38 GEOLOGICAL BIOLOGY. 



assign them to one or other of these systems upon evidence 

 of the fossils found in them. The character of the rocks 

 themselves, their composition, or their mineral contents have 

 nothing to do with settling the question as to the particular 

 system to which the new rocks belong. The fossils alone are 

 the means of correlation. It thus happens that each geolog- 

 ical system, which is a local aggregation of strata, of particular 

 composition, structure, and thickness, becomes a standard of 

 chronological period and duration by virtue of the fossils 

 which it contains. The fossils are characteristic of some 

 particular period in the history of organisms, and the strata 

 containing them were deposited during that period. 



