178 



PRINCIPLES OF CLASSIFICATION. 



[Ch. xni. 



CHAPTER XIII. 



CLASSIFICATION OF TERTIARY FORMATIONS. PLIOCENE PERIOD. 



Order of succession of sedimentary formations — Imperfection of the record — Defec- 

 tiveness and obscurity of the monuments greater in proportion to their antiquity 

 — Reasons for studying the newer groups first — General principles of classifica- 

 tion of tertiary strata — Detached formations scattered over Europe — Strata of 

 Paris and London — More modern groups — Peculiar difficulties in determining the 

 chronology of tertiary formations — Increasing proportion of living species of 

 shells in strata of newer origin — Eocene, Miocene, and Pliocene terms explained 

 — Formations of the Newer Pliocene period — Island of Ischia — Eastern base of 

 Mount Etna — Newer Pliocene strata of great height and extent in Sicily — For- 

 mations of same age in the Upper Val d'Arno — Norwich Crag — Chillesford 

 beds — Bridlington beds — Older Pliocene strata — Red Crag of Suffolk — White 

 or coralline Crag — Successive refrigeration of climate proved by tho pliocene 

 shells of Suffolk and Norfolk — Antwerp Crag — Subapennine strata — Aralo-Cas- 

 pian formations. 



The post-tertiary formations, comprising the Post-pliocene and 

 Recent, having been described in the last three chapters, I have- 

 now to give an account of the strata called tertiary and the several 

 groups into which they have been subdivided. 



The annexed diagram will show the order and superposition of the 

 principal sets of fossiliferous deposits enumerated in the table, page 



Fig. 140. 



ARY 



TE RTIARY 



POST-TERTIARY 

 Sea 



101, assuming them all to be visible in one continuous section. In 

 nature, as before hinted, page 98, we have never an opportunity of 

 seeing the whole of them so displayed in a single region ; first, be- 

 cause sedimentary deposition is confined, during any one geological 

 period, to limited areas ; and secondly, because strata, after they have 

 been formed, are liable to be utterly annihilated over wide areas by 

 denudation. But wherever certain members of the series are pres- 

 ent, they overlie one another in the order indicated in the diagram, 



