AGES OF AQUEOUS ROCKS. 
121 
The doctrine here laid down will be more readily under¬ 
stood, if we reflect on what is now going on in the Mediter¬ 
ranean. That entire sea may be considered as‘one zoological 
province; for although certain species of testacea and zo¬ 
ophytes may be very local, and each region has probably 
some species peculiar to it, still a considerable number are 
common to the whole Mediterranean. If, therefore, at some 
future period, the bed of this inland sea should be converted 
into land, the geologist might be enabled, by reference to 
organic remains, to prove the contemporaneous origin of 
various mineral masses scattered over a space equal in area 
to half of Europe. 
Deposits, for example, are well known to be now in prog¬ 
ress in this sea in the deltas of the Po, Rhone, Nile, and other 
rivers, which differ as greatly from each other in the nature 
of their sediment as does the composition of the mountains 
which they drain. There are also other quarters of the Med¬ 
iterranean, as off the coast of Campania, or near the base of 
Etna, in Sicily, or in the Grecian Archipelago, where another 
class of rocks is now forming; where showers of volcanic 
ashes occasionally fall into the sea, and streams of lava over¬ 
flow its bottom; and where, in the intervals between vol¬ 
canic eruptions, beds of sand and clay are frequently derived 
from the waste of cliffs, or the turbid waters of rivers. 
Limestones, moreover, such as the Italian travertins, are here 
and there precipitated from the waters of mineral springs, 
some of which rise up from the bottom of the sea. In all 
these detached formations, so diversified in their lithological 
characters, the remains of the same shells, corals, Crustacea, 
and fish are becoming inclosed; or, at least, a sufficient num¬ 
ber must be common to the diferent localities to enable the 
zoologist to refer them all to one contemporaneous assem¬ 
blage of species. 
There are, however, certain combinations of geographical 
circumstances which cause distinct provinces of animals and 
plants to be separated from each other by very narrow lim¬ 
its ; and hence it must happen that strata will be sometimes 
formed in contiguous regions, differing widely both in min¬ 
eral contents and organic remains. Thus, for example, the 
testacea, zoophytes, and fish of the Red Sea are, as a group, 
extremely distinct from those inhabiting the adjoining parts 
of the Mediterranean, although the two seas are separated 
only by the narrow isthmus of Suez. Calcareous formations 
have accumulated on a great scale in the Red Sea in modern 
times, and fossil shells of existing species are well preserved 
therein ; and we know that at the mouth of the Nile large 
