l deposits_, or by the more specialised names of the Drift,
the Northern Drift, the Boulder-clay, the Till, &c.

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 formations 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 transported
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 _Boulder-clay_, or _Till_.

2. Sands, gravels, and clays, often more or less regularly
_stratified_, but containing erratic blocks, often of large size,
and with their edges _unworn_, 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, comprising
a greater or less 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 rearrangement 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-deposits," with the
bones of extinct Mammals, truly belong to the Glacial period,
being formed during warm intervals when the severity of the Arctic
cold had become relaxed. It is further believed that some, at
any rate, of the so-called "high-level" river-gravels and
"brick-earths" have likewise been deposited during mild or warm
interval