HAMILTON ON THE GEOLOGY OF TUSCANY. 283 



that he is near a block of alabaster, and by following the direction of 

 the crust, he removes the gypsum all round until he has nearly de- 

 tached the whole nodule, which is at last carefully separated from 

 the parent rock. Gunpowder is occasionally used to blast the 

 rock when no black crust indicates the existence of the alabaster. 

 This crust in connection with the pure alabaster is perhaps one of 

 the most curious features of the mine. On close examination it 

 appears to be laminar and concentric, and to consist of layers of 

 blue clay and gypsum. Now the whole formation of gypsum 

 contains a small portion of clay which gives it the greyish colour, 

 and it is probable that, when that peculiar principle, whether crys- 

 tallisation, attraction or electricity, which caused the aggregation 

 of the particles of gypsum in greater purity and in a more crystal- 

 line state was in operation, one of its chief effects was to expel 

 to the circumference all the particles of argillaceous matter pre- 

 viously mixed up with the gypsum ; a process which would continue 

 until either the crust itself opposed a resistance to the further 

 action of this principle, or until two opposing spheres nearly came 

 in contact with each other. Very fine crystals of selenite, and some- 

 times of a large size, are not unfrequently found in the fissures of 

 the gypsum. They are used for the purpose of making the fine 

 Scagliola cement, and are consequently sold at a much higher 

 price than the more ordinary gypsum. The price of the fine ala- 

 baster is 5 Tuscan lire the 100 lb. Tuscan at the quarry, or 8 if 

 delivered in Leghorn. 



b. Leghorn. — The plain immediately to the S. E. of Leghorn 

 consists of a hard calcareous rock in horizontal beds extending from 

 the mountains to the sea-shore and even into the sea, forming some- 

 times the low flat beach, and at others the broken cliff above. It is 

 generally very compact and hard, and not unfrequently contains 

 pebbles of alberese and other secondary rocks, and also a few 

 marine shells. It appears to belong to the formation of areno- 

 calcareous rocks, which caps the summit of the Volterra and other 

 hills, and has received from some of the Italian geologists the 

 name of Panchina. 



c. Poggibonzi. — The third district in which marine tertiary de- 

 posits are found is that of Poggibonzi, including the country from 

 Colle to San Casciano, with a portion of the valley of the Elsa and of 

 the Pesa. I have made this a distinct district from that of Volterra ; 

 because it appears to be entirely cut off from it by a chain of hills of 

 secondary formation : geologically speaking, it is probably a por- 

 tion of the same district. It consists, in its western portion, of 

 thick beds of yellow sand ; while towards the N. E., and particu- 

 larly towards the north, and near San Casciano, it becomes gradually 

 more gravelly, and, at length, consists almost entirely of thick 

 banks of pebbles, increasing in size towards the north as they 

 approach the rocks of the secondary age against which they rest, 

 and from the breaking up or wearing away of which they were, in 



