246 J. W. Judd—On Vohanos. 



The island of Ischia, like those of Vivara, Procida, and Nisita, is 

 merely an outlying portion of the Cam pi Phlegrsei of Naples ; 

 and the conclusions to which we are led concerning the nature 

 and age of the tuffs and lavas of the island we are especially 

 describing, are almost equally applicable to those of the whole dis- 

 trict. The two principal volcanos of this area are Vesuvius (including 

 under that name the more ancient Somma as part of the same moun- 

 tain) and Epomeo ; the former of which consists of a central cone 

 within a semi-circular crater-ring, composed of trachy tic- tuffs enve- 

 loped by lava and agglomerates of leucitic basalt, which have been 

 the products of all its later eruptions ; the latter is a ruined tuff-cone, 

 surrounded by numerous parasitical vents, the lavas and fragmentary 

 materials produced by which have all been of trachytic character. 



A line drawn through Epomeo and Vesuvius would, if produced, 

 strike the volcanic region of Monte Vultur in one direction, and that 

 of the Ponza Islands in the other; and the numerous cones, crater- 

 rings, and crater-lakes of the Campi Phlegrasi all appear to be 

 arranged along lines parallel to this series of grand volcanos. 



We can scarcely doubt, therefore, that this linear and parallel 

 arrangement points to the existence of a corresponding series of sub- 

 terranean fissures, by means of which the igneous products, whether 

 of recent date or belonging to older geological periods, were enabled 

 to reach the surface. 



The oldest portions of the volcanic rocks of Ischia are unquestion- 

 ably the series of pumiceous tuffs of a greenish colour which make 

 up the great central cone of Epomeo. That this tuff has been 

 formed from an ordinary trachytic lava, through the distension and 

 dispersion of portions of its mass at periods of eruption by the im- 

 prisoned gaseous and liquid materials, there is clear proof afforded in 

 its general characters, and chemical composition. We may indeed 

 call this tuff the " froth " of trachyte, and compare its relations to 

 that rock with those which subsist between the wave and the wreaths 

 of foam driven from its summit. The following analyses illustrate 

 the composition of these tuffs of Epomeo, and prove its general iden- 

 tity with those of the Phlegrsean fields, of which numerous analyses 

 have been published : — 



Analysis of the Analysis of the 



Green tuff of Epomeo same by 



by C. W. C. Fuchs, 1872. H. Abich, 1837. 

 Silica 54-69 54-57 



Alumina 

 Ferric Oxide 

 Ferrous Oxide 



Lime 



Magnesia 

 Manganese ... 



Potash 



Soda 



Phosphoric acid 

 "Water and loss 



20-00 17-93 



313 \ 5-49 



2-26 j ° iV 



2-17 0-77 



0-70 077 



0-02 — 



4-77 5-23 



0-28 6-40 



0021 — 



11-61 8-19 



99-65 99-35 



Specific gravity 2-17 2-52 



