SUBDIVISIONS OF TUE CRETACEOUS SYSTEM. 175 



an insulated portion of an ancient sea-bottom, 

 or in other words a mass of consolidated sedi- 

 ments formed in the profound depths of the 

 ocean, in a very remote period of the earth's 

 physical history. This detritus is made up of 

 inorganic and organic materials : the former con- 

 sist 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 

 freshwater currents ; and of chemical deposits 

 thrown down from mineral solutions. The or- 

 ganic substances are the durable remains 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 forming such an 

 assemblage of sedimentary deposits as would pro- 

 bably 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. 



Subdivisions of the Cretaceous system. — 

 The nomenclature of the chalk formation has 

 undergone so many alterations, that to avoid con- 

 fusion, it is necessary to define the precise mean- 

 ing which the terms employed in the following 

 pages are intended to convey. 



