106 THE FAIRY-LAND OF SCIENCE. 
is made of chalk or carbonate of lime, which the watei 
took out of the rocks when it was passing through 
them. Professor Bischoff has calculated that the river 
Rhine carries past Bonn every year enough carbonate 
of lime dissolved in its water to make 332,00x5 million 
oyster-shells, and that if all these shells were built into 
a cube it would measure 560 feet every way. 
Imagine to yourselves the whole of St. Paul's 
churchyard filled with oyster-shells, built up in a large 
square till they reached half as high again as the top 
of the cathedral, then you 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 'lill-side down which 
the rain found its way in a thin thread-like stream. 
But by and by, as the stieam carried down some of 
