OEDER OF STRATA. 
137 
CHAPTER IX. 
CLASSIFICATION OF TEETIAKY FORMATIONS. 
Order of Succession of Sedimentary Formations.—Frequent Unconformability 
of Strata.—Imperfection of the Record.—Defectiveness of the Monuments 
greater in proportion to their Antiquity.—Reasons for studying the newer 
Groups first.—Nomenclature of Formations.—Detached Tertiary Forma¬ 
tions scattered over Europe.—Value of the Shell-bearing Mollusca in Class¬ 
ification.—Classification of Tertiary Strata.—Eocene, Miocene, and Plio¬ 
cene Terms explained. , 
By reference to the tables given at the end of the last 
chapter the reader will see that when the fossiliferous rocks 
are arranged chronologically, we have first to consider the 
Post-tertiary and then the Tertiary or Cainozoic formations, 
and afterwards to pass on to those of older date. 
Order of Superposition. —The annexed diagram will show 
the order of superposition of these deposits, assuming them 
all to be visible in one continuous section. In nature, as be¬ 
fore hinted, page 107, we have never an opportunity of see- 
Fig. 86. 
ing the whole of them so displayed in a single region; first, 
because 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 present, they overlie one 
another in the order indicated in the diagram, though not 
always in the exact manner there represented, because some 
of them repose occasionally in unconformable stratification 
on others. This mode of superposition has been already ex¬ 
plained at pp. 94, 111, where I pointed out that the discord¬ 
ance which implies a considerable lapse of time between two 
