
Parr V. Szor. i. §1.] PLEISTOCENE. — 883 
PART V.—Post-TERTIARY OR QUATERNARY. 
Under this division are included the various superficial deposits 
in which all the mollusca are of still living species. It is usually 
subdivided into two series—(1) an older group of deposits in which 
many of the mammals are of extinct species,—to this group the 
names Pleistocene, Post-Pliocene, and Diluvial have been given; 
and (2) a later series, wherein the mammals are all or nearly all 
of still living species, to which the names Recent, Alluvial, and 
Human have been assigned. These subdivisions, however, are con- 
fessedly very artificial, and it is often exceedingly difficult to draw 
~ any line between them. 
In Europe and North America a tolerably sharp demarcation can 
usually be made between the Pliocene formations and those now to 
be described. ‘The Crag deposits of the south-east of England show 
_traces of a gradual lowering of the temperature during later Plio- 
cene times. This change of climate continued to augment until at 
last thoroughly arctic conditions prevailed, under which the oldest 
of the Post-Tertiary or Pleistocene deposits were accumulated. 
It is hardly possible to arrange the Post-Tertiary deposits in a 
strict chronological order, because we have no means of deciding, in 
many cases, their relative antiquity. In the glaciated regions of the 
northern hemisphere the various glacial deposits are grouped as the 
older division of the series under the name of Pleistocene. Above 
them lie: younger accumulations such as river-alluvia, peat-mosses, 
lake-bottoms, cave-deposits, blown-sand, raised lacustrine and marine 
terraces, which, merging insensibly into those of the present day, 
are termed Recent or Prehistoric. 
Section i.—Pleistocene or Glacial. 
§1. General Characters. 
Under the name of the Glacial Period or Ice Age, a remarkable 
geological episode in the history of the northern hemisphere is 
denoted. The Crag deposits (p. 873) afford evidence of a gradual 
refrigeration of climate at the close of the Tertiary ages. This 
change of temperature affected the higher latitudes alike of the Old 
and the New World. It reached such a height that the whole of the 
north of Europe was buried under snow and ice, extending southwards 
even as far as Saxony. The Alps and Pyrenees were loaded with 
1 No section of geological history now possesses a more voluminous literature than 
the Glacial Period, especially in Britain and North America. For general informa- 
tion the student may refer to Lyell’s Antiquity of Man, J. Geikie’s Great Ice Age, J. 
Croll’s Climate and Time, and for detailed descriptions, to the Quart. Journ. Geol. Soe., 
Geol. Mag., and Amer. Journ. Science, for the last fifteen or twenty years. 
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