196 G. Poulett Scrope — On the Cause of Volcanic Action. 



deferred acquisition of a specimen so much desired by Agassiz in 

 1836.1 w^Q main prevision so sagaciously deduced from fragmentary 

 groups of the fossil teeth by the founder and chief builder of the fair 

 edifice of Palichthyology is confirmed, viz. : — that " the genus 

 StropJiodus had a less considerable number of teeth in the jaw than 

 the genus Cestracion : " ^ and the only emendation which this fossil 

 suggests is, that " it does not appear, with regard to the rows in 

 Cestracion homologous with those present in Strophodus, that there 

 are fewer teeth in such rows in the fossil genus." ^ Of the teeth of 

 Strophodus figured by Agassiz, those in Tab. 18, Yol. 3, op. cit., 

 figs. 5 and 6, ascribed to Str. suhreticidatus, and those figs. 12, 13, 

 ascribed to Str. magnus, resemble teeth in rows a and b of the present 

 specimen so closely as to indicate specific identity ; perhaps the term 

 Strophodus medius may conveniently indicate the species of the Caen 

 Oolite, which includes more than one of the species originally pro- 

 posed for detached teeth. 



DESCRIPTION OF PLATE VII. 



Fig. 1. Jaw of Strophodus from the Oolite of Caen (drawn of the natural size). 

 The arrow indicates the line of symphysis and fore part of the jaw. 



Fig. \a. View of the section across the principal row of teeth, a, showing the curve 

 of their attached bases. The figures and letters indicate the corresponding rows in the 

 two rami of the jaw. 



[A figure of the Jaw of recent Cestracion is given at page 236 of this Number, — 

 Edit.] 



II. — On the Supposed Influx of Water to the interior of the 



GUOBE, AS THE CaUSE OF VOLCANIC ERUPTIONS. 

 By G. Poulett Scrope, Esq., F.E,S., F.G.S. 



IT is now generally recognised that the power which forces up lava 

 from a depth of miles through narrow and crooked fissures 

 broken across the solid crust of the globe, is no other than steam, 

 developed in the interior of the lava by vaporisation of water inti- 

 mately disseminated throughout its substance. I am not aware that 

 this view of the volcanic phenomena was put forward by any writer 

 previously to the publication of my volume on Volcanos in 1825. 

 But I had derived a conviction of its truth from observation of the 

 great Vesuvian eruption of 1822, and a study of Etna and Stromboli 

 in the years 1819-21. This was subsequently confirmed by the dis- 

 coveries of Sorby, Scheerer, and others, of the existence of water in 

 intimate molecular combination with the crystalline or granular 

 minerals that compose the granitic rocks from the fusion of which 

 lava is supposed to proceed. I had expressed my belief that the 

 aqueous particles so confined, whether m a liquid or a solid state, 

 would, upon the occurrence of increased heat, or diminished 

 pressure, exert an elastic separating force upon the solid 



' " Alors il m'importait essentiellement de connaitre leur disposition et leurs formes 

 diverses." — Recherches sur les Poissons Fossiles, iii. p. 118. 



2 "Le genre Strophodus av ait a ses m^choires un nombre de rangees moins con- 

 siderable que le genre Cestracion." — Tome. cit. p. 120. 



^ " II me parait egalement probable que chaque rangee contenait aussi moins de 

 deuts placeea les unes derriere les autres, de dehors en dedans." — Tome. cit. p. 122. 



