12 J. W. Judd — On Volcanos. 



part of Stromboli, which Abich found to possess a specific gravity of 

 2-7307, and a per-centage of silica of 61*78. 



The lavas of the second period may be divided into three classes, 

 examples of all of which may be found in each of the islands in 

 which the products of this period are developed. 



A. — The most abundant of these varieties are the ordinary 

 trachytes, usually rendered of a highly porphyritic character, by 

 the dissemination through their mass of scattered crystals of sani- 

 dine, but occasionally compact and granular in texture, and sometimes 

 exhibiting banded and ribboned structures. These old trachytes are 

 often found assuming red and purplish tints on weathering, and then 

 exactly resemble in appearance, as they also do in chemical constitu- 

 tion, many of the " porphy rites " of ancient geological periods. 



B. — A somewhat less common but very beautiful form assumed 

 by these trachytes is that of a dark grey or almost black granular 

 base, through which crystals of sanidine are diffused ; by the passage 

 of the granular or stony base into a more or less perfect vitreous 

 condition, the rock assumes the well-known characters of a " pitch- 

 stone-porphyry." This rock — of which beautiful examples are found 

 in the lava-streams issuing frorn the old ruined crater of Monte 

 Sant' Angelo, constituting the highest point of Lipari, above Tivoli 

 in the same island, and also near La Malfi in Salina — forcibly recalls 

 to the mind the precisely similar varieties of rocks, so abundant at 

 Beinn Shiant and the Sour of Eigg in the Western Highlands of 

 Scotland. 



C. — The third variety of the Lipari trachytes finds its exact 

 analogue in the celebrated Arso lava of Ischia, which has been so 

 admirably described by Fuchs. Its base is similar to that of ordinary 

 trachytes ; but scattered through its mass in greater or less abundance 

 occur crystals of augite, mica, and magnetite, with grains of olivine, 

 which impart to the rock a more basic composition, and cause it to 

 approximate towards the trachy-dolerites of Abich. Trachytes of 

 this third class are found in Monte Rosa in Lipari, near Rinella in 

 Salina, and in great abundance and variety in the southern part of 

 Vulcano. 



One of the most interesting features of the Lipari Islands is the 

 series of wonderful changes which their rocks have undergone, in 

 consequence of the passage through them, subsequently to their 

 eruption, of acid gases and vapours. By this means the hard and 

 crystalline trachytic lavas of Lipari have, over very large areas, 

 been reduced to a soft, white, earthy material, to the eye exactly 

 resembling chalk ; in other cases they have assumed the carious and 

 open crystalline texture of the " alaunstein " of German petrologists ; 

 while in others again they are found less altered, and presenting the 

 most beautiful variegated tints. Similar changes may be seen taking 

 place in the lava of Olibano, where it issues from the crater of the 

 Solfatara of Naples, but in Lipari they are far more complete in 

 character, and on a much grander scale. 



The accumulations of fragmentary materials which constitute the 

 larger portions of the mass of the Lipari Islands exhibit also many 



