No. Wl,^^ Islands of Procida and Ischiu. 345 



as We have already remarked, be at least as recent as the ter» 

 tiary series. It is called Creta on the island. 



This clay must be carefully distinguished from the " pietra 

 aluminosa'"* or alum rock. The latter seems at one period to 

 have been very abundant, and actually to have given rise to the 

 first alum-work in Italy, though now so entirely superseded by 

 thatof Tolfa in the Roman states, to the rock of which it bears 

 much resemblance. It was first wrought by oneBartolemeoPer- 

 nix, in 1459, and was continued for a considerable time, and it is 

 not even known why it was given up, but probably from 

 the increasing scarcity of the rock which is now found only in 

 insulated masses on the north side of Monte Epomeo. Breis- 

 lak characterizes the aluminous rock as a lava in a state of de- 

 composition, but I am more disposed to consider it as real alum 

 slate elevated from below ; probably the great abundance and 

 plasticity of the clay is connected with the occurrence of this 

 " pietra aluminosa.'' Remains of the ancient alum-works ex- 

 ist at the spot called the " Piazza della Pera," but there the 

 material seems to be exhausted. 



We must not confound either of these substances with that 

 named " Terra dMschia," which is merely an extremely fine 

 pozzuolana, which is much esteemed as a cement in nice oper- 

 ations, such as ornamental pavements. 



The occurrence of gold in Ischia has been repeatedly assert-, 

 ed, but the report has been apparently derived solely from the 

 authority of Strabo, who seems to have stated it merely from the 

 report that the Erythraeans wrought that precious metal when 

 they were in possession of the island. But though no gold be 

 now found, and although the whole may probably have arisen 

 in a mistake, there is no need for ridiculing the idea, as some 

 have done, as utterly absurd. Gold is not so unknown in vol- 

 canic districts, as Andria asserts,* and is even there found 

 in abundance, in one instance at least on record, -f W^ 

 have likewise had occasion to mention the belief which was 

 long entertained of its occurrence in this very neighbourhood, 

 namely in Vesuvius itself.J With respect to Ischia, however, 



• Acgite Minerali, ii. 67. 



t Breislak, ii. 188. 



+ See this Journal, vol ix. p. 206. and x. 136. 



