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DI8TEIBUTI0N OF MOLLUSCA IN TIME. / . ^ ,^ 117 



CHAPTER III. 



ON THE DISTRIBUTION OF THE MOLLUSCA IN TIME 



The liistorian of modem geology, Sir Charles Lyell, has taught 

 us to regard the stratified rocks as so many monuments, record- 

 ing the physical condition and living inhabitants of the earth in 

 past ages. 



Each formation consists of a similar and more or less complete 

 series of limestones, sandstones, clay, coal, and other strata, 

 representing the deep and shallow seas, the fresh-waters, and 

 the terrestrial portions of the surface of the globe, at one par- 

 ticular period of time.* 



The organic remains found in the strata exhibit no such 

 repetitions, but are changed gradually and regularly, from the 

 earliest to the latest formations ; so that the mass of species in 

 each period must have been peculiar and distinctive. 



The important theory, that strata may be identified by fossils, 

 was taught by William Smith, early in the present century, and is 

 thus expressed in his Stratigraphical System : — ' ' Organised fossils 

 are to the naturalist as coins to the antiquary ; they are the 

 antiquities of the earth ; and very distinctly show its gradual, 

 regular formation, with the various changes of inhabitants in 

 the watery element." — " They are chiefly submarine, and as 

 they vary generally from the present inhabitants of the sea, so at 

 separate periods of the earth's formation they vary as much 

 from each other; insomuch that each layer of these fossil 

 organised bodies must be considered as a separate creation ; 

 or how could the earth be formed, stratum super stratum, and 

 each abundantly stored with a different race of animals and 

 plants."t 



The " Prodrome" of M. D'Orbigny is a catalogue of the shells 

 (and radiate animals) of each formation, from which it appears 

 that the mass of the living population of the globe has been 

 changed twenty times since the close of the First or Palaeozoic 

 Age ; and although the fossils of the older rocks have not been 

 generally classified with the same minuteness, yet enough is 



* The coal-measures and chalk of England cannot indeed be called similar, but the 

 Cretaceous formations of the whole world afford mineral tj'pes, corresponding to, per- 

 haps, every variety of Carboniferous rock. 



t Stratigraphical System of Organised Fossils, 4to., Lond. 1817. 



