112 THE FAIRY-LAND OF SCIENCE. 
will soon answer this question for yourself. For you 
will find that even from river water which looks quite 
clear, a thin layer of mud will fall to the bottom of 
the glass, and if you take the water when the river is 
swollen and muddy you will get quite a thick deposit. 
This shows that the brooks, the streams, and the rivers 
wash away the land as they flow over it and carry it 
from the mountains down to the valleys, and from the 
valleys away out into the sea. 
But besides earthy matter, which we can see, there 
is much matter dissolved in the water of rivers (as we 
mentioned in the last lecture), and this we can not see. 
If you use water which comes out of a chalk coun- 
try you will find that after a time the kettle in which 
you have been in the habit of boiling this water has 
a hard crust on its bottom and sides, and this crust 
is made of chalk or carbonate of lime, which the water 
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 332,000 million 
oyster-shells, and that if all these shells were built into 
a cube it would measure 560 feet. 
Imagine to yourself a building, perhaps larger than 
any you have ever seen as large, for example, as the 
State, War, and Navy Department buildings at Wash- 
ington an edifice that extends over a space measur- 
ing five hundred and sixty-seven feet in one direction 
and four hundred and seventy-one in the other, com- 
pletely filled up, covered over, and deeply buried in 
a great square mound of oyster shells extending many 
times the height of the building above it; then you 
