and keep the fall upright instead of its being sloped away at the
top, and becoming a mere steam. 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 entirely cut out by waterfalls. But the best
know 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 tons of water fall
over them every minute, making magnificent clouds of spray.