420 A TREATISE ON METAMORPHISM. 



the surface is very difficult to estimate. It is everywhere an important 

 force, but is more important in proportion as there is lack of vegetation. 

 In the complete desert regions this force acts alone in returning the water 

 to the surface. In such regions practically the entire precipitation may be 

 returned to the surface by capillarity. This is likely to occur where, on 

 account of sudden and abundant precipitation, a saturated layer forms 

 above a layer not saturated, and especially where the layer below is dry. 

 It has already been explained that under such conditions the water works 

 its way downward with great difficulty, and, remaining near the surface, it 

 is rapidly evaporated. 



While much the larger part of the water returned to the surface by 

 molecular attraction is the water of imbibition in the belt of weathering, 

 a portion of the water may be derived from the belt of cementation, and 

 thus pass upward entirely through the belt of weathering in reaching the 

 surface. The effect of capillarity in raising the level of ground water above 

 the normal level is spoken of on page 412 and on pages 150-152. It is also 

 explained that, due to the attraction between the molecules of rock and 

 water, water is drawn along the walls of the openings of the mineral 

 particles beyond the height of the free surface. 



The amount of water which may be transported upward a given dis- 

 tance by molecular attraction is a function of the absolute amount of pore 

 space and the size of the openings. With a given pore space the amount 

 increases with fineness of subdivision to a certain limit. The amount of 

 surface of the particles increases as subdivision increases. Therefore, in 

 finely subdivided material the wall space along which the water can creep 

 is very much greater than in coarse material. To illustrate, it will be 

 readily appreciated that the amount of water which capillarity would carry 

 up from the free surface through a distance of 1 meter in a coarse grained 

 sandstone is small as compared with that which would be transported 

 upward for the same distance through a soil having the same absolute 

 amount of pore space. But where the openings are so small as to be subcap- 

 illary the water adheres to the walls, and the amount of water transmitted 

 decreases. Thus, through a fine clay having the same pore space as an 

 ordinary soil, the amount of water transported upward would be less than 

 in the soil. 



The quantity of water which may thus move upward from the belt of 

 saturation has been experimentally determined by King for some materials 



