AN OLD SEA-FLOOR 313 



subsequently piled up to a height of several thousand 

 feet. Lastly succeeded the epoch in which the volcanic 

 platform, no longer increased by fresh eruptions, was 

 carved by subaerial agencies into the topography which 

 it presents to-day. Each of these three phases has 

 had its history legibly graven in the rocky framework 

 of the Campagna, and some of its memorials may be 

 recognised even within the walls of Rome. 



I. The records of the first period lie beneath the 

 Seven Hills on the left bank of the Tiber, but rise 

 high above the plain on the right bank, where they 

 form the chain of heights that culminates in Monte 

 Mario, 455 feet above the level of the Mediterranean. 

 These records, forming the series known to geologists 

 by the name of Pliocene, consist of a lower bluish- 

 grey clay and an upper group of yellow sands and 

 gravels, the whole being probably a good deal more 

 than 450 feet thick. The clay (Plaisancian) has been 

 found to extend, with a remarkable persistence of 

 aspect and contents, from the north to the south of 

 Italy. It has yielded several hundred species of mollusks 

 and other organisms, which show it to be a thoroughly 

 marine silt, deposited on the bottom of a sea of some 

 little depth. 



At the time of the deposition of this clay the 

 mountainous backbone of the country had already 

 undergone the greater part of that prolonged series 

 of terrestrial disturbances whereby solid sheets of lime- 

 stone were folded, crushed, ruptured and driven to- 

 gether into a series of parallel ridges, having a general 

 trend from northwest to southeast, and forming the 

 nucleus of what is now the chain of the Apennines. 



