62 J. W. Judd — On Volcanos. 



(Sciaras) which surround the island. Sometimes this lava assumes 

 the finely columnar structure so common in rocks of this class. 

 Thus, a very fine series of columns is exhibited at Punta Labronzo, 

 the northern point of Stromboli, and ruder ones at Punta del Uomo, 

 on the south-east of the island. On the extremest verge of this 

 latter lava-stream is situated one of those little shrines, which, in 

 spite of the apparent inaccessibility of its position, has its burn- 

 ing lamp constantly replenished. The voyager in these seas is 

 startled when, on reaching these spots, the wild cries and strange 

 songs of the boatmen are suddenly hushed, all engaging for a few 

 moments in silent devotion to the saint who is supposed to warn, by 

 means of this primitive and not very efficient lighthouse, the mariner 

 who approaches these inhospitable shores. 



The products of the modern eruptions along the southern line of 

 fissure — that, namely, which extends beneath the islands of Lipari 

 and Vulcano — offer, as we have already remarked, the most striking 

 contrast to those of Stromboli. These lavas belong to that highly 

 silicated class so well illustrated in the Ponza Islands, the Euganean 

 Hills, and Hungary. The highly acid lavas, to which the name 

 of quartz-trachyte is usually applied, but which by Roth were called 

 " Liparite," and by Eichtofen " Ehyolite," are in their ultimate 

 composition almost identical with the granites ; and when highly 

 crystalline, are seen to be composed of precisely the same constituent 

 minerals — namely, several species of felspar, orthoclase being always 

 predominant, free quartz, and variable quantities of hornblende or 

 mica. By the peculiar arrangement of their materials, however, the 

 highly silicated lavas are well characterized ; and in their internal 

 structure they present features which almost always serve to dis- 

 tinguish them from the granites, with which they were by early 

 geologists so frequently confounded. 



In illustration of the ultimate composition of these highly acid 

 lavas of Lipari, we give the following analyses of Abich, with which 

 others by Berthier and Klaproth closely agree : 



Obsidian of Lipari. Pumice of Lipari. 



Silica ... 74-05 7370 



Alumina 12-97 12-27 



Oxide of Iron 273 2-31 



Lime 0-12 0-65 



Magnesia 0-28 0-29 



Soda 4-15 4-52 



Potash 5-11 4-73 



"Water 0-22 1-22 



Chlorine 0-31 0-31 



The specific gravity of the obsidian is 2-3702, and of the pumice 

 2*3771. When in its most completely stony condition, the rock has 

 a specific gravity of 2-53, and consists almost entirely of orthoclase 

 felspar, quartz, and hornblende, in about the following proportions : 



Felspar 77 per cent. 



Quartz 18 ,, 



H ornblende or Mica 5 „ 



In the less compact or stony and more cavernous varieties of 



