M. Brongniart on the Fresh-water Formations of Italy. 9$ 
as we have characterized them. It presents especially, those 
singular sinuous canals, so constant in the lacustrine limestone 
of all countries, tubular cavities, which have not escaped the ob- 
servation of so acute a naturalist as M. Von Buch ; for this geo- 
logist has described them with perfect precision, before being 
aware of their importance, as furnishing a general character of 
these formations. 
The extent of this fresh-water formation in the south of Italy 4 
together with its importance in a geological point of view, and 
with reference to the arts, induce and authorise me to enter into 
some details upon the subject, in order to determine the circum- 
stances of its formation and its position, relatively to the other 
formations. 
M. Omalius d’Halloy, has discovered these formations at the 
entrance of the Pontine marshes near Cisterne, at the foot of the 
volcanic hills of Velletri, in a low plain. It is a compact and 
solid white limestone, perfprated by a great number of tubular 
cavities, containing limnasse and globular helices : he supposes it 
eovered in many points, like that of Auvergne, by volanic 
breccise. It appears that this limestone occurs farther south 
towards Calabria ; for the temples of Paestum, in the Gulf 
of Salerno, are said to be built of a brecciated stone, which is 
undoubtedly travertine. 
This formation first occurs indistinctly at Monte- Verde, to the 
south of Rome. It is found, again, very well characterised in 
Rome itself, then of considerable extent and thickness at some 
distance from that city, towards the east of the Tivoli side, and 
to the north-west of the Civita Vecchia side. It is by examining 
it at these different points, that I purpose to distinguish the diffe- 
rent circumstances of its geognostical position. 
At Monte- Verde, it shows itself only in a thin bed, which is 
interrupted and even destitute of its essential characters ; it is 
placed upon a siliceous sand, mixed with some augites, which 
covers a perfectly homogeneous earthy volcanic tufa. 
In Rome, M. Brocchi, with whom I have had the valuable 
advantage of visiting these places., pointed out to me the fresh*, 
water limestone at the eastern foot of the Aventine Hill, on the 
banks of the Tiber, in the place called the Cavern of Cacus , 
It is compact, contains some fresh-water shells, lies upon a red* 
