2 
. INTRODUCTION. 
Lyell himself does not appear ever to have regarded it as anything 
more than this. 
There is, however, no difficulty in fixing the lower limit of the 
Pliocene system in England, for unfortunately no deposits of 
Miocene age are yet known here, and we commence with strata 
yielding over 50 per cent, of recent mollusca. The higher limit 
cannot well be settled by the percentage test, for we have too 
small a number of marine mollusca from the highest Pliocene and 
lowest Pleistocene deposits to allow of comparison. The effects 
of the marked change of climate that occurred after the Cromer 
Forest-bed was deposited seem, however, to form a natural close 
to the period, for the refrigeration apparently caused the extinction 
of a large proportion of the higher animals, though the inverte¬ 
brates and plants lived on. 
Thus under the term Pliocene we may include ail strata deposited 
after the sub-tropical Miocene period, and before the incoming of 
the intense cold of the earliest Pleistocene ages. A change from 
warm temperate to cold temperate occurred in Britain during the 
Pliocene period. There may have been minor oscillations; but 
the change from conditions like those now existing in the Mediter¬ 
ranean region, to a climate like that now found in England, 
seems to have been fairly continuous and regular. The alternation 
in the uppermost Crag of a boreal marine fauna with land- 
animals and plants of the temperate zone, is probably due, as we 
shall show, to local causes, for the temperate land fauna and the 
arctic marine fauna occur also mixed in the same estuarine 
deposits. 
In Britain this change of climate during the period may be used 
even further in the classification, for our Pliocene strata fall 
naturally into two groups—those of the warm temperate and 
those of the cold temperate period—with a break and distinct un¬ 
conformity between them. The division of the newer Pliocene 
deposits into a lower marine and an upper freshwater and estuarine 
series completes the main outlines of the classification, though 
several minor zones can be recognized. The following are the 
subdivisions adopted in this Memoir:— 
Base of the 
Pleistocene 
; { 
Newer Pliocene 
(cold temperate).' 
Older Pliocene . 
(warm temperate).’^. 
Arctic Freshwater Bed (with Salix iwlaris, Betula 
nana, &c. 
Leda-myalis Bed (classed provisionally with the 
Pliocene). 
Fnrp^t hprl f Freshwater I Gravels with 
Ses 1 ^Elephas meridion- 
L Lower Freshwater J alis at Dewlish. 
Weybourn Crag (and Ohillesford Olay?). 
Chillesford Crag. 
Norwich Crag and Scrobicularia Crag. 
Red Crag of Butley, &c. 
Walton Crag (lower Red Crag). 
St. Erth Beds. 
Coralline Crag. 
Lenham Beds. 
Box stones and phosphate beds (with remanie early 
Pliocene fossils). 
