Professor Silliman's Geological Notes. 143 



found it to contain one volume of carbonic acid, and less than 

 a half volume of sulphuretted hydrogen. Tracing the canal 

 from where it escapes, the "Via Appia Nova," up to the 

 lake, a mile and a quarter, the observer is much struck with 

 the copious escape of large volumes of gas from the whole 

 surface of the lake, which is such as to convey the idea of 

 ebullition. A stone cast into the water is soon followed by 

 a very violent escape of gas, causing the whole adjoining 

 area to boil with great energy, and a loud sound of efferves- 

 cence. The water is very clear, a slight milky, or opaline 

 appearance it has, being due, as we thought, to the reflection 

 of the white sides and bottom of the lake. Its temperature 

 is 80° F., and it has, when drunk from a glass, an agreeable 

 pungent taste of free carbonic acid, with no more of sulphu- 

 retted hydrogen than is pleasant. It is perfectly clear in the 

 glass, and effervesces briskly. The escape of the gas does 

 not appear to be confined to any particular part of the bed of 

 the lake, but takes place equally over all its surface. The 

 grass and reeds which grow luxuriantly at its margin, are 

 constantly encrusted with carbonate of lime from the water, 

 and it is from this cause that the area of the lake appears to 

 have been contracted, and not from any partial drainage, 

 which is evidently inexplicable, since the level of the lake is 

 now only a few inches below that of the adjoining and level 

 Campagna. Breaking off a mass and clods from the shore, it 

 was found that the roots of the plants were also encased ; 

 while the stone walls of the fields adjoining were wholly 

 composed of loose blocks of travertine, made up entirely of 

 casts of the stems of growing grapes and aquatic plants. 

 It required but casual observation of the travertine in other 

 and distant parts of the Campagna, to detect the same species 

 of plants imbedded in every part of its mass. Blocks of 

 travertine in the Colosseum at Rome and other ancient build- 

 ings, also shewed us the same evidence. A large space on 

 the Campagna, near the sulphur lakes, lately fell in, owing 

 to the cavernous and unsupported nature of the rock ; and it 

 was easy to see, in the freshly fractured portions of the rock, 

 the same stems of aquatic plants, such as now grow on the 

 margin of the water. 



