350 HISTORICAL PALAEONTOLOGY. 



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 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, compris- 

 ing 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 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- 

 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 ex- 

 tinct animals and the implements of man, having been shown 

 to -be overlaid by true Boulder-clay. 



The fossils of the undoubted Glacial deposits are principally 

 shells, which are found in great numbers in certain localities, 

 sometimes with Foraminifera, the bivalved cases of Ostracode 

 Crustaceans, &c. Whilst some of the shells of the "Drift" 



