Site of the Ancient City of the Aurunci. 237 



tions more or less vitreous, and generally scaly, with a pearly 

 lustre arising from the commencement of a kind of crystal- 

 lization in the mass, or, where this is wanting, passing into a 

 stony structure, or into a semivitreous one corresponding with 

 that of pitchstone, which latter mineral seems to be nearly 

 allied to it. 



6. Trachytic tuff, the principal rock covering the Phlegrean 

 fields, the analysis of which proves that it is, like pumice, 

 only a metamorphosed condition of trachyte. Thus tuff, pu- 

 mice, and obsidian, are all modifications of the same volcanic 

 basis ; and all, except obsidian, contain water chemically com- 

 bined — ^}'ellow tuff three atoms — white tuff two atoms — ^pu- 

 mice one. 



Now lava, although commonly accompanied by abundance 

 of steam at the time of its eruption, and containing, even for 

 several months afterwards,* entangled within it a large 

 quantity of aqueous vapour, holds no water in chemical com- 

 bination, so that the fact stated with respect to tuff and pu- 

 mice shews, that these formations have been placed under 

 circumstances in some respects different from modem lavas. 

 We must, therefore, regard the three former as caused 

 by water operating in a different manner from the steam 

 which accompanies a flow of lava, inasmuch as the latter 

 never contains any water in a state of chemical combination. 

 All these varieties, then, of volcanic products, which Abich 

 has classed under the general name of trachyte, approximate 

 to granite, in the circumstance of containing a trisilicate of 

 alumina or of some corresponding base, and hence may be 

 supposed to be more immediately derived from the latter rock, 

 than other igneous formations are. Nevertheless, in one 

 variety of it, namely, in the species distinguished by Beudant 

 as trachytic porphyry, quartz is present ; and, accordingly, 

 this modification would seem to present the nearest approxi- 

 mation to granite, the chief difference indeed between the 

 two being the partial substitution of glassy felspar for ortho- 



* See my Memoir on the Eruption of 1834, in Ph. Trans, for 1845. 



