40 CONTEMPORANEOUS ORIGIN OF ROCKS [Ch. IV. 
—if the same species abounded in every climate, or even in 
every part of the globe where a corresponding temperature, 
and other conditions favourable to their existence were found, 
the identification of mineral masses of the same age, by means 
of their included organic contents, would be a matter of much 
greater facility. 
But, fortunately, the extent of the same zoological provinces,; 
especially those of marine animals, is very great, so that we 
are entitled to expect^ from analogy, that the identity of fossil 
species, throughout large areas, will often enable us to connect 
together a great variety of detached and dissimilar formations. 
Thus, for example, it will be seen, by reference to our first 
volume, that deposits now forming in different parts of the 
Mediterranean, as in the deltas of the Rhone and the Nile, 
are distinct in mineral composition ; for calcareous rocks are 
precipitated from the waters of the former river, while pebbles 
are carried into its delta, and there cemented, by carbonate of 
lime, into a conglomerate; whereas strata of soft mud and fine 
sand, are formed exclusively in the Nilotic delta. The Po, 
again, carries down fine sand and mud into the Adriatic ; but 
since this sediment is derived from the degradation of a differ- 
ent assemblage of mountains from those drained by the Rhone 
or the Nile, we may safely assume that there will never be an 
exact identity in their respective deposits. 
If we pass to another quarter of the Mediterranean, as, for 
example, to the sea on the coast of Campania, or near the base 
of Etna in Sicily, or to the Grecian archipelago, we find in all 
these localities that distinct combinations of rocks are in pro- 
gress. Occasional showers of volcanic ashes are falling; into 
the sea, and streams of lava are flowing along its bottom ; 
and in the intervals between volcanic 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, while shells and corals accumulate 
in various localities. Yet the entire Mediterranean, where the 
