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TEAVEETIN OF SAN VIGNONE. 



401 



few hundred yards from the high road between Sienna and 

 Borne. The spring issues from near the summit of a rocky 

 hill, about 100 feet in height. The top of the hill stretches 

 in a gently inclined platform to the foot of Mount Amiata 

 a lofty eminence, which consists in great part of volcanic 

 products. The fundamental rock, from which the srvrino- 



Fiff. 28. 



Baths of San Vignone. 



ma 



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Section of travertin, San Vignone. 



issues, is a black slate, with serpentine (b b, fig. 28), belong- 

 ing to the older Apennine formation. The water is hot, has 

 a strong taste, and, when not in very small quantity, is of a 

 bright green colour. ~ " " " 



So rapid is the deposition near the 

 source, that in the bottom of a conduit-pipe for carrying off 

 the water to the baths, and which is inclined at an angle of 

 30', half a foot of solid travertin is formed every year. A 

 more compact rock is produced where the water flows slowly ; 

 and the precipitation in winter, when there is least evapora- 

 tion, is said to be more solid, but less in quantity by one 

 fourth, than in summer. The rock is generally white ; some 



*V~\ C\ *l/t -4- r*4 *-^ J- -_ J t — . 



^ L& Ui n are compact, and ring to the hammer ; others are 

 cellular, and with such cavities as are seen in the carious 

 part of bone or the siliceous millstone of the Paris basin. A 

 portion of it also below the village of San Vignone consists 

 of incrustations of long vegetable tubes, and may be called 

 tula Sometimes the travertin assumes precisely the botry- 



Auversm 



e, 



of a much 



common to similar 



them, it often 



scales off in thin, slightly undulating layers. 



A large mass of travertin (c, fig. 28) descends the hill from 

 uie point where the spring issues, and reaches to the distance 



VOL. I. 



D D 



