Ch.XIY.] MIOCENE FORMATIONS. 175 



and Sicily wliere the date of its origin is very distinct, may be explained 

 if we consider that it may represent the deltas of rivers and torrents, which 

 gained upon the bed of the sea where blue marl had previously been de- 

 posited. The latter, being composed of the finer and more transportable 

 mud, would be conveyed to a distance, and first occupy the bottom, over 

 which sand and pebbles would afterwards be spread, in proportion as 

 rivers pushed their deltas farther outwards. In some large tracts of yel- 

 low sand it is impossible to detect a single fossil, Avhile in other places 

 they occur in profusion. Occasionally the shells are silicified, as at San 

 Vitale, near Parma, from whence I saw two individuals of recent species, 

 one freshwater and the other marine [Lymnea palustris, and Cytherea 

 concentrica, Lam.), both perfectly converted into flint. 



Home. — The seven hills of Kome are composed j artly of marine ter- 

 tiary strata, those of Monte Mario, for example, of the Older Pliocene 

 period, and partly of superimposed volcanic tufi", on the top of which are 

 usually cappings of a fluviatile and lacustrine deposit. Thus, on Mount 

 Aventine, the Vatican, and the Capitol, we find beds of calcareous tufa 

 with incrusted reeds, and recent terrestrial shells, at the height of about 

 200 feet above the alluvial plain of the Tiber. The tusk of the mammoth 

 has been procured from this formation, but the shells appear to be all of 

 living species, and must have been imbedded when the summit of the 

 Capitol was a marsh, and constituted one of the lowest hollows of the 

 country as it then existed. It is not without interest that we thus dis- 

 cover the extremely recent date of a geological event which preceded an 

 historical era so remote as the building of Rome. 



Aralo-Caspian formations. — This name has been given by Sir R. Mur- 

 chison and M. de Verneuil to the limestone and associated sandy beds, of 

 brackish-water origin, which have been traced over a very extensive area 

 surrounding the Caspian, Azoff, and Aral Seas, and parts of the northern 

 and western coasts of the Black Sea. The fossil shells are partly fresh- 

 water, as Paludina, JVeritina, &c., and partly marine, of the family Car- 

 diicice and Mytili. The species are identical, in great part, with those 

 now inhabiting the Caspian ; and when not living, they are analogous to 

 forms now found in the inland seas of Asia, rather than to oceanic types. 

 The limestone rises occasionally to the height of several hundred feet above 

 the sea, and is supposed to indicate the former existence of a vast inland 

 sheet of brackish water as large as the Mediterranean, or larger. 



The proportion of recent species agreeing with the fauna of the Caspian 

 is so considerable as to leave no doubt in the minds of the geologists above 

 cited, that this rock, also called by them the " Steppe Limestone," belongs 

 to the Phocene period.* 



MIOCENE FORMATIONS. 



Faluns of Touraine. — The strata which we meet with next in the de- 

 scending order are those called by many geologists " Middle Tertiary," 



* Geol. of Russia, p. 279, &c. 



