350 THE CANADIAN NATURALIST. [Sept. 



and before any satisfactory theories can be established regarding 

 the methods of their formation. 



The old motto for statesmen dealing with barbarous tribes on 

 the frontiersof an empire was, " Divide and command ;" and this 

 must also be the motto for students of the apparently confused 

 accumulations of glacial deposits. We must divide to command. 

 I. The oldest boulder clay I at present believe to be 

 represented by that which underlies the shell clay of the West 

 of Scotland. 



It is only in the lower districts, however, that the shell clay 

 rests upon it, while it reaches to a height in Scotland (1,500 feet 

 or more) far beyond that at which any shells have yet been 

 discovered. Its thickness extends from a mere covering of the 

 rock, to the depth of even 300 feet, and is excessively variable. 



It is closely compact, as though subjected to immense pressure 

 and difficult to work even with the pick-axe. 



Although occasionally containing patches of sand, of greater 

 or less extent, it has no stratification. The included stones are 

 in large numbers, polished and striated, and have not been broken 

 by the process through which they have passed. Even thin and 

 brittle pieces of shell are found finely striated. 



These polished and striated surfaces are so freshly preserved 

 that the stones could not have been rolled on a beach subsequently 

 to their production. Any trituration would at once destroy the 

 fineness of the glaciated surfaces. 



The included stones are chiefly traceable to the heights nearest 

 the locality in which the special bed is found, although a certain 

 proportion have travelled from distances in the direction along 

 which a glacier would naturally have moved, according to the 

 general conformation of the country. 



This boulder clay, we suggest, belongs to the period when 

 the cold of the glacial epoch reached its intensest point. 



It preceded the development of the arctic fauna, now fossil 

 in our glacial clays, since the shell beds again and again most 

 decisively rest upon it. 



The highest point at which arctic shells are found in Scotland 

 is 510 feet (Airdrie) ; from that point downwards at various 

 levels to half- tide m; rk and beneath the sea. 



The boulder clay, however, is found destitute of shells to the 

 height of at least 1,600 feet, in every hollow and nook, on 

 mountain flanks, through Scotland. 



