100 M. Brongniart on the Fresh-water Formations of Italy. 
formation sometimes compact with siliceous parts, which adhere 
in its mass, presenting the sinuous cavities and shells which 
characterise the lacustrine formation, sometimes porous, even 
tufaceous, and seemingly composed of stems of chara and my- 
riophyllum, petrified into limestone. This formation re-appears 
on the opposite hill, that is to say, on the left bank of the Elza. 
It cannot be considered as of modern formation, for it occurs 
at an elevation, to which the Elza, in its highest risings, never 
attains, and it rests upon the upper marine formation, which 
has itself a very great thickness. 
The waters which have deposited this formation no longer 
rise to its level ; but we see issuing from the foot of the emi- 
nences on the left bank of the Elza, streamlets which turn a 
mill, and which have the property of depositing a great quantity 
of calcareous matter. It may be presumed, that before the 
opening of the valley or watel’-course of the Elza, these 
springs issued at the level of the summits of the hills which 
form its sides. This change has taken place at a period pre- 
ceding the commencement of historical records, and has hap- 
pened, without doubt, at the same epoch when the valleys were 
scooped out, and when the continents, partly laid bare, have as- 
sumed, in their lower parts, the forms which they at present 
possess. 
A fresh-water limestone, similar to the more compact parts of 
the preceding, appears even towards the summit of the hills, 
above the marine formation, almost every where in the neigh- 
bourhood of Volterra. Also at Gastello di Santo Juliano , we 
see very high cliffs, which are composed, at their base, of 
bluish argillaceous marl, with marine shells ; towards their mid- 
dle, of reddish sand with pebbles, sometimes containing oysters, 
pectens, and some other marine shells ; and, at their summit, 
a Very thick deposit of fresh- water limestone, with its tubular 
perforations, shells, &c. 
At Pomarance, to the south-east of Volterra, and, conse- 
quently, at a considerable distance from the two places where 
I have mentioned the fresh-water limestone as occurring, the 
same formation still presents itself, with the same characters, 
but not altogether in the same position ; for here it seems to 
have flowed upon the southern declivity of the hill, at the 
