296 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Cerboli, and the remarkable phenomena therewith connected ; for 

 I have no doubt but that many of the actual geological features 

 of Tuscany must be referred to agencies and to causes similar to 

 those which are now exhibited in this locality. The works have 

 been already in some degree described by Mr. Babbage in Murray's 

 Handbook of Central Italy, p. 178., and by Dr. Bowring in his 

 Report on the Commercial Relations of Tuscany, laid before Par- 

 liament in 1837. I shall therefore confine my observations to a 

 few of the principal phenomena. 



The numerous and violent jets of vapour from which the 

 boracic acid is extracted, rise, with considerable noise and in large 

 volumes, from a narrow rocky valley in the secondary cretaceous 

 limestone, about 15 miles S.W. of Volterra. Huge blocks of this 

 rock and its associated indurated marls cover the surrounding hills, 

 and add to the desolation of the scene. The vapour naturally 

 leaves a considerable deposit ; but this is much increased in con- 

 sequence of its being compelled by artificial means to pass through 

 water collected into numerous reservoirs. By this process, the 

 water is impregnated with the boracic acid previously held in 

 solution in the vapour ; while the greater part of the sulphur, 

 lime, and carbonic acid gas, which it also contains, is deposited in 

 the muddy bottoms of the pools, and assumes, when dry, a crys- 

 talline form, being, from time to time, thrown out in the course of 

 the operations : sulphate and carbonate of lime are also deposited in 

 the cauldrons and cooling pans where the boracic acid is obtained 

 by evaporation from the saturated water. Amongst the neigh- 

 bouring rocks, I saw a remarkable instance, where a large fissure 

 or crack, with several smaller ramifications, had been completely 

 filled up by the matter deposited by the vapour which must once 

 have escaped through it. The sides were coated with a hard 

 compact calc-sinter, while the central portions were filled with a 

 more porous substance, so that the passage of the vapour had been 

 obstructed before the central parts had become so densely consoli- 

 dated as the sides, thereby explaining at least one of the causes 

 by which these vents are constantly changing their positions, and 

 how the jets of vapour escape sometimes in one place and some- 

 times in another. 



The simple mode by which the boracic acid is obtained is as 

 follows. Small reservoirs, from 15 to 30 feet in diameter, are dug 

 round the most convenient and powerful of the many steam vents ; 

 and into these reservoirs a small- stream of water is conducted 

 from the mountain side. After being for some time exposed to 

 the action of the rising vapour the water is let off from one 

 reservoir into another, until it has passed through five or six, in 

 each of which it remains about 24 hours, the vapour boiling and 

 bubbling up through it the whole time with much noise and vio- 

 lence. By this time the water is sufficiently impregnated with 

 the boracic acid; and after being allowed to settle in another 

 reservoir to deposit the mud, it is led off into the evaporating 

 houses, where, after undergoing a slow and gradual process of 



