138 NEWER PLIOCENE PERIOD. [Ch. XI. 



of recent species. The fresh-water shells which I collected 

 near Colle are in a very perfect state, and the colours of the 

 Neritinae are peculiarly brilliant. The following six species, 

 all of which now inhabit Italy, were identified by M. Deshayes: 

 Paludina impura, Neritina fluviatilis, Succinea amphibia, Lim- 

 neus auricularis, L. pereger. and Planorbis carinatus. 



Travertins of Rome. Many of the travertins and calca- 

 reous tufas which cap the hills of Rome may also belong to 

 the same period. The terrestrial shells inclosed in these masses 

 are of the same species as those now abounding in the gardens 

 of Rome, and the accompanying aquatic shells are such as are 

 found in the streams and lakes of the Campagna. On Mount 

 Aventine, the Vatican, and the Capitol, we find abundance of 

 vegetable matter, principally reeds encrusted with calcareous 

 tufa, and intermixed with volcanic sand and pumice. The 

 tusk of a mammoth has been procured from this formation, 

 filled in the interior with solid travertin, wherein sparkling 

 crystals of augite are interspersed, so that the bone has all the 

 appearance of having been extracted from a hard crystalline 

 rock *. 



These Roman tufas and travertins repose partly on marine 

 tertiary strata, belonging, perhaps, to the older Pliocene era, 

 and partly on volcanic tuff of a still later date. They must 

 have been formed in small lakes and marshes, which existed 

 before the excavation of the valleys which divide the seven hills 

 of Rome, and they must originally have occupied the lowest 

 hollows of the country, whereas now we find them placed upon 

 the summit of hills about 200 feet above the alluvial plain of 

 the Tiber. We know that this river has flowed nearly in its 

 present channel ever since the building of Rome, and scarcely 

 any changes in the geographical features of the country have 

 taken place since that era. 



When the marine tertiary strata of this district were formed, 

 those of Monte Mario for example, the Mediterranean was 

 already inhabited by a large proportion of the existing species 

 * This fossil wag shown me by Signer Riccioli at Rome. 



