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 



