ANNIVERSARY ADDRESS OF THE PRESIDENT. XXXVll 



Lyel] published the third vohime of his * Principles of Geology/ and 

 he there brought forward a division of the tertiary formations into 

 four groups, founded on " the comparative proportion of living spe- 

 cies (^shells found fossil in each */' which he termed Eocene, Miocene, 

 Older Pliocene, and Newer Pliocene. He adopts the same divisions in 

 the second edition of his 'Elements of Geology,' published in 1841, 

 then adding a fifth di\dsion, the *' Post-Pliocene, including the fossili- 

 ferous strata of the Recent, or human period." f " I have adopted the 

 term Post-Pliocene," he says, "for those strata which are sometimes 

 called modern, and which are characterized by having all the imbedded 

 fossil shells identical with species now living ;" — he takes no account 

 of their containing, or being free from, remains of extinct mammalia. 

 The proportions of living species of shells in each division he states 

 to be as follows : — 



Post-Pliocene 99 to 100 per cent. 



Newer PHocene 85 — 90 „ 



Older Pliocene 60 — - 70 „ 



Miocene 20 — 30 „ 



Eocene 1 or 2 „ 



A definite order of superposition, from the eocene upwards, was of 

 course implied as an essential condition of the classification through- 

 out the whole range of deposits. The term pleistocene had been 

 proposed by Mr. Lyell three years before, for his fourth division ; 

 although he dropped it himself, it has since been occasionally used by 

 other writers. 



These terms have usually been understood to mean certain periods or 

 measures of past time. If they apply to time, they apply to every 

 part of the earth's surface ; that is, as generally understood, at the 

 time pliocene deposits were forming in Europe, it was pliocene time 

 all over the world. If we inquire how this scale of geological chrono- 

 logy has been formed, we find that it has been graduated by the results 

 of the examination of deposits in certain localities, by different ob- 

 servers, and by a careful comparison of the remains of mollusca con- 

 tained in these deposits, with those now living in the neighbouring 

 seas. 



The application of the terms pliocene, miocene, &c. to time generally, 

 presupposes that the numerous causes which led to the extinction of 

 existing species and favoured the introduction of new species, had been 

 going on over the whole globe, both in respect of kind and degree, 

 although not necessarily simultaneously in different regions ; that is, 

 that the same changes might be brought about in periods of longer 

 duration in one region than in another : that the causes operated in 

 one region on certain species, in another on analogous or representative 

 species ; the general effect being, that an uniformity/ in the character 

 of the result, during the epoch in question, was produced all over the 

 globe. Facts already collected appear to some geologists to lead to 

 this conclusion ; they maintain that if we extend the period of time 

 sufficiently, a certain class of changes will have taken place, havui^ 



* Preface, p. xiii, t Vol. 1. page 210. i 



d2 



