II.  GLACIAL DEPOSITS.—­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 Glacial 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 intervals in the great age of ice; and in two or three instances this has apparently been demonstrated—­deposits of this nature, with the bones of extinct animals and the implements of man, having been shown to be overlaid by true Boulder-clay.