68 J. W. Judd — On Volcanos. 



over the glassy, while in the latter the reverse is the case. Old 

 craters can be traced at Fossa del Monte, Monte della Guardia, and 

 other points in the district. 



Some of these lavas have undergone a certain amount of alteration 

 from the passage through them of acid gases, as is shown by the fol- 

 lowing analysis by Abich of a Liparite from Monte della Guardia : 



Silica 



Alumina 



Oxide of Iron 

 Lime ... .... .... 



Magnesia 



Potash 



Soda 



Volatile materials, principally Sulphur and Sul 



68-35 

 13-92 

 2-28 

 0-84 

 2-20 

 3-24 

 4-29 



4-64 



phuric Acid 



While the action of the acid gases upon the ordinary trachytes of 

 the second period of eruption in Lipari gives rise to the formation 

 of selenite and basic sulphates of iron, — sulphate of alumina and 

 free sulphur are the products of the same action on the later formed 

 quartz-trachytes. 



To those who regard the fluidity of lava as the result of simple 

 fusion, nothing can be more startling than the behaviour of these 

 obsidian currents of Lipari. While, as is well known, some of the 

 highly crystalline lavas of Vesuvius have flowed with the most 

 astonishing rapidity, these glassy masses have evidently possessed 

 only the most imperfect fluidity. In proof of their viscosity I may 

 point to the manner in which the modern obsidian stream of Vulcano 

 is confined to the steep slope of the cone, at the bottom of which it 

 has piled itself up in great hummocky masses, instead of spreading 

 out in a fan-shaped manner, or continuing to flow in a stream over 

 the smaller slopes. The same fact is more or less strikingly illus- 

 trated by all the glassy lava-streams. But even more decisive 

 evidence of this slow movement of the obsidian lavas, and of the vast 

 amount of tension and pressure to which their masses have been 

 subjected, is afforded by their internal structure. Every conceivable 

 condition of plication, crumpling and puckering, is illustrated by the 

 sections afforded either in sea-cliffs or the ravines cut by mountain 

 torrents in these obsidian lavas. The appearance presented at two 

 different portions of the same lava-streams, as exposed in a steep 

 escarpment at Porto delle Genti, south of the city of Lipari, are 

 shown in Fig. 9 : in A the mass has been bent into large but sharp 

 folds ; in B the folding has been accompanied by the most intense 

 crumpling and puckering. As we shall show on a future occasion, 

 these mechanical forces have combined with the forces producing 

 devitrification to produce some most interesting phenomena in the 

 minute internal structure of the rocks. 



There can be little doubt that the last great effort of volcanic 

 activity in the island of Lipari was that which produced the present 

 crater of Campo Bianco, and the lava-stream of Eocche Eosse. In 

 spite of traditions and obscure historical allusions, I find it difficult 



