1.18 



MANUAL OF THE MOLLUSCA. 



known to show 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 Formations or Periods ; the second contains those by which 

 the principal strata are known. 



I. GEOLOGICAL TABLE. 



It must be observed that the number and magnitude of the 

 •' Formations " was determined by accident in the first instance, 

 and afterwards modified to suit the requirements of theory, and 

 to make them more nearly equal in value.* 



* The names of formations are in gi-eat measure provisional, and open to criticism. 

 Some of them were given by Brongniart and 0, D'Halloy ; others have been more 



