CALCAREOUS SPRINGS. 



179 



At the hill of San Vignone, in Tuscany, a ther- 

 mal spring issues from the summit of a rocky hill 

 100 feet high, composed of black slate and serpen- 

 tine. 



Fig. 40. 



B, Serpentine and Slate, a a, Limestone Deposite. 



A large mass of tufa, a a, or travertin in regular 

 strata, descends on each side of the hill ; one of the 

 strata being compact, and about 15 feet thick, serves 

 as an excellent building stone ; while the other, de- 

 scending 250 feet in length, forms a mass about 200 

 feet deep. So rapid is the deposition of lime at 

 these springs, that half a foot of solid limestone is 

 deposited every year in a conduit pipe, inclined at 

 an angle of 30 degrees. Where the water flows 

 more slowly, the deposite is more compact, and 

 rings when struck with a hammer ; but where its 

 current is swifter, it is cellular and porous, like the 

 Paris buhr millstone ; sometimes it assumes a mam- 

 miliary form, and scales off in thin layers, like the 

 coats of an onion. 



At the baths of San Filippo, among the Apen- 

 nines, the water which supplies the baths falls into 

 a pond, where it has been known to deposite a 

 solid mass 30 feet thick in 20 years. A manufac- 

 tory of medallions, in basso-relievo, is carried on 

 at these baths. The water is conducted by canals 



