280 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



a few light-coloured spots, which, by degrees, become more nu- 

 merous, and occur even in the shells themselves. Other specimens, 

 in which decomposition or alteration is more advanced, show larger 

 spots ; until at length they assume an entirely pisolitic character, 

 the grains being perfectly detached, whether they occur in the 

 mass of the rock itself, or in the shells. I believe it to be the blue 

 marl altered by the eruption of the igneous rock. 



But if the blue marl near the centre of the basin is deficient in 

 organic remains, it abounds in numerous productions of mineral- 

 ogical importance, which, both from their own intrinsic value, 

 and from their connection with other phenomena still occurring 

 in the same vicinity, deserve to be particularly noticed. 



About five miles S. S. E. from Volterra, in the deep valley of 

 S. Giovanni, situated in the blue marl, and watered by a small 

 stream, are extensive salt-works, now the property of the Govern- 

 ment, but from which, in former times, the inhabitants of Volterra 

 chiefly derived those riches by which their town flourished and 

 preserved its independence and importance. These salt-works are 

 fed by springs of brine collected from nine different spots where 

 the saturated water is pumped up from deep pits. It has been 

 ascertained by sinking wells and by Artesian borings that this 

 brine is derived from beds of rock salt, which, at different depths 

 below the surface, varying from 50 to 100 feet, are found alterna- 

 ting with the blue marl, in the same manner as the alabaster 

 occurs in other parts of the same formation ; and in the boilers, 

 during the process of evaporation, much sulphate of lime is depo- 

 sited. The average annual production of salt is stated to be from 

 18,000,000 to 19,000,000 Tuscan pounds :— about 140,000 cwt., 

 or 7000 tons per annum. 



The other productions found in the blue marl are gypsum, 

 alabaster, and selenite, — the various forms in which the sulphate 

 of lime so abundant in this district has been deposited. They are 

 particularly met with in the neighbourhood of Volterra, where, 

 however, all the varieties of gypsum do not occur. The pure 

 white quality known as alabaster, and in such demand at Florence, 

 is only found in the neighbourhood of Castellina, about twenty 

 miles W. N. W. from Volterra, at the western extremity of the 

 hills of Monte Catini. It may, however, be observed that the 

 great developement of gypseous matter appears to be confined to a 

 narrow line extending from W. N. W. to E. S. E., in which direc- 

 tion a band of only a few miles in width drawn from Castel- 

 lina to somewhere near Monte Miccioli would comprise all the 

 gypsum and alabaster quarries in the country, as well as the salt- 

 works already alluded to. 



The deposits of Volterra, Picchiaiola, S. Lorenzo, and Castellina 

 may be described as giving the best types of the different forms in 

 which the gypsum of this country occurs. 1. That of Volterra, 

 which is called the variegated alabaster, is most frequently found in 

 detached irregular masses of greater or less size, penetrated by red 



