PETRIFYING SPRINGS. 195 



edges of this receptacle, the water spreads over the 

 ground, forming numerous ponds and plashes, and in 

 these it becomes hard, and produces a beautiful trans- 

 parent stone, commonly called Tabreez marble. The 

 petrifying process may here be traced from its begin- 

 ning to its close. In one part the water is clear ; in 

 a second it appears thicker and stagnant ; in a third 

 quite black ; and at last it is white, like hoar-frost. 



The petrified ponds look like frozen water ; a stone 

 slightly thrown upon them breaks the crust, and the 

 black water issues forth ; but, when the operation is 

 complete, a man may walk upon the surface without 

 wetting his shoes. A section of the mass appears 

 like sheets of rough paper in accumulated layers. Such 

 is the constant tendency of this water to form stone, 

 that the bubbles become hard, as if they had been 

 arrested in their course and changed into marble. 



A considerable number of buildings of ancient and 

 modern Rome are built of a substance called traves- 

 tine, obtained from quarries which must have originated 

 from a lake of the same kind. Sir Humphrey Davy 

 says, " The waters of these lakes have their rise at 

 the foot of the Apennines, and hold in solution car- 

 bonic acid, which has dissolved a portion of the calca- 

 reous rocks through which it has passed ; the carbonic 



