GLEN ROY, IN SCOTLAND. 
53 
terials is very different. Examine the shingle 
along our beaches; we find it so distributed as 
to show that the fading tide-wave has carried 
the lighter materials farther than the heavier 
ones, and the successive deposits exhibit an 
imperfect cross-stratification resulting from 
changes in the height of the tide and the di¬ 
rection of the wind. Moreover, in any mate¬ 
rials collected under water we find the heavier 
ones at the bottom, the lighter on the top. It 
is true that large angular boulders may occa¬ 
sionally be found resting upon beach-shingle, 
but their presence in such a connection is 
easily explained. They may have been dropped 
there by floating icebergs, or have fallen from 
crumbling drift-cliffs. 
I should add, in speaking of drift-materials, 
that, while we find the large angular boulders 
resting above them, we occasionally find boul¬ 
ders of unusual size mingled with them; but, 
when this is the case, such massive fragments 
are more or less rounded, polished, and marked 
in the same way as the smaller pebbles, or as 
the surfaces over which the glacier has passed. 
This is important to remember, because, when 
we examine the drift in countries where the 
