IOO PLANTS 



a conservative force in protecting the surface of the land from 

 erosion by holding the soil in position and breaking the force 

 of the rush of surface waters, which rapidly wears away naked 

 or unprotected soils. It is even a constructive agency in many 

 cases, as, for example, in the formation of deposits of coal, peat, 

 iron ore and other minerals, and in filling up swamps, ponds, 



FIG. 56. A solid-rock surface covered with lichens, mosses and ferns. 



lakes and sluggish streams with the wash from the land, and 

 even in places building out the land into the" shallow open sea. 

 223. The general climatic conditions of a region may also 

 be modified by changes in the vegetation so as to determine 

 very materially the social and economic conditions of man. The 

 deforestation of a region has in more than one instance resulted 

 in the practical destruction of a highly developed civilization. 



