10 J. W. Jadd — On Volcanos. 



isolated fragments, such as Basiluzzo, Dattilo, Bottaro, Lisca nera, 

 and Lisca bianca, each of which presents the peculiar wedge-like 

 forms so characteristic of the denuded segments of old crater rings. 

 Some of the lava masses in this tract, especially those of Basiluzzo 

 and Dattilo, exhibit a rudely columnar structure. Panaria was sup- 

 posed by Dolomieu to afford traces of an old crater in its central 

 valley, but this point seems to me, at best, very doubtful. The great 

 mass of highly crystalline rocks of which this island is composed is 

 probably, like the central trachytic bosses of Astroni ancl Rocca 

 Monfina, to which it presents striking resemblances in chemical and 

 petrological characters, the product of an outburst of highly viscid 

 materials which have accumulated immediately around the volcanic 

 vent, instead of flowing as lava streams ; this result being due, as in 

 the analogous examples of the domitic puys of Auvergne, to the 

 imperfect liquidity of the rock at the time of its emission. 



The lavas of the central volcano of the Lipari Islands have long 

 been celebrated for their remarkable petrological characters. Com- 

 posed of one or more species of felspar, with hornblende or mica, 

 and some free quartz, their highly crystalline character led some of 

 the early observers to class them as granites. On the other hand, 

 that they were erupted near the surface, and in many cases under 

 no very great pressure, is shown by the glassy and pumiceous cha- 

 racters which portions of their mass assume. Hence they have been 

 described as exhibiting all the transitions from granite to pumice. 



In their chemical characters these peculiar rocks offer points of as 

 great interest as in their petrological structure. Prom true or ordi- 

 nary ("quartzfrei") trachytes, with a specific gravity of 2*6, and an 

 average per-centage of silica of about 62, they graduate in one 

 direction up to the rocks designated by Abich as trachy-dolerites, 

 with a specific gravity of 2-75., and a silica per-centage of 57 ; while 

 in the other, by exhibiting a smaller specific gravity with a higher 

 per-centage of silica and some free quartz, they approach the quartz- 

 trachytes. These extremes of composition are exhibited in the rocks 

 of Lisca nera, Lisca bianca, and Dattilo, on the one hand, and in 

 those of Basiluzzo on the other. They are illustrated by the follow- 

 ing analyses made by Abich : — 



Lava of Lisca nera, etc. Lava of Basiluzzo, etc. 

 Specific gravity 27752 2-4008 



Silica 



.. 57-67 



67-09 



Alumina 



11-94 



17-36 



Oxides of Iron and Manganese 



6-71 



0-81 



Lime 



7-72 



1-23 



Magnesia 



7-02 



1-20 



Soda 



\ not 



4-10 



Potash 



J determined. 



8-27 



Two specimens of trachyte taken from Panaria were determined 

 by the same geologist to have specific gravities of 2 - 6754 and 2-7225, 

 and per-centages of silica of 64-37 and 61-39 respectively. A third 

 variety from the same island was found to closely approximate to 

 the rock of Basiluzzo in composition. 



