14 THE BOOK OF FORESTRY 
covering a tin roof. If water is allowed to fall upon 
a steep roof, it runs immediately to the spout, whereas 
if this same roof were covered with a layer of spongy 
material a very large amount of the shower would be 
absorbed and very gradually released. 
In a light summer shower it is estimated that from 
twenty-five to sixty per cent of the rainfall never 
reaches the ground at all, but is caught upon twigs 
and limbs and trunks of the trees. After the shower 
has ceased this moisture is evaporated directly into the 
air without reaching the soil. 
Of the rain that does reach the ground, a very 
large amount is absorbed by the humus, that organic 
layer of partly decomposed leaves and twigs which lies 
on top of the mineral soil. This humus has a marvelous 
capacity for holding water; in fact it can hold many 
simes its own weight. The trunks of the trees and 
supporting roots form countless little basins on every 
hillside and when the humus has become thoroughly 
soaked so that it can no longer contain another drop, 
these little basins gently fill and overrun one into the 
other, allowing the water to reach the brook very 
gradually. 
When the forest cover is removed the humus is 
decomposed by the air; the leaves, twigs and branches 
no longer catch the rain and as a consequence the water 
rushes immediately from the steep slope to the river 
bed below. In case of heavy rains the soil may be 
badly washed and gullied and large amounts of gravel 
and stones may be washed from the steep slopes upon 
the flats and fertile fields farther down the river. In 
many of the Southern States the heavy clay soils are 
often badly eroded during torrential rains, and it is 
no uncommon sight to see rolling fields located just at 
