222 Dr Daubeny on the Spring at Torre del Annunziata. 



pump-room or bath-house, is 68 feet ; of which from five to ten 

 feet on the top consist of vegetable mould intermixed with de- 

 composing lava. 



Immediately underneath this is a bed of lava, averaging five 

 feet in thickness, hard and compact internally, but scoriform at 

 bottom. It is throughout more or less cellular, and contains 

 here and there some considerable cavities. 



I am indebted to Colonel Robinson, an English officer of 

 great enterprise and intelligence, at present in the Neapolitan 

 service, for the knowledge of a fact relating to this lava, which 

 seems of some theoretical importance. He assures me, that, in 

 one of the cavities alluded to, he discovered a large quantity of 

 a white pulverulent earthy matter, which, upon examination, 

 turned out to be carbonate of magnesia. I regret that, as Co- 

 lonel Robinson had parted with the whole quantity which he 

 collected, it is out of my power to give to his statement all the 

 authenticity which the exhibition of the substance itself would 

 afford ; but I may offer this corroboration of it, namely, my 

 having myself found a white superficial coating on one part of 

 this same lava, which certainly did appear to consist of, or at 

 least to contain, a very large proportion of the earth in question 

 combined with carbonic acid. Colonel Robinson also wrote me 

 word, in a letter I received from him just before I quitted 

 Naples, " that the large quantity of magnesia ascertained by 

 the analysis, induced him to endeavour to find out its origin, 

 and, for that purpose, he excavated to the depth of forty feet two 

 miles up the mountain, in a direction with the spring, and found 

 large pieces of pumice, of a brownish-red colour, very much re- 

 sembling an honeycomb, the cavities of which were completely 

 filled up with a perfectly white powder, which proved on exa- 

 mination to be carbonate of magnesia." 



Now the occurrence of magnesia in such situations would 

 seem to imply the fact of its sublimation, and to concede the 

 possibility of this being one step at least towards the admission of 

 Von Buch's much contested theory respecting the origin of do- 

 lomite. 



Underneath the lava bed just mentioned, the cliff is princi- 

 pally made up of a succession of beds of rapilli and scoriae, 

 sometimes agglutinated through the medium of volcanic sand, 



