208 THE OPEN BOOK OF NATURE 
ash-bed. Why, we can almost image the whole 
affair in operation ! 
Down in the bed of the stream there are some 
“‘not-holes.” These have been made by whirlpools 
or eddies which have whirled boulders round and 
round like millstones. As the boulders turned they 
gradually wore out circular holes. In that hole 
there you can see a rounded stone, which was once 
big and rough, but has been gradually ground down 
and smoothed to its present size; its whirling, 
grinding motion has not only caused it to become 
smaller and smaller, but it has also made the pot-hole 
in which it lies. The stream is low to-day, and 
you cannot see the process in action; but after 
heavy rains the stream thunders down its course, 
and boulders may then be seen whirling and grind- 
ing in the holes. These pot-holes are sometimes 
called ‘‘ witches’ cauldrons.” Notice these two 
holes that are side by side. The partition between 
them has been cut through by the grinding action 
of stones, and a hole has been made at the base of 
each of them, allowing water to escape into the 
pool below. 
This stream is a favourite haunt of the Dipper 
(Cinclus aquaticus), called in England the Water 
Ouzel and in Scotland the Water Crow. The Dipper 
is about seven inches in length. The colour above 
is dark grey, all the feathers being edged with black ; 
but the bird is well known by its cheeks, throat, 
and chest being pure white. It is a very shy bird, 
and, unless in the breeding season, it is usually seen 
alone. If you desire to watch it you need to hide 
under cover of a rock or in some other advantageous 
