188 



STUDIES OF TKEES 



own quantity of water; i.e., for every foot in depth of soil 

 there can be six inches of water and, when thus saturated, 

 the soil will act as a vast, underground reservoir from whic^h 

 the s])rine;s and streams are su])])lied (V\^. 125). Cut the 

 forest down and the land becomes such a desert as is shown 

 in Fig. 126. The soil, leaves, Ijranches and fallen trees 



Fig. 128.— Flood in Pittsburgh, Pa. 



dry to dust, are carried off by the wind and, with the fall 

 of rain, the soil begins to wash away and gullies, such as 

 are shown in Fig, 127, are formed. Streams generally 

 have their origins in mountain slopes and there, too, the 

 forests, impeding the sudden run off of the water which is 

 not immediately absorbed, prevent soil erosion. 



Where the soil is allowed to wash off, frequent floods 



