J. W. Judd—On Volcanos. 249 



marl, produced by aqueous and organio agencies from the igneous 

 products. But in its central parts, and within the great crater, the 

 mass is composed of much coarser materials, and includes angular 

 blocks and bombs of large size. 



The causes of the destruction of the vast crater of Epomeo we 

 have not far to seek. Its walls being wholly built up, as we 

 have seen, of loose tuffs, it is evident that the weight of any 

 great mass of lava rising through the vent would inevitably, if it 

 did not force an exit for itself nearer the base of the mountain, carry 

 away the weakest side, and thus breach the crater. That the crater 

 of Epomeo was thus breached on its eastern side we have very 

 clear evidence. It is true that the great streams of lava which have 

 flowed from this side of the crater down the slope have been cut up 

 into a number of isolated plateaux by aqueous erosion, but a com- 

 parison of the slopes and elevations of these surviving portions 

 furnishes us with the clearest evidence of their former continuity. 

 Thus Monte Vetta exhibits the great bed of trachyte, which is 

 nearly 200 feet thick, at an elevation of 1651 feet ; in Monte 

 Trippia it rises to 1341 feet ; while in the hill of Moscardine it is 

 only about 900 feet. In Monte G-arofali we find another great 

 mass, perhaps a portion of the same sheet. And all have a similarly 

 inclined position dipping from the great central crater of Epomeo. 



All the lavas of Ischia, from the oldest to those more recent ones 

 which we shall hereafter describe, are of trachytic character ; but 

 although their ultimate chemical composition is almost identical, 

 they nevertheless offer some very interesting peculiarities with 

 respect to the minerals into which their elements are combined. 

 The trachytes of Ischia are made up of a crystalline base composed 

 of both orthoclastic and plagioclastic felspar, throughout the mass of 

 which large and brilliant crystals of sanidine, with smaller ones of 

 sodalite, hornblende, mica, mellilite, magnetite, and ilmenite are 

 scattered in various proportions. Sometimes the whole passes into 

 a compact rock ; at others we find a stony base containing the 

 sanidine and other crystals ; while occasionally the base becomes 

 vitreous and the rock passes into a porphyritic obsidian ("obsidian- 

 porphyry" or " pitchstone-porphyry "). Sometimes again, as in the 

 case of the quartz-trachyte or Liparites of Ponza, Lipari, etc., the 

 true trachytes of Ischia exhibit compact and vitreous portions com- 

 bined in alternating bands. 



As examples of the composition of the coarsely crystalline light- 

 coloured trachytes of Ischia, we may cite the following analyses 

 from Euchs and vom Eath : — 



1 



/Tonte Vetta. 



Scanella. 



Scarrupata. 



Marecocco, 



Specific gravity .. 



2-45 



— 



I. 



II. 



2-43 



Silica 



. 61-87 



59-12 



62-95 



65-75 



61-49 



Alumina 



. 18-33 



21-46 



18-26 



17-87 



20-02 



Ferric Oxide 



3-23 



2-68 



4-46 



4-25 



3-11 



Ferrous Oxide 



2-51 



2-72 











2-72 



Lime 



2-11 



2-16 



0-84 



1-33 



1-88 



Magnesia 



0-65 



0-84 



0-63 



0-52 



0-52 



Potash 



6-51 



7-66 



6-06 



3-48 



7-13 



Soda 



5-07 



3-78 



7-17 



5-36 



3-39 



