338 Mr Forbcs's Physical Notices of the Bay of Naples. 



by disintegration, and the action of the waves, forms the mag- 

 netic sand whicli on some parts of the shore abounds, and 

 which Spallanzani noticed to consist of octohedra or their frag- 

 ments. A similar sand we have mentioned as t)ccurring in the 

 Bay of Pozzuoh, at the base of the Monte Olibano, whicli has, 

 beyond doubt, a similar origin, and the lava of which nearlv 

 approximates to those of Ischia. So high does the felspathose 

 character of the Ischian Javas sometimes reach, as to form 

 beds of undoubted trachyte, which occur near Foria in huge 

 masses, extending through the tufa. But the most remarka- 

 ble locality is the Monte Taborre, between Casamicciola and 

 Celso, which is composed of trachyte allied to clinkstone por- 

 phyry. * So much subterranean heat still exists here, that 

 from the fissures of the rock watery vapours at a temperature of 

 4-9° R. = 142° F. still rise, f Of the age of the trachyte we 

 have no historical data for judging, but, geologically consider- 

 ed, it overlies a bed of clay undoubtedly not older than the 

 tertiary series, from the shells which it contains. \ 



Not a few other craters, n^ore or less distinct, appear upon 

 the island, and in detecting them Breislak has shown his usual 

 ingenuity ; but it would be to little purpose to discuss them 

 more minutely : — suffice it to say that we have some examples 

 of cinereous cones resembling the Monte Nuovo. 



Among the next most important volcanic products, though 

 rather simple minerals than rocks, we must reckon the inter- 

 esting substances of obsidian and pumice, which, though so 

 different in appearance and structure, are intimately allied in 

 their origin and composition. Both of these substances we 

 have seen to be rare productions of Vesuvius, nor have we 

 had occasion to mention the former in any part of the Phle- 

 graean fields, except in the crater of Astroni ; § in Ischia it 

 has been overlooked by many, or most authors, yet it occurs 

 in considerable abundance in one part of the island, and its 

 appearance is sufficiently remarkable to attract even the super- 

 ficial observer. Dr Daubeny says, " At Castiglione the 



• Daubeny on FoIcanoSf^. 181. 



f Breislak, Campanie, ii. ^4. 



4: Brocchi Conchiologia Subapennina, Pp. G5 and 354. 



§ See No. iii. of these Notices in vol. x. of this Journal, p. 264. 



