250 /. W. Judd — On Vokanos. 



I have omitted from these analyses the small proportions of sodic 

 chloride, phosphoric acid, etc., which they contain. 



As examples of the compact and dark-coloured trachytes, we may 

 notice the following :- 





Monte 



Monte dell' 



Punta 





Campagnano. 



Imperatore. 



della Cima. 



Silica 



... 63-06 



61-05 



61-55 



Alumina 



... 18-32 



18-35 



17-81 



Ferric Oxide 



... 3-22 



4-21 



3-01 



Ferrous Oxide 



— 



2-12 



2-60 



Lime 



... 2-53 



205 



1-69 



Magnesia 



... 0-93 



0-90 



0-47 



Potash 



... 7-52 



5-28 



7-51 



Soda 



... 3-08 



5-94 



408 



Specific gravity ... 2-48 2-53 2-46 



These trachytes are all apparently of considerable antiquity. 

 That of Monte Vetta forms, as we have seen, a number of isolated 

 plateaux, once a connected lava-stream, which breached the great 

 crater of Epomeo on its western side. The other lavas also form 

 plateaux of ancient date, which flowed from some portion of 

 Epomeo, or from lateral cones on its flanks. The original points of 

 outburst of these lava-streams can now, however, be no longer 

 traced, so greatly have they suffered from denudation. . Some of 

 these lavas, like those of Scarrupata and Monte Vetta, are remark- 

 able for containing sodalite, as shown by vom Eath. Others, like 

 those of Monte Marecocco, etc., are distinguished by the presence 

 of mellilite. The compact lava of Monte Campagnano in places 

 becomes vitreous, and passes into obsidian, and it occasionally ex- 

 hibits a banded or ribboned structure, veins of vitreous alternating 

 with others of stony lava. 



These lava-streams frequently cap masses of tuff, and are some- 

 times interbedded with such materials. In some cases these tuffs 

 are made up of fragments of trachyte distended by gas ; in others 

 they are decidedly pumiceous, and have been evidently formed from 

 a glassy rock. As an example of the former, we may cite the 

 trachyte tuff of Punta S. Angelo ; of the latter, the pumice-tuff and 

 pumice of Monte Vico : — 





Trachyte -tuff of 



Pumice-tuff of 



Pumice of 





Punta S. Angelo. 



Monte Vico. 



Monte Vico 



Silica 



... 53-71 



54-02 



60-06 



Alumina 



... 16-35 



18-18 



16-42 



Ferric Oxide 



... 2-82 



3-64 



301 



Ferrous Oxide 



... 2-19 



2-23 



2-33 



Lime 



... 1-38 



2-01 



1-37 



Magnesia ... 



... 0-55 



0-79 



0-40 



Potash 



... 6-73 



3-86 



8-05 



Soda 



... 2-53 



1-71 



3-20 



"Water 



... 14-43 



14-30 



5-27 



The whole of the rocks which we have now described as occurring 

 in Ischia are of such ancient date that they have suffered very greatly 

 from the denuding action of subaerial forces. The lavas which now 

 cap the highest hills must have originally flowed along the bottoms 

 of valleys, the sides of which have been since removed. Streams 



