lt>0 OLDER PLIOCENE PERIOD. [Ch. XII. 
of the places where I have seen it, a border formation near the 
junction of the tertiary and secondary rocks. In some cases, 
as near the town of Sienna, we see sand and calcareous gravel 
resting immediately on the Apennine limestone, without the 
intervention of any blue marl. Alternations are there seen of 
beds containing fluviatile shells, with others filled exclusively 
with marine species ; and I observed oysters attached to many 
of the pebbles of limestone. This locality appears to have 
been a point where a river, flowing from the Apennines, en- 
tered the sea in which the tertiary strata were formed. 
Between Florence and Poggibonsi, in Tuscany, there is a 
great range of conglomerate of the Subapennine beds, which is 
seen for eleven miles continuously from Casciano to the south of 
Barberino. The pebbles are chiefly of whitish limestone with 
some sandstone. On receding from the older Apennine rocks, 
the conglomerate passes into yellow sand and sandstone, with 
shells, the whole overlying blue marl. In such cases we may 
suppose the deltas of rivers and torrents to have gained upon the 
bed of a sea where blue marl had previously been deposited. 
The upper arenaceous group above described sometimes 
passes into a calcareous sandstone, as at San Vignone. It 
contains lapidified shells more frequently than the marl, owing 
probably to the more free percolation of mineral waters, which 
often dissolve and carry away the original component elements 
of fossil bodies and substitute others in their place. In some 
cases the shells imbedded in this group are silicified, as at San 
Vitale, near Parma, from whence I saw two species, one fresh- 
water and the other marine (Limnea palustris, and Cytherea con- 
centrica, Lamk.), both recent and perfectly converted into flint. 
On the other hand, the shells of Monte Mario, near Home, 
which are probably referrible to the same formation, are 
changed into calcareous spar, the form being preserved not- 
withstanding the crystallization of the carbonate of lime. 
Mode of formation of the Subapennine beds.- — The tertiary 
strata above described have resulted from the waste of the 
secondary rocks which now form the Apennines, and which 
