240 Dr Charles Daubeny on the 



hence various minerals being produced in which this alkali 

 is predominant.* 



These facts may, perhaps, suffice to shew, that the original 

 material out of which volcanic rocks of whatever age have 

 been elaborated, was of a granitic nature — a strong confir- 

 mation, as it appears to me, of the old opinion, that this rock 

 stands lowest in the series of formations, and serves as the 

 foundation upon which the rest repose. 



The same circumstances may, likewise, be alleged as proofs 

 that the igneous operations actually going on are, in many 

 respects, different from those which produced the primeval 

 granite ; to which conclusion we shall also be led, by consider- 

 ing the differences that exist between the composition of the 

 ancient volcanic products of the Monte Somma, and those 

 resulting from the operations of Vesuvius at the present day. 



Thus, if we go back to the period when the materials which 

 constitute the tuff about Naples were ejected, we shall find 

 that pumice was then one of the principal products ; whereas 

 it is now never found amongst the ejected masses at Vesuvius. 



Now, pumice has been shewn to be merely an altered con- 

 dition of trachyte, and not to be derivable from felspars so 

 poor in silica as labradorite, or anorthite. Moreover, M. 

 Dufrenoy has ascertained that the lavas of Monte Somma 

 are almost unattackable by acids, whilst in those of Vesuvius 

 the proportion of the soluble to the insoluble part is in gene- 

 ral about as four to one. The former lavas contain a larger 

 proportion of potass, whilst in the latter soda predominates. 



It is also a well ascertained fact,t although disputed in an 

 English work of authority, that this volcano was formerly 



* As Natrolite, Nepheline, Thomsonite, &c. 



t For this I need not go further than the Guida de Napoli, already quoted, 

 the geological portion of which was contributed by Professor Scacchi, a very 

 accurate mineralogist, who has done a service to science, not only by the dis- 

 covery of many new species at Vesuvius, but also by identifying several of those 

 which Monticelli had created with substances previously discovered. From his 

 enumeration of the minerals found about Vesuvius, it will be perceived, that, 

 with the exception of felspar, augite, hornblende, and brieslakite, they all ap» 

 pear to be derived from the extinct volcano of Monte Somma. 



