THE 
PLIOCENE D E P O S1 r S 
OP 
B E I T A I N. 
CHAPTER 1. 
INTRODUCTION. 
During the past sixty years the study of the deposits, once 
vaguely spoken of as Supra-Cretaceous ” or Tertiary,” has led 
to a more and more exact classification. But though a great 
advance has been made, it is still necessary to define the sense in 
which the term Pliocene is used in the following pages, for custom 
has not yet fixed the limits of the period which it denotes. 
It is needless for this purpose to go back further than the first 
edition of Lyell’s Principles of Geology,” for in the third volume 
of that work, published in 1833, the Tertiary rocks were for 
the first time divided into groups approximating to those now in 
use. Lyell subdivided the Tertiary strata into three series, 
according to the relative proportion of living and extinct mollusca 
found in them, and named them Eocene, Miocene, and Pliocene. 
Subsequently the Eocene series was divided into two, and the 
upper part named Oligocene; and the original Newer Pliocene 
group of Lyell was raised to the rank of a distinct formation, 
under the name of Pleistocene. 
The Pliocene, which is the only formation dealt with in this 
Memoir, was defined by Lyell as including all strata which con- 
~ tain from 36 to 95 per cent, of recent marine mollusca. This test, 
though readily applied, is somewhat arbitrary ; for the percentages 
in strata really contemporaneous may vary considerably from local 
causes. We must regard the percentage test, like the Linnean 
system in botany, not as fixing a rigid standard by which 
all questions must be decided, but as avaluable key, to be used 
temporarily until a more natural system becomes available. 
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