M. Brongniart on the Fresh-water Formations of Italy. 99 
tions 5 and that it issues from a limestone which may be referred 
to the transition formation. 
In the places which I have here described for illustration, the 
origin of the fresh-water limestone is not at all doubtful, and 
when one sees it forming under his eyes, if we may so speak, at 
Tivoli, at Terni, and Especially at Saint Philippo, and at the lake 
of Solfatara, he cannot hesitate to attribute the same origin to 
that which is on th0 route of Civita Vecchia, the source of which 
is no longer visible. Now, as the fresh-water limestone, which 
we find in Tuscany, in the neighbourhood of Colle and Vol- 
terra, is absolutely similar, in many of its parts, to that of die 
plain of Tivoli, there is no need of seeing its source, in order to 
assign to it the same origin. 
This formation is here very remarkable for its extent, for its 
evident position, and for the great quantity of fresh-water and 
land shells, which it contains in a state little altered from the 
original. 
It is at the bridge of the Podernia, which is some miles to the 
north-west of St Philippo, on the route of Sienna, that the hard 
and compact travertine begins to shew itself anew ; but it is at 
Sienna itself that the fresh-water limestone re-appears in consi- 
derable quantity. It continues thus covering the summits of 
the hills, sometimes descending along their declivities, but 
ceasing in the valleys, to the passage of the Staggia, on the 
route of Poggibonzi. It often presents a soft texture, and en- 
velopes stems of aquatic plants and fresh-water shells. These 
places are now much elevated about the water, as it exists at 
present, and, from their form and position, cannot receive any 
stream of water, or retain it in any considerable quantity. 
On descending in the valley of Elza toward -Colle, we find 
the same arrangement, that is to say, at first, and on the heights, 
a very extended, very thick and compact fresh-water limestone, 
then a fine sand of the same formation, for it incloses a multi- 
tude of fresh-water shells, in excellent preservation, some of 
them retaining even their original colours, and these are Nerites. 
The other shells are Physae, Limneag, Planorbes, some Helices 
and Paludinae. We find next, that is to say, on descending to- 
wards the valley, not precisely beneath the preceding beds, but 
always at a level much higher than that of the highest waters, a 
g 2 
