142 TERTIARY ROCKS OF AUSTRALASIA, 



This is the reason why Lyell based his original classifi- 

 cation of the European Tertiary strata — Eocene, Miocene, 

 and Pliocene — mainly upon the evidence of the testaceous 

 mollusca. 



By relation to existing species he arranged the known 

 European beds into three principal groups, named on the 

 basis of the percentage of living forms to be found in 

 them. 



Thus, the order of the three divisions was classed as 

 follows, beginning with the oldest : — 



Contained Percentage of 

 Species still existing. 



Eocene fdawn of (recent) life) 1 to 3| 



Miocene fless (proportion of recent) life J 17 to 30 

 Pliocene (more (proportion of recent) life) 36 to 98 



In the first edition of the Principles of Geology, Lyell 

 recognised four distinct groups by the same method ; 

 thus : — 



Contained Percentage of 



Species still existing. 



Eocene 3| 



Miocene 17 



Older PHocene (Pliocene) 35 to 50 



Newer Pliocene (Pleistocene) 90 to 95 



The two latter groups are now for the most part linked 

 together under the name Pliocene, and a fresh subdivision, 

 absorbing a portion of the Lower Miocene and Upper 

 Eocene, has been adopted very generally in Europe, 

 termed " Oligocene." 



The percentages given, however, are not to be taken too 

 rigidly, for Lyell has himself stated that since the time 

 when the original classification was made (1830) "the 

 number of known shells, both recent and fossil, has largely 

 increased, and their identification has been more accu- 

 rately determined. Hence some modifications have been 

 required in the classifications founded on less perfect 

 materials." 



Generally, then, it may be well to consider that the 

 Eocene includes those formations in which living species 

 represent an extremely small proportion of the testaceae ; 

 Miocene, those in which the extinct species exceed those 

 haying living representatives ; and Pliocene, those in 

 which the extinct species bear a smaller proportion than 



