422 A TREATISE ON METAMORPHISM. 



the day mainly affects the cultivated layer. This contains but little mois- 

 ture, and consequently evaporation is much slower than in areas where the 

 capillary openings extend unbroken to the heated surface. Consequently 

 soils cultivated as soon after precipitation as the water has had time to 

 make its way to the subsoil may retain considerable amounts of water 

 a short distance below the surface during long periods of drought. 



VEGETATION. 



In proportion as vegetation is present, this becomes an important 

 influence working in conjunction with molecular attraction in the circula- 

 tion of water in the belt of weathering. In another connection it will 

 be shown that roots commonly permeate the soil both laterally and 

 vertically. Ordinary herbaceous plants frequently have roots extending 

 to a depth of 1 to 2 meters, and the roots of trees penetrate to a 

 depth of 5 to 6 meters, or even 9 to 12 meters. The lateral extent of 

 many plant roots is even greater than their vertical extent. In areas of 

 abundant vegetation the upward movement of water is much more largely 

 the result of plant roots than of molecular attraction. According to 

 Merrill, in forested areas the total amount of evaporation and transpiration 

 from the trees amounts to about 75 per cent of the total precipitation. In 

 areas covered by other vegetation the total varies from 70 to 90 per cent, 

 depending upon the character of the plants." If these figures be correct, 

 they show how important an agent the roots of plants are in the upward 

 transfer of ground waters to the surface. In proportion as the region 

 is arid the volume of the roots decreases. But even in the deserts the 

 amount of plant life is surprisingly great. Moreover, in deserts the roots 

 are often several times as extensive as the parts of the plants above 

 ground, so that in such regions roots are very important factors in the 

 circulation of the small amounts of water of the belt of weathering*. It 

 is only in the comparatively small areas where the deserts are absolute 

 that roots are altogether wanting. 



On the average, much the larg'er quantity of water brought to or above 

 the surface by plants is derived from the belt of weathering. But it is 

 clear that in many areas, and especially in low-lying areas, where the level 



"Merrill, George P., Rocks, rock-weathering, and soils, Macmillan Co., New York, 1S97, pp. 

 280-281. 



