122 THE FAIRY-LAND OF SCIENCE. 
rock lying at the foot of the cliff which are most active 
in wearing it away. Have you never watched the 
waves breaking upon a beach in a heavy storm? How 
they catch up the stones and hurl them down again, 
grinding them against each other! At high tide in 
such a storm these stones are thrown against the foot 
of the cliff, and each blow does something toward 
knocking away part of the rock, till at last, after many 
storms, the cliff is undermined and large pieces fall 
down. These pieces are in their turn ground down 
to pebbles which serve to batter against the remain- 
ing rock. 
Professor Geikie tells us that the waves beat in 
a storm against the Bell Rock Lighthouse with as 
much force as if you dashed a weight of 3 tons against 
every square inch of the rock, and Stevenson found 
stones of 2 tons' weight which had been thrown dur- 
ing storms right over the ledge of the lighthouse. 
Think what force there must be in waves which 
can lift up such a rock and throw it, and such force 
as this beats upon our sea-coasts and eats away the 
land. 
Fig. 30 is a sketch on the shores of Arbroath which 
I made some years ago. You will not find it diffi- 
cult to picture to yourselves how the sea has eaten 
away these cliffs till some of the strongest pieces which 
have resisted the waves stand out by themselves in 
the sea. That cave in the left-hand corner ends in a 
narrow dark passage from which you come out on 
the other side of the rocks into another bay. Such 
caves as these are made chiefly by the force of the 
waves and the air, bringing down pieces of rock from 
