No. II. — Ilerculaneum, Pompeii^ aiid Stabice. 125 



to add in the way oi facts to what has already been published 

 on the subject. 



During the last century it was usual to call the stony mat- 

 ter which envelopes this unfortunate city a lava. The word 

 is still occasionally employed, and, in the present want of defi- 

 nitive terms in the science, perhaps it would be difficult to ob- 

 ject to any particular designation which implies a volcanic pro- 

 duction. Yet certainly, according to the authorized use of 

 the term, it cannot be called a lava. In composition, it may 

 rather be supposed to fall under the head of tufa, volcanic 

 dust, or decomposed trachyte. As far as we are acquainted 

 with the constitution of the substance, it would seem improper 

 to place it along with the last rock, and I have always preferred 

 considering it as a tufa ; though, whether it flowed originally 

 liquid from the mountain, or in the state of ashes afterwards 

 consolidated by moisture, there has been much dispute. Delia 

 Torre, in his work on Vesuvius, seems not very decided on 

 this point, as the short casual notices of the subject which oc- 

 cur throughout his book are somewhat contradictory; but 

 from one very explicit statement,* I think he rather leant to 

 the idea, that a shower of volcanic matter fell, which was af- 

 terwards brought to a consistence by atmospheric moisture. 

 Sir William Hamilton gives a masterly sketch of his theory of 

 the origin of the Herculaneum tufa ; and, I am happy to say, 

 that it agrees perfectly with the results I myself deduced 

 from the examination of the spot, before I was biassed by 

 any theory on the subject, or had become acquainted with his 

 observations. The resemblance it bears to the tufas of Pausi- 

 lippo, and the vicinity of Naples, is too striking not to demand 

 an attentive consideration ; and I was speedily impressed with 

 the conviction, that their origin must have been extremely si- 

 milar. Every fact of appearance and structure corroborates 

 the idea, with this only exception, (as I humbly conceive,) 

 that the Kerculaneum tufa is more uniform in its structure, 

 and less broken into layers, so as to give us the idea of one 



* " Si vede sopra le case di questa antica citta (Ercolano) un masso di 

 materia che non e se non che I'unione d'arena, cenere, lapilli e piettruzze 

 insieme uniti coll'acqua e divenute consistenti per Tumido continuo delle 

 acque piovane."^ — Storia del Fesuvio, 4to. 



