THE WORK OF RUNNING WATER = 15 
ness is due to little particles of soil which the water 
has picked up, and is carrying along “ in suspension,”’ 
as the geologists say. Look at the bottom of the 
pool that has been formed ; towards the middle you 
will see a very thin layer of mud over the cinder-grit. 
Of course, you can easily see how the mud came 
there ; it is a collection of the fine particles of soil 
that have been carried by the little streams from 
the bank, and dropped on to the bottom of the pool. 
Now, if the pool were to remain for some years, and 
if the streams continued to deposit little particles of 
soil in it, the mud would get thicker and thicker. 
Suppose the mud were deposited at the rate of only 
a twentieth of an inch per month ; in the course of 
five years a thickness of three inches would be 
accumulated, and in a century the deposit would be 
vastly increased. | 
There is an overflow from the pool. The water is 
not scattering over the road, but is making a crooked 
channel for itself through the soft cinder-dust. 
The little stream is rushing at high speed down the 
sloping road towards a drain. Ten hours ago the 
road was almost smooth, but now the water has 
broken its even surface, and just where we stand it 
has made a channel quite six inches deep. Come to 
the bottom of the road where the water enters the 
drain, Just round the grating, which is a little 
higher than the road, there is a collection of mud 
and cinder-grit ; it is material from the road which 
the stream has carried away in making its channel, 
and deposited where you see it. I calculate that 
this deposited material will weigh at least fifty 
pounds, and we can realize that quite that weight 
