THE TWO GREAT SCULPTORS. 113 
will have some idea of the amount of chalk carried 
invisibly past Bonn in the water of the Rhine every 
year. 
Since all this matter, whether brought down as 
mud or dissolved, comes from one part of the land 
to be carried elsewhere or out to sea, it is clear that 
some -gaps and hollows must be left in the places from 
which it is taken. Let us see how these gaps are 
made. Have you ever clambered up the mountain- 
side, or even up one of those small ravines in the hill- 
side, which have generally a little stream trickling 
through them? If so, you must have noticed the 
number of pebbles, large and small, lying in patches 
here and there in the stream, and many pieces of 
broken rock, which are often scattered along the sides 
of the ravine ; 'and how, as you climb, the path grows 
steeper, and the rocks become rugged and stick out 
in strange shapes. 
The history of this ravine will tell us a great deal 
about the carving of water. Once it was nothing 
more than a little furrow in the hill-side down which 
the rain found its way in a thin thread-like stream. 
But by and by, as the stream carried down some of 
the earth, and the furrow grew deeper and wider, the 
sides began to crumble when the sun dried up the 
rain which had soaked in. Then in winter, when the 
sides of the hill were moist with the autumn rains, 
frost came and turned the water to ice, and so made 
the cracks still larger, and the swollen stream rushing 
down, caught the loose pieces of rock and washed 
them down into its bed. Here they were rolled over 
and over, and grated against each other, and were 
