254 



J. W. Judd — On Volcanos. 



of the issue of a lava-stream from the foot of Monte Eotaro, by the 

 flow of which its northern side was swept away, while many of its 

 lighter materials have also been removed by atmospheric denudation. 

 The lava-stream which flowed from the cone to the sea is very 

 extensively quarried, and thereby favourably exposed for study. The 

 rock consists of a sanidine-trachyte of a pink, reddish, greyish, bluish, 

 or greenish tint. The pink colour of certain parts of the rock is due 

 to numerous crystals of Mellilite (Humboldtilite). The sanidine crys- 

 tals of this rock are usually small, and some of oligoclase are scat- 

 tered through the basis of the rock. The other minerals contained 

 in the rock are magnetite, hornblende, mica, and a little augite. This 

 interesting rock has been analysed by Fuchs with the following 

 result : — 



Silica 



Alumina ... ... 



Ferric Oxide ... 



Ferrous Oxide ... 



Manganese 



Lime 



Magnesia 



Potash 



The upper surface of this lava-stream is composed of a scoriaceous, 

 almost pumiceous mass, containing porphyritic crystals entangled 

 in it, and often also inclosing blocks of lava and fragments of the 

 older tuffs which have been borne along in its flow. Where it 

 first issues, this upper scoriaceous portion of the lava-stream is not 

 less than 15 feet thick, but it gradually diminishes to a few feet only 

 towards the end of the current. 



A little to the east of Monte Tabor another and smaller vent has 

 been opened, which has given rise to another lava that has flowed 

 down to the sea. At its source is the still active stufa known as 

 Acqua Castiglione al mare, the water of which has a temperature of 

 from 160° to 170° F. The lava of Castiglione is a trachyte of darker 

 colour than that of Monte Tabor, and, like it, is exhibited in sections 

 in the cliffs of the island. 



62-17 



Soda 



. 4-76 



20-83 



Phosphoric Acid ... . 



. 0-024 



2-26 



Chlorine 



. 0-25 



2-16 



Loss ... 



. 0-25 



trace 









1-68 





101-23 



0-45 









6-40 



Specific gravity . 



. 2-45 



FlG 16.— The Crater-lake of Lago del Bagno, Ischia, -with Montagnone and Eotaro in the 

 distance. 



The Lago del Bagno is evidently a small crater-lake ; it is of 

 regularly circular form, and about a quarter of a mile in diameter. 

 It has clearly been formed, like the similar craters occupied by 

 lakes in the Campi Phlegraei, by explosive action. A few years ago 

 the tuffs forming the northern margin of this lake were cut through, 



