STRATIFIED ROOKS. 209 



are two modes of determining the limits of the divisions 

 of the rocks, and corresponding divisions of time, viz., 

 by unconformity of the rocks, and by change of the fossils. 

 These two usually occur together, because they are pro- 

 duced by the same cause, viz., change in physical geogra- 

 phy and climate ; but, if there be discordance between 

 the two, then we follow the change in the fossils rather 

 than unconformity of rocks. By means of the most gen- 

 eral unconformity and greatest change in fossil forms, the 

 primary divisions are established ; and then, by less gen- 

 eral unconformity and less important changes in organic 

 forms, these are divided and subdivided. A generalized 

 schedule of the divisions and subdivisions of the rocks 

 and corresponding divisions of time which will be used 

 in this work, is given on the preceding page. 



LE CONTE, GEOL. 14 



