26 Account of the 



of conjecture), save those changes which the slow but constant 

 action of existing forces has produced, such, for example, as are 

 seen in the delta formed at the mouth of the Tiber, and the 

 partial depositions of travertino. 



Without carrying our views beyond a comparatively recent 

 geological period, we see three distinct conditions of this region : 

 viz. the period when the surface was formed of the clay, with 

 marine shells ; next, that period when the clay received a cover- 

 ing of volcanic ashes ; and, lastly, the deposition of the beds, 

 whose included fossils show that they were formed under tran- 

 quil lakes of fresh water. There is yet another state of things, of 

 which records are left, when the whole country was acted upon 

 and eroded by running streams and floods, which scattered 

 blocks of stone upon the surface of the fresh- water deposits, in 

 situations where the existing rivers could not have carried them, 

 even if their waters could have transported such masses. 



Marine shells are found imbedded in the tufa or volcanic 

 ashes on the summit of Mons Albanus, an elevation of 3000 

 feet above the level of the present sea. It is demonstrable, there- 

 fore, that a change, to that extent at least, has taken place be- 

 tween the relative level of the sea and land now and formerly *. 

 Volcanic ashes cover the country more or less on the western 

 side of the Apennines, from the Umbro to Calabria. They are 

 invariably deposited in horizontal beds, or nearly so, whether in 

 their loose incoherent state, or agglutinated in the form of the 

 stony tufa. That they were deposited by water, is not only in- 

 dicated by this horizontality, but by their having been carried 

 into the sinuosities of the valleys in the Apennines, as is seen 

 in the valley of the Anio, to within a short distance from 

 Subiaco. It is found in the same manner in the valleys of 

 the Volscian mountains. Another remarkable circumstance is, 

 that between beds of stony lava are frequently found layers of 

 rolled pebbles, not only of pumice-stone and other volcanic sub-' 

 stances, but of Apennine limestone. Elephants' bones have also 

 been met with imbedded in the tufa. 



This vast mass of volcanic matter, must therefore have been 



• More than one argillaceous stratum, containing marine shells, occurs 

 within 800 feet of the summit of Epomeo, in Ischia, a mountain 2605 feet 

 above the sea. — Lyell. 



