302 J. W. Judd—On Volcanos. 



place. As the result of these latter, at least three volcanic cones, 

 composed of similar materials to those of Epomeo and the older por- 

 tion of Vesuvius and Somma, were formed — namely, those of which 

 we see the relics in Yentotiene and San Stefano, in the Botte Rock, 

 and in the Monte della Gruardia of Ponza, respectively. 



The volcanic tuffs (whether of the older or younger volcanic series 

 in the Ponza Islands) have as yet yielded no organic remains ; so 

 that some doubt still remains, both as to the conditions under which 

 they were formed, and their exact geological age. The newer 

 trachytic lavas and stratified tuffs are not improbably of the same 

 age as the rocks of identical composition constituting Epomeo and 

 the nucleus of Vesuvius ; the older series of rocks of highly acid 

 composition belong to some period between the Cretaceous and the 

 Pliocene. 



It is on account of the peculiar and very interesting characters 

 presented by these highly acid or siliceous rocks that the Ponza 

 Islands have attracted so much attention from geologists. The 

 ultimate chemical composition of these rocks is exhibited in 

 the three subjoined analyses, for which we are indebted to Abich. 

 They illustrate three of the most important modifications of character 

 assumed by the rock. 



I. 



Silica 73-46 



Alumina 13-05 



Oxide of Iron 1-49 



Oxide of Manganese ... trace. 



Lime 0-45 



Magnesia 0-39 



Potash 4-39 



Soda 6-28 



Loss 



99-51 



II. 



III. 



74-54 



75-09 



13-57 



13-26 



1-74 



1-10 



0-10 



— 



0-34 



0-18 



0-24 



0-16 



3-68 



8-31 



4-86 



1-67 



0-20 





99-27 



99-77 



2-5293 



.... 2-6115 



Specific Gravity 2-5398 



I. is a porphyritic rock with crystals of mica and glassy felspar 

 from Ponza ; it represents the more granitic forms of the rock. II. 

 is the interesting, curiously laminated, rock of Palmarola, " the 

 banded and ribboned trachyte " of Mr. Scrope ; it contains only 

 traces of mica and hornblende. Abich regards these two rocks as 

 made up of about 50 per cent, of orthoclase, 25 per cent, of free 

 quartz, and 25 per cent, of albite. The small proportion of lime and 

 the large per-centage of soda make it extremely probable that albite 

 is a very important constituent of this rock. III. is a more porous 

 rock from Zannone inclining to the vitreous structure, in which 

 nearly the whole of the felspar appears to be orthoclase, while the 

 free quartz amounts to 28-4 per cent. 



The microscopic study of these rocks of Ponza brings to light 

 many features of the highest interest. A series of specimens may 

 easily be collected, exhibiting every variation from a vitreous rock 

 to one of the most highly crystalline character ; some of the examples 

 of the latter, indeed, approach so closely in character to granite that 



