FOREST SOCIETIES. 547 



the air with vapor. In mountainous countries where we can 

 obtain a bird's-eye view of large forest areas, it is a common and 

 interesting sight to see the cloud formation. Here and there, as 

 the invisible water-vapor rises and comes in contact with the 

 cooler air, one can see forming clouds of white mist in the morn- 

 ing which grow in size as they rise, unite with others, become 

 more dense and center around and obscure some large mountain 

 peak as they change into black "thunder-clouds," then move and 

 precipitate their moisture over valley and mountain in the after- 

 noon. Even when the air is not cool enough to cause precipita- 

 tion of the moisture, the mist coming in contact with the cool 

 surfaces of leaves, twigs, and branches condenses into water 

 which drips from their surface. 



1032. Importance of the forest in the disposal of rainfall. 

 The importance of the forest in disposing of the rainfall is very 

 great. The great accumulation of humus on the forest floor 

 holds back the water both by absorption and by checking its flow 

 so that it does not immediately flow quickly off the slopes into the 

 drainage system of the valley. It percolates into the soil. Much 

 of it is held in the humus and soil. What is not retained thus 

 filters slowly through the soil and is doled out more gradually 

 into the valley streams and mountain tributaries, so that the 

 ilood period is extended, and its injury lessened or entirely pre- 

 vented, because the body of water moving at any one time is 

 not dangerously high. The winter snow is shaded and in the 

 spring melts slowly, and the spring freshets are thus lessened. 

 The action of the leaves and humus in retarding the flow of the 

 water prevents the washing away of the soil; the roots of trees 

 bind the soil also and assist in holding it. 



1033. Absence of forest encourages serious floods. The great 

 floods of the Mississippi and its tributaries are due to the rapid- 

 ity with which heavy rainfall flows from the rolling prairies of 

 the west, and from the deforested areas west of the Alleghany 

 system. The serious floods in recent years in some of the South 

 Atlantic States are in part due to the increasing area of deforesta- 

 tion in the Blue Ridge and Southern Alleghany system. The 



