160 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 Rome, 

 which are probably referrible to the same formation, arc 

 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 



