

 pected from observations made during any particular form of eruption; 

 as for example, in respect to the eruption of Vesuvius in 1856, M. 

 Deville, from the facts he had observed, did not hesitate to announce 

 the probability of a series of small eruptions from the summit of the 

 volcano, — a prediction which was fully verified. 



But it was yet to be determined whether the fissures, which per- 

 form so important an office in volcanic eruptions, are merely acci- 



