PALEONTOLOGY 



315 



employ a series of terms like those in the accompanying time scale. It 

 is given in sufficient detail to be used as a reference table, and the larger 

 divisions of geological time must be mastered even by the beginning 

 student. 



Geological Time Scale' 



' The periods of a geological time scale are arranged in the same order as the 

 strata belonging to them, that is, with the oldest at the bottom. To read such a 

 table in chronological order one must therefore begin at the bottom. 



In determining the period to which a given rock stratum belongs the 

 character of the rock itself is usually of little value. The fossil animals 

 and plants are used as recognition marks. Though there is some irregu- 

 larity in the occurrence of some kinds of animals, certain fossils are so 

 characteristic of a given geological age that they are termed "index fossils." 

 It is assumed that, the world over, these animals lived at approximately 

 the same time, and that their remains are now found in contemporaneous 

 rocks. While this assumption is probably incorrect in some cases, as 

 pointed out below, it almost certainly is true as a general proposition. 



