OLDER PLIOCENE PERIOD. 
533 
and Davallia Canariensis^ ferns now abundant in Madeira. 
Thirdly—The great age of this leaf-bed of San Jorge, which 
was perhaps originally formed in the crater of some ancient 
volcanic cone afterwards buried under lava, is proved by its 
belonging to a part of the eastern extremity of Madeira, 
which, after the close of the igneous eruptions, became cov¬ 
ered in the adjoining district of Cani 9 al with blown sand in 
which a vast number of land-shells were buried. These fos¬ 
sil shells belonged to no less than 36 species, among which 
are many now extremely rare in the island, and others, about 
five per cent., extinct or unknown in any part of the world. 
Several of these of the genus Helix are conspicuous from the 
peculiarity of their forms, others from their large dimensions. 
The geographical configuration of the country shows that 
this shell-bed is considerably more modern than the leaf-bed; 
it must therefore be referred to the Newer Pliocene, accord¬ 
ing to the definition of this period given in a former chap¬ 
ter (p. 143). 
Older Pliocene Period. — Italy, —In Tuscany, as at Radico- 
fani, Viterbo, and Aquapendente, and in the Campagna di 
Roma, submarine volcanic tuffs are interstratified with the 
Older Pliocene strata of the Sub-apennine hills in such a man¬ 
ner as to leave no doubt that they were the products of erup¬ 
tions which occurred when the shelly marls and sands of the 
Sub-apennine hills were in the course of deposition. This opin¬ 
ion I expressed* after my visit to Italy in 1828, and it has 
recently (1850) been confirmed by the argument adduced 
by Sir R. Murchison in favor of the submarine origin of the 
Tertiary volcanic rocks of Italy.f These rocks are well 
known to rest conformably on the Sub-apennine marls, even 
as far south as Monte Mario, in the suburbs of Rome. On 
the exact age of the deposits of Monte Mario new light has 
recently been thrown by a careful study of their marine fos¬ 
sil shells, undertaken by MM. Rayneval, Vanden Hecke, and 
Ponzi. They have compared no less than 160 species with 
the shells of the Coralline Crag of Suffolk, so well described 
by Mr. Searles Wood; and the specific agreement between 
the British and Italian fossils is so great, if we make due al¬ 
lowance for geographical distance and the difference of lati¬ 
tude, that we can have little hesitation in referring both to 
the same period, or to the Older Pliocene of this work. It is 
highly probable that, between the oldest trachytes of Tus¬ 
cany and the newest rocks in the neighborhood of Naples, a 
* See 1st edit, of Principles of Geology, vol. iii., chaps, xiii. and xiv., 1833; 
and former editions of this work, chap. xxxi. 
t Quart. Geol. Jour., vol. vi., p. 281. 
