THE TWO GREAT SCULPTORS. 109 
at the top, and becoming a mere stream. Secondly, 
you may often see curious cup-shaped holes, called 
" pot-holes," in the rocks on the sides of a waterfall, 
and these also are concerned in its formation. In these 
holes you will generally find two or three small 
pebbles, and you have here a beautiful example of 
how water uses stones to grind away the face of the 
earth. These holes are made entirely by the falling 
water eddying round and round in a small hollow of 
the rock, and grinding the pebbles which it has 
brought down, against the bottom and sides of this 
hollow, just as you grind round a pestle in a mortar. 
By degrees the hole grows deeper and deeper, and 
though the first pebbles are probably ground down 
to powder, others fall in, and so in time there is a 
great hole perforated right through, helping to make 
the rock break and fall away. 
In this and other ways the water works its way 
back in a surprising manner. The Isle of Wight gives 
us some good instances of this ; Alum Bay Chine 
and the celebrated Blackgang Chine have been en- 
tirely cut out by waterfalls. But the best known and 
most remarkable example is the Niagara Falls, in 
America. Here, the River Niagara first wanders 
through a flat country, and then reaches the great 
Lake Erie in a hollow of the plain. After that, it 
flows gently down for about fifteen miles, and then the 
slope becomes greater and it rushes on to the Falls of 
Niagara. These falls are not nearly so high as many 
people imagine, being only 165 feet, or about half the 
height of St. Paul's Cathedral, but they are 2700 feet 
or nearly half-a-mile wide, and no less than 670,000 
