96 M. Brongniart on the Fresh-water Formations of Italy* 
the formation of fresli-water deposits, and perhaps even of many 
others, and to induce us to presume that many calcareous for- 
mations have been produced, like the travertines of the plain 
of Tivoli, by numerous springs issuing from the ground im- 
pregnated with a solution of carbonate of lime, which they have 
deposited with greater or less' rapidity at the surface of the 
soil. 
These considerations seemed to me of sufficient importance to 
engage our attention for a few moments ; for they contribute 
essentially to complete the history of a formation which was 
first observed in the neighbourhood of Paris. 
But the travertine or fresh-water formation of the plain of 
Tivoli is not utterly destitute of the remains of mollusca. I 
have seen them near the Villa Adriana, even at the foot of the 
hill, in a limestone cliff, which , besides discloses in a very evi- 
dent manner its position with regard to the other formations. 
The fundamental rock of the Tivoli mountains is a fine 
compact limestone, including uninterrupted beds or nodules of 
hornstone, and which appeared to me to have a very perfect re- 
semblance to that of Jura. Sometimes the fresh-water limestone 
rests immediately upon this ancient rock, sometimes it is placed 
upon a volcanic breccia* which itself reposes upon this compact 
limestone. The place which I have mentioned shows this inter- 
position in the most evident manner. We find, on proceeding 
from the surface of the soil downwards, 1st, A compact traver- 
tine with sinuous tabular perforations and some shells ; 2d, A 
mixture of friable travertine and debris or volcanic breccia ; 3d, 
A pretty large bed of this breccia. 
Thus, all the facts observed by M. Brocchi, and which I have 
hail an opportunity of re-examining along with him, together 
with those which I have observed myself, establish for the posi- 
tion of formations analogous to those of the neighbourhood of 
Paris, as well at Home as in its neighbourhood, the following is 
the order of succession, proceeding from below upwards. 
Istf, A compact limestone, analogous either to the Jura lime- 
stone, or perhaps even to chalk. The petrifactions alone, when 
they occur, and they are very seldom seen in it, might resolve 
these doubts. 
