M. Brongniart on the Fresh-water Formations of ‘Italy. 95 
The lake of Solfatara nearer the foot of the hills is alto- 
gether different. It is a considerable mass of whitish waters, 
from which bubbles of air, and a very distinct smell of sulphu- 
retted hydrogen-gas are continually disengaged. They deposit 
on the vegetables which grow on the banks and in the canal, by 
which they find an exit, a thick layer of white limestone, with 
a compact texture, a true travertine. 
When the bottom of this lake is stirred, a considerable dis- 
engagement of gas is produced. The water acquires along the 
line of passage of the gas, a degree of limpidity which is, with- 
out doubt, owing to the solution of the limestone by the car- 
bonic acid which is disengaged. There is not a single living 
creature in the waters or on the banks of the lake, at least we 
saw none. 
The differences which these two lakes present, seem to be in 
relation with the differences often presented by the lower and 
upper parts of fresh-water formations. That of Solfatara shows 
the phenomenon of the calcareous formation in its period of 
activity. The waters are too much impregnated with gas and 
earthy matter to allow animals to live in them ; and in conse- 
quence these first calcareous deposits should not contain them at 
least in the places near the issuing of the spring ; but in pro- 
portion as the mass of mineral matters diminishes, or, as the ca- 
nals are filled up by the depositions, the waters become less 
impregnated with gas and calcareous matter, the deposition less 
rapid and more crystalline, animals can live, and this deposit, 
containing the remains of these animals, is necessarily superior 
to the former. This is probably the state in which the Lago 
de Tartaro actually exists; and this relative position of the fresh- 
water formation without shells, and of that containing them, is 
precisely what has been observed in all places where these two 
formations are known to exist : thus, in the basin of Paris, the 
siliceous limestone without shells is placed beneath the fresh- 
water shelly limestone ; the millstone without shells is inferior to 
that containing them, &c. 
We find, therefore, here an association of phenomena, and 
circumstances, which permit the employment of direct observa- 
tions equivalent to facts, to lay the foundation of the theory of 
