418 A TREATISE ON METAMOEPH1SM. 



If precipitation comes suddenly and abundantly an upper layer of the 

 soil may become saturated, and this layer be separated from the belt of 

 saturation below by a layer in which the pore spaces are largely occupied 

 by air. The air below the surface layer of saturation prevents the ready 

 passage of the water downward, and if precipitation continues a large part 

 of the falling water may be prevented by the air from easily making its 

 way into the soil, and consequently joins the run- off. But, as explained 

 later, where the soil is cultivated deeply, so as to leave a considerable 

 percentage of large openings near the surface, and from this supply a 

 temporary reservoir is furnished, a considerably larger proportion of the 

 precipitation may get into the ground than in uncultivated areas. 



The water in an upper saturated layer may make its way downward 

 in two ways, (1) the confined air below slowly escapes, either by upward 

 creep or through the rare larger passages which are not filled with water, 

 and the water consequently moves downward; and (2) if the soil particles 

 below the layer of saturation are moistened or contain water of imbibition, 

 the water driven by gravity and capillarity slowly creeps downward along 

 the surfaces of the particles. 



If, however, the particles below the upper layer of saturation be entirely 

 dry, this condition exerts a strong retarding influence. Says Whitney: 

 "Water does not readily spread through a previously dry soil, because the 

 tension or contracting power of the surface of the water is greater than the 

 attraction of the soil grains, which tends to cause its diffusion through 

 the mass. One may see, therefore, a nearl}* saturated layer closely adjacent 

 to a perfectly dry and dusty mass." The explanation of this retardation 

 is expressed differently by Wolff, who attributes it to the elastic meniscus 

 at the front of the moving water. 6 (See p. 141.) 



The above condition of affairs is especially likely to occur in the arid 

 and semiarid regions, where a large part of the belt of weathering is very 

 dry. According to Whitney, even where the annual rainfall is as much as 

 50 cm., nearly the entire amount may remain within a few meters of the 



« Whitney, Milton, Conditions in soils of the arid region: Yearbook of the Dept. of Agric, 1894, 

 p. 160. King, F. H., Principles and conditions of the movement of ground water: Nineteenth Ann. 

 Kept. U. S. Geol. Survey, pt. 2, 1899, p. 93. Merrill, George P., Bocks, rock-weathering, and soils, 

 Macmillan Co., New York, 1897, pp. 379-380. 



b Wolff, H. C, The unsteady motion of viscous liquids: Trans. Wisconsin Acad. Sci., Arts, and 

 Letters, vol. 12, pt. 2, 1900, pp. 552-553. 





