DISINTEGRATING WORK OF PLANTS. 447 



rock, may be lifted 1, 3, or 5 meters above the surface level. When the 

 trees decay the inorganic material again joins the soil, producing a mound. 

 The material of the mound is in a much disintegrated and decomposed 

 state ; for it has been in a position to be effectively acted upon by all forces 

 of weathering. How important this effect is can be appreciated only when 

 one travels through the original forests. In such places there are seen 

 almost everywhere hollows where trees have been uprooted and mounds 

 where the material has fallen to the surface. Where tornadoes have swept 

 through the forests, all the trees in their paths have been overthrown at once 

 and it seems as if almost the entire mass of soil and rock to a depth ol 

 several meters had been upturned. This process is well illustrated in the 

 Lake Superior region. The paths of the tornadoes vary from 30 meters 

 to 2 kilometers or more in width. In traveling through the forests of this 

 region one may find the paths of recent tornadoes, those a few years old 

 and those many years old. Where tornadoes have recently cut throu°'h 

 the forests the giant trees have been uprooted and thrown down in a tano-led 

 mass like jackstraws. Where the tornadoes occurred a few years ago the 

 fallen trees are dead and a tangle of briers, brush, and saplings is between 

 the tree trunks, presenting a smooth surface above, but the whole makino- 

 an interwoven mass of live and dead vegetation, which presents all but an 

 impassable obstacle to travel. Still later the fallen trunks show marked 

 decay, and at this time the saplings have become small trees. After many 

 years the great tree trunks and roots are ridges of rotten wood, so decayed 

 that one's foot may pass through them, and the inorganic material held by 

 the roots has fallen to the earth again. The saplings have become trees, 

 but they may be discriminated from the older trees of the adjacent forest. 

 In the final stage of decay one may recognize the path of a tornado by 

 numberless earth mounds, where the soil and rocks have been lifted up by 

 the roots and fallen back, but the second-growth trees can scarcely be 

 discriminated from those of the adjacent forest. 



Burrowing animals are very numerous, and they, like the plants, have 

 an important mechanical effect upon the rocks in the belt of weathering. 

 The effect of animals is, however, somewhat different from that of plants, in 

 that the animals frequently move the soil in a definite direction rather than in 



