408 5IANUAL OF THE MOLLtTSCA. 



Chapter IIT. 



ON THE DISTRIBL'TION OF THE MOLLUSC A IN TIME. 



The historian of modern geology, Sir Chas. Lyell, has taught us to regard 

 the stratified rocks as so many monuments, recording 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 particular 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 forma- - 

 tions ; so that the tnass of species in each period must have been peculiar 5 

 and distinctive. \ 



The important theory, that strata may be identified by fossils, was taught J 

 by William Smtih, early in the present century, and is thus expressed in ■; 

 his Stratigra'pMcal System : " Organized fossils are to the naturalist as coins ■ 

 to the antiquary ; they are the antiquities of the earth; and very distinctly i 

 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 j 

 the earth's formation they vary as much from each other ; insomuch that j 

 each layer of these fossil organized bodies must be considered as a separate 

 creation ; or how could the earth be formed, stratum super stratum, and . 

 each abundantly stored with a diflPerent race of animals and plants."t j 



The " Prodrome " of j\t. 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 known to shew that at least ten great changes had taken place ^ 

 before the Secondary epoch. \ 



In the following Table, the first column gives the names of the Forma- \ 

 tions, or Periods ; the second contains those by which the principal strata ; 

 are known. ! 



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

 Cretaceous formations of the whole icorld aiford mineral types corresponding to perhaps ' 

 every variety of Carboniferous rock. j 



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



