120 THE FAIRY-LAND OF SCIENCE. 
and so it carries most of the mud away and prevents 
a delta growing up there. If you will look about when 
you are at the seaside, and notice wherever a stream 
flows down into the sea, you may even see little min- 
iature deltas being formed there, though the sea gen- 
erally washes them away again in a few hours, unless 
the place is well sheltered. 
This, then, is what becomes of the earth carried 
down by rivers. Either on plains, or in lakes, or in 
the sea, it falls down to form new land. But what 
becomes of the dissolved chalk and other substances? 
We have seen that a great deal of it is used by river 
and sea animals to build their shells and skeletons, 
and some of it is left on the surface of the ground by 
springs when the water evaporates. It is this car- 
bonate of lime which forms a hard crust over any- 
thing upon which it may happen to be deposited, and 
then these things are called " petrified." 
But it is in the caves and hollows of the earth 
that this dissolved matter is built up into the most 
beautiful forms. If you have ever been to Buxton in 
Derbyshire, you will probably have visited a cavern 
called Poole's Cavern, not far from there, which when 
you enter it looks as if it were built up entirely of 
rods of beautiful transparent white glass, hanging from 
the ceiling, from the walls, or rising up from the floor. 
In this cavern, and many others like it,* water comes 
dripping through the roof, and as it falls slowly drop 
by drop it leaves behind a little of the carbonate of 
lime it has brought out of the rocks. This carbonate 
of lime forms itself into a thin, white film on the roof, 
* See the picture at the head of the lecture. 
