THE QUATERNARY PERIOD. 337 
dition, these, and some other similar accumulations, are some- 
times considered as referable to the Pliocene period. 
If. GiactaL Deposirs.— Under this head is included a 
great series of deposits which are widely spread over both 
Europe and America, and which were formed at a time when 
the climate of these countries was very much colder than it is 
at present, and approached more or less closely to what we see 
at the present day in the Arctic regions. ‘These deposits are 
known by the general name of the Glacad deposits, or by the 
more specialised names of the Drift, the Northern Drift, the 
Boulder-clay, the Till, &ce. 
These glacial deposits are found in Britain as far south as 
the Thames, over the whole of Northern Europe, in all the 
more elevated portions of Southern and Central Europe, and 
over the whole of North America, as far south as the 39th 
parallel. They generally occur as sands, clays, and gravels, 
spread in widely-extended sheets over all the geological forma- 
tions alike, except the most recent, and are commonly spoken 
of under the general term of “ Glacial drift.” They vary much 
in their exact nature in different districts, but they universally 
consist of one, or all, of the following members :— 
1. Unstratified clays, or loams, containing numerous angular 
or sub-angular blocks of stone, which have often been trans- 
ported for a greater or less distance from their parent rock, 
and which often exhibit polished, grooved, or striated surfaces. 
These beds are what is called Bouwlder-clay, or Til. 
2. Sands, gravels, and clays, often more or less regularly 
stratified, but containing erratic blocks, often of large size, and 
with their edges wnuzworn, derived from considerable distances 
from the place where they are now found. In these beds it is 
not at all uncommon to find fossil shells; and these, though of 
existing species, are generally of an Arctic character, compris- 
ing a greater or Jess number of forms which are now exclusively 
found in the icy waters of the Arctic seas. These beds are 
often spoken of as “ Stratified Drift.” 
3. Stratified sands and gravels, in which the pebbles are 
worn and rounded, and which have been produced by a re- 
‘arrangement of ordinary glacial beds by the sea. ‘These beds 
are commonly known as “ Drift-gravels,” or ‘ Regenerated 
Drift.” 
Some of the last-mentioned of these are doubtless post- 
glacial ; but, in the absence of fossils, it is often impossible to 
arrive at a positive opinion as to the precise age of superficial 
accumulations of this nature. It is also the opinion of high 
authorities that a considerable number of the so-called “ cave- 
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