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 or travertin in regular 
strata, descends on each side of the hill ; one of the 
strata being compact, and about l5 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- 
millary 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 
