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



from the deep-seated bases of Apennines, which in different 

 parts, as Lom hardy, Tuscany, Calabria, and Sicily, display 

 this rock in remarkable perfection. * But it is undoubtedly a 

 mistake which seems to have sometimes prevailed, that magne- 

 sian minerals are in any way inconsistent with a volcanic origin. 

 Steatite, which is most intimately allied to serpentine, is not un- 

 frequently found in such formations, and olivine, the very vol- 

 canic crysolite itself, is a magnesian mineral. I fear I have 

 extended too far these observations on the Ischian serpentine, 

 but I consider the subject to be one of great interest at the 

 present moment. I will only add, that these masses of noble 

 serpentine, whatever be their origin, have an uncommonly fine 

 mineralogical character : when cut thin (as they always are 

 for ornamental purposes) they are very transparent, and show 

 a beautifully variegated structure, with nearly black clouds on 

 a fine deep grass green ground ; it yields to the nail as usual. 

 The substance we have next to notice, and which is one of 

 the most characteristic productions of Ischia, is the peculiar 

 clay just alluded to. Great beds of this substance exist at the 

 north-eastern part of the island, and it was as much used by the 

 ancients as now for the manufacture of pottery, from whence 

 indeed, according to Pliny, the island took its name.-|- I re- 

 gret that I have no analysis to ofier of this earth. Breislak,J 

 however, tells us that it consists of alumina, silica, a little mag- 

 nesia and less lime. It abounds near Casamicciola, and was 

 dug out by means of subterranean pits, being not merely used 

 on the island, but exported for the purposes of manufacture. 

 Its close analogy to the materials used in constructing the Et- 

 ruscan vases, may be argued from the following analysis of 

 these by Vauquelin : silica 53, alumina 15, lime 8, oxide of 

 iron 24.§ The Ischian bed of clay underlies the trachyte of 

 Monte Taborre, and from the shells which it contains, must, 



• Brongniart has given a very full account of the Serpentines and Dial- 

 lage of Italy, in the Annahs des Mines for 1821. It is translated in Mr 

 De La Beehe's excellent volume of Foreign Geological Memoirs. 



+ Pithecusa, Plin iii. .5. 



X Campanie, ii. 208. 



§ Hausmann on the Etruscan vases, Ed. Phil. Jonm. xiii. 46. Perhaps 

 the iron was derived from a mixtnre of Pozzuolana. 



3 



