Ch. X.] POST- PLIOCENE FORMATIONS. 117 



by having all the imbedded fossil shells identical with species now living, 

 whereas even the Newer Pliocene, or newest of the tertiary deposits 

 above alluded to, contain always some small proportion of shells of ex- 

 tinct species. 



These modern formations, thus defined, comprehend not only those 

 strata which can be shown to have originated since the earth was inhab- 

 ited by man, but also deposits of far greater extent and thickness, in 

 which no signs of man or bis works can be detected. In some of these, 

 of a date long anterior to the times of history and tradition, the bones 

 of extinct quadrupeds have been met with of species which probably 

 never co-existed with the human race, as, for example, the mammoth, 

 mastodon, megatherium, and others, and yet the shells are the same as 

 those now living. 



That portion of the post-pliocene group which belongs to the human 

 epoch, and which is sometimes called Recent, forms a very unimportant 

 feature in the geological structure of the earth's crust. I have shown, 

 however, in "The Principles," where the recent changes of the earth 

 illustrative of geology are described at length, that the deposits accumu- 

 lated at the bottom of lakes and seas within the last 4000 or 5000 years 

 can neither be insignificant in volume or extent. They lie hidden, for 

 the most part, from our sight ; but we have opportunities of examining 

 them at certain points where newly gained land in the deltas of rivers 

 has been cut through during floods, or where coral reefs are growing 

 rapidly, or where the bed of a sea or lake has been heaved up by sub- 

 terranean movements and laid dry. Their age may be recognized either 

 by our finding in them the bones of man in a fossil state, that is to say, 

 imbedded in them by natural causes, or by their containing articles fab- 

 ricated by the hands of man. 



Thus at Puzzuoli, near Naples, marine strata are seen containing frag- 

 ments of sculpture, pottery, and the remains of buildings, together with 

 innumerable shells retaining in part their color, and of the same species 

 as those now inhabiting the Bay of Baise. The uppermost of thfcse 

 beds is about 20 feet above the level of the sea. Their emergence can 

 be proved to have taken place since the beginning of the sixteenth cen- 

 tury.* Now here, as in almost every instance where any alterations of 

 level have been going on in historical periods, it is found that rocks contain- 

 ing shells, all, or nearly all, of which still inhabit the neighboring sea, may 

 be traced for some distance into the interior, and often to a considerable 

 elevation above the level of the sea. Thus, in the country round Na- 

 ples, the post-pliocene strata, consisting of clay and horizontal beds of 

 volcanic tuff, rise at certain points to the height of 1500 feet. Although 

 the marine shells are exclusively of living species, they are not accom- 

 panied like those on the coast at Puzzuoli by any traces of man or his 

 works. Had any such been discovered, it would have afforded to the 

 antiquary and geologist matter of great surprise, since it would have 



* See Principles, Index, " Serapis." 



