No. VII. — Islands of Procida and Isckia. 335 



species of tufa, on the true nature of which geologists are un- 

 decided. Were I to offer an opinion, I should be disposed 

 to coincide with those who consider it a mechanical aggrega- 

 tion of pre-existent materials, such as comminuted pumice, 

 forming of a compound such as may be found in some parts of 

 the Phlegraean fields, though generally more intimate and of 

 a more decided character. It seems admitted, however, that 

 its nature is essentially the same with the conglomerates of 

 Hungary and some of the volcanic rocks of the Rhine, where 

 these formations approach nearest to the trachytic character. 

 But I must repeat, what I have formerly had occasion to re- 

 mark, that the uncertainty of the characters of volcanic rocks 

 is still so great, that a controversy about the name or class of 

 a rock like that of Ischia is little better than a quarrel respect- 

 ing words : the modes of formation and ejection are still ob- 

 scure, and we are wholly in the dark as to the relative ages of 

 most volcanic rocks, of which the most difficult seem to be 

 thos^ connected by infinitely fine shades of difference between 

 Trachyte or Greystone, and the apparently sedimentary depo- 

 sitions of Pozzuolana, Trap, and Tufa. The essential cha- 

 racters of the tufa of Epomeo appear to be a homogeneous 

 texture, rather fine in the grain, a powerful tendency to dis- 

 integration generally taking place in crusts, but the fracture 

 of the massive rock is frequently conchoidal ; alumina is an 

 abundant component part, and the rock is soft and friable, 

 with rather a smooth feel. It has occasionally an apparent 

 stratified structure, and differs conspicuously from the tufas 

 in the Bay of Naples in not generally containing fragments of 

 lava or pumice. This seems to favour the views of Brocchi, 

 Spallanzani, Breislak, and others who consider it to be a de- 

 composed lava or trachyte. In this rock large felspar masses 

 or crystals are sometimes found, and these are occasionally of 

 a rose colour. Hornblende, is, I believe, also a production of 

 the higher parts of the island. It is not unimportant, in judg- 

 ing of the nature of the aggregate rock, (as Dr Daubeny sup- 

 poses it to be) to know that it is used along with lime as an 

 excellent mortar : * this could not be the case were it a decom- 

 posed trachyte, and bespeaks the character of pozzuolana. The 

 colour of this substance, we have already seen, is, at least on 

 * Siano, Notizie deU'Isola d' Ischia, § 48. 



