§ 35. LAKE OF THE SOLFATARA. 77 



35. Lake of the Solfatara. — One of the most cele- 

 brated travertine springs is the lake of the Solfatara, which 

 lies in the Campagna between Rome and Tivoli, and is fed 

 by a stream of thermal water that flows into it from a 

 neighbouring pool. The water is of a high temperature, and 

 saturated with carbonic acid gas, which, as the water cools, 

 is constantly escaping, and keeping up an ebullition on the 

 surface. The stream that flows out of the lake fills a canal, 

 which is conspicuous at a distance, from the line of vapour 

 that emanates from the water. The formation of travertine 

 is so rapid, that not only the vegetables and shell-fish are 

 surrounded and destroyed by the calcareous deposit, but 

 insects also are frequently incrusted. In the beautiful 

 specimens of travertine before us from the Solfatara, vege- 

 table impressions are distinctly seen ; the cavities in these 

 masses have been produced by the decomposition of the 

 vegetable matter.* 



A considerable number of the edifices of both ancient 

 and modern Rome, are constructed of travertine obtained 

 from the quarries of Ponte Luccano, which has clearly 

 originated from a lake of this nature. Psestuni is also 

 built of calcareous tufa, derived from similar deposits. 

 " The waters of these lakes," says Sir Humphry Davy,f 

 " have their rise at the foot of the Apennines, and hold in 

 solution carbonic acid, which has dissolved a portion of 

 the calcareous rocks through which it has passed ; the car- 

 bonic acid is dissipated by the atmosphere, and the marble, 

 slowly precipitated, assumes a crystalline form, and pro- 

 duces coherent stone. The acid originates in the action of 

 volcanic fires on the calcareous rocks of which the Apen- 

 nines are composed, and carbonic acid being thus evolved, 

 rises to the source of the springs, and gives them their 



* See Appendix B. 



f Consolations in Travel ; or, the Last Days of a Philosoph3i\ 



