UPLIFT OF LAND 317 



striking contrast which, in the nature of their material, 

 they present to the clay, plainly points to a great 

 alteration of the geography of the coasts at the time 

 when they were deposited. The sea must have become 

 rapidly shallower. Not improbably one of the uplifts 

 now took place, whereby the land has been raised at 

 intervals to its present height. The steepness of the 

 descent of the mountains into the sea might not lead 

 at once to much gain of land along the western coast ; 

 but instead of the grey mud that had previously 

 accumulated in the deeper water, coarser sediment, 

 brought down by numerous torrents from the hills, 

 now spread out over the sea-bottom. Such a 

 transition from the finest silt to gravel and sand could 

 not fail to affect the distribution of the animals liv- 

 ing along the coast-line. Accordingly, on comparing 

 the fossils in the sands with those of the clay, we 

 see that while some of the shells, especially the larger 

 and more massive kinds, continued to flourish in 

 abundance ; others, which found their most congenial 

 haunts in tranquil waters, were driven further out 

 to sea. 



As the Pliocene deposits so well displayed at Rome 

 are known to preserve throughout Italy the same 

 twofold character, with the same types of sediment 

 and of organic remains, the observer who tries to 

 follow their development in the Campagna is soon 

 puzzled by the way in which they there suddenly 

 disappear and allow their place to be taken by later 

 deposits of volcanic origin, which are known by the 

 general designation of Tuff. The most astonishing 

 example of this local peculiarity is to be found at 



