14. ORGANIC REMAINS OF THE CHALK FORMATION. 



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14. Organic remains of the chalk formation. — 

 I would remind the reader, that the numerous groups of 

 strata comprised in the cretaceous system, are to be 

 regarded as an ancient ocean-bed ; — a vast accumulation 

 of sedimentary deposits, more or less consolidated by sub- 

 sequent chemical changes, that was formed in the profound 

 depths of the sea, in periods of unfathomable antiquity. 

 This mass is made up of inorganic and organic materials : 

 the former consist of the debris of the cliffs and shores which 

 encompassed the ancient sea ; of the spoils of islands and 

 continents brought into the ocean by floods and currents of 

 fresh water ; and of mineral deposits thrown down from 

 chemical solutions: the organic substances are the durable re- 

 mains of animals and plants which lived and died in the ocean, 

 and of fluviatile and terrestrial species that were transported 

 from the land by rivers and their tributaries. The whole 

 forms such an assemblage of sedimentary deposits as 

 would probably be presented to observation, if a mass of 

 the bed of the Atlantic, 2000 feet in thickness, were elevated 

 above the waters, and became dry land ; the only essential 

 difference would be in the generic and specific characters of 

 the imbedded animal and vegetable remains. 



The fossils of the cretaceous formation are exceedingly 

 numerous, and offer examples of all the usual forms of 

 marine existence except Cetaceans, of which no vestiges 

 have as yet been found in strata older than the eocene. 



Particular genera and species appear, however, to be 

 restricted to certain subdivisions of the system ; thus, in the 

 white chalk, there are many species of shells that do not 

 occur in either of the other groups. The marl and gait also 

 have peculiar fossils, and the greensand abounds in shells and 

 zoophytes, that are wanting in the upper series of deposits. 

 The genera and species of the mollusca must therefore have 

 varied during the period of the deposition of the chalk ; 

 some kinds prevailed at the commencement of the forma- 



