SUCCESSION AMONG ROCKS. 293 



their contents shows also, that these changes have 

 been posterior to the time of their consolidation. 

 This case offers an excellent example of partial revo- 

 lutions ; and serves to prove how imperfect a criterion 

 for distinguishing the artificial classification of primary 

 and secondary strata, is derived from such circum- 

 stances alone. 



The Chalk, which is the last of the strata esteemed 

 secondary? is followed by various substances which 

 have been grouped in different manners by different 

 Geologists, but of which the localities are partial and 

 limited. The plastic clay occurs first, and consists, 

 not only of various clays, but of sand and gravel of 

 various qualities. The London clay, which succeeds, 

 comprises also calcareous sandstone ; besides which 

 it contains many organic remains, terrestrial and 

 marine. The strata which succeed, consist chiefly 

 of marls and clays ; but as the details of objects so 

 local are too minute for the present purpose, I may 

 refer to Mr. Webster's accurate account ; particularly 

 as the importance of the tertiary or partial deposits, 

 is such as to demand a separate consideration. 



It must now be observed that the series of secon- 

 dary strata in England, thus given in a complete 

 form, as it is thought, by those who have investigated 

 this subject most minutely, is far from exhibiting this 

 succession in any one place. It is not merely that 

 the whole series terminates at some point beneath the 

 uppermost or London clay, as for example, at the 

 coal series, or the red marl, or the lias, but nu- 

 merous members are in many places wanting. This 

 succession must therefore be considered as, in some 

 sense, an artificial one ; constructed according to 

 some presumed principles in the science, and a picture 



