94 M. Brongniart on the Freshwater Formations of Italy. 
dish and earthy volcanic breccia, and is not covered by any 
rock. 
The plain which extends from Rome to the foot of the moun- 
tains on which Tivoli is situated, is covered in a large portion of 
its extent by a great deposit of travertine ; it commences at Mar- 
tellone, on the road from Rome to Tivoli, and continues almost 
without interruption, to the foot of the Tivoli mountains. This 
plain, in which are situated the quarries of Ponte Lucano, which 
furnish the travertine employed in building, may be considered, 
as M. Omalius d’Halloy says, as the bottom of a great lake, tra- 
versed at present by the Tiverone, bordered by volcanic brec- 
cias, raised by calcareous depositions, and almost dried by this 
raising ; for it is not completely dried, and with M. Omalius 
d’Halloy, we may consider as the remains of this vast mass of 
water, the small lakes of Tartaro , Solfatara , and others, which 
seem still to exist, for the purpose of showing us some of the cir- 
cumstances under which fresh- water formations have been pro- 
duced. 
The geologists who have examined this formation, and par- 
ticularly Messrs de Buch and Omalius d'HalJoy, have observed, 
1st, That the lower and old travertine, whose formation 
does not go on at the present day, is the only one employed for 
building, as possessing sufficient compactness and solidity ; that 
which is daily formed by the waters of the lake of Solfatara 
and of the Teverone, is not dense enough : 2dly, That the 
fresh-water shells are of extremely rare occurrence in it, for not 
only was M. Omalius d’Halloy unable to discover any, but he 
thinks that their absence is owing to the influence of the sul- 
phuretted hydrogen gas existing in these waters in a state of 
solution, and which precludes the possibility of lacustrine mol- 
lusca living in them. 
The great difference of circumstances between the lake of Tar- 
taro and the lake of Solfatara, agrees very well with this theory. 
The waters of the first are clear, its banks are covered with 
calcareous incrustations, having a crystalline structure, but we 
see that they are old, and it does not appear that the present 
waters have the property of depositing them. The bottom of this 
lake is also covered with vegetables, and its waters are peopled 
with all sorts of animals, frogs, insects, &c. 
