J. W. Judd — On Volcanos. 



253 



are the obsidians like those of Ischia, which are of the same chemical 

 composition as the ordinary trachytes ; while the basic lavas only 

 very rarely assume the glassy condition (tachylite, etc.). The 

 following analyses of an Ischian pumice by Abich, and of the 

 "obsidian" of Monte Eotaro by Fuchs, may be compared with 

 those of the more acid glasses of Lipari, which analyses have been 

 already given on page 62. 









Pumice of the 



Obsidian of 



Island of Ischia. 



Monte Rotaro. 



Silica ; 62-29 



60-77 



Alumina 





»•• ... 



.. 16-89 



19-83 



Ferric Oxide 





. ... 



— 



4-14 



Ferrous Oxide 





. ... 



.. 4-15 



2-43 



Lime 









1-24 



1-63 



Magnesia 







.. 0-50 



0-34 



Potash ... 





.. ... 



.. 3-98 



6-27 



Soda ... 





•• ... 



.. 6-21 



4-90 



"Water, etc 









.. 389 



0-24 



Specific gravity 2-4172 2-44 



The bottom of the crater of Monte Eotaro is a small level plain 

 about 70 yards in diameter ; around this the walls of the crater rise 

 to heights varying from 400 to 100 feet. The irregular height 

 of the crater- walls appears to be the result of denudation ; for though 

 several lava-streams have issued from it, notably one on its southern 

 side, yet the crater seems to have been reformed subsequently to the 

 outflow of the last of them. Indeed, as we shall now proceed to 

 show, the streams of lava of the latest eruptions of Monte Rotaro 

 appear to have proceeded from its base, and not from its summit, 

 the crater remaining unbreached. 



On the northern slope of Monte Eotaro are situated two small 

 ruined cinder-cones, which bear to that volcano the same relation 

 which it does to the central mass of Epomeo. These smaller and 

 more modern vents are Monte Tabor and the Castiglione. (See Fig. 

 .15.) The little cone of Monte Tabor is made up of an agglomerate of 



SEA 



Fig. 15.— Monte Rotaro -with Monte Tabor at its foot, as seen from Casamk:ciola. 

 a, the crater of Rotaro. *, the ruined cone of Tabor, c, the lava-stream flowing to the sea 

 and well exposed in a series of quarries. 



blocks, some of which are of great size, and are always more or less 

 scoriaceous. Mingled with the scoriae are many fragments ^ of the 

 ordinary tuffs of Ischia, and of the same peculiar trachyte which has 

 flowed from the cone. These are often burnt to a bright-red colour. 

 Near the south foot of Monte Tabor an old " stufa " occurs. Monte 

 Tabor is evidently the remains of a small cone thrown up at the time 



