180 M.BRONCtiiART on the position 



of Genoa ; at Castellamonte and Baldissero, near Turin ; and 

 notwithstanding the difficulty of recognising, or even the 

 impossibility of seeing the rock beneath tliese serpentines ; 

 notwithstanding the differences they present, I consider 

 that their position may be presumed the same as that of 

 the serpentines of Rochetta, Pietramaia, and Monteferrato. 



To the south of Volterra, and beyond Pomerance in Tus- 

 <any, on the way to the lagonis of Monte Cerboli, a high 

 Jiill is traversed, of some extent, bearing the name of Poggio 

 del Gabbro,* (fig. 1.), and which is entirely composed of 

 diallage serpentine. On the ascent we find a blackish com- 

 pact limestone at the foot of the hill, afterwards rolled 

 pebbles of serpentine and jasper, then serpentine in a thick 

 mass. Descending from the col of Monte-Cerboli, towards 

 the S.S.E. a bed of gypsum is first met with, which appears 

 to rest against the serpentine, and covered in one spot by a 

 conglomerate in thick and nearly horizontal beds, composed 

 of all kinds of rocks, and especially fragments of serpen- 

 tine ; continuing (o descend in order to reach the valley of 

 the lagonis we quit, after passing Monte Cerboli (fig. 2), 

 the serpentine, and meet with diallage rock, and afterwards 

 limestone, the numerous and regular beds of which, sepa- 

 rated by beds of marly limestone, dip under the mountain, 

 and consequently under the diallage rock. 



In this limestone are situated what are called the Lagonis 

 «f Tuscany (fig. 3). Their description and the account of 

 their position, are foreign to my subject j I nevertheless 

 cannot avoid remarking, that the vapours of boiling water, 

 which rise with great violence from the clefts of this lime- 

 stone, and which contain boracic acid among the substances 

 that they carry up with them, take their rise beneath this 

 rock, or at least in it. As not any mineral is known, in the 

 mass of this limestone, either here nor elsewhere, which 

 could give rise to phenomena that are so powerful, so ex- 

 tensive, and so general throughout all this country, nor to 



* A new proof that Gabbro is tlie Italian name for serpentine, and not 

 diallage rock, which is generally called granitone. 



