152 A TREATISE ON METAMORPHISM. 



tively slow, and that the amount of water which is thus transferred for 

 a given surface is small. But in the soils very large surfaces are available 

 for creep, and therefore this process is a very important one, especially 

 in connection with plant growth. The process is one which especially 

 pertains to the belt of weathering and is therefore later considered. (See 

 pp. 412-423.) 



The rise of the free surface of ground water above the normal level 

 saturating the rocks, the creep of water along the walls without saturation, 

 and the flowage of water through small tubes where there is no free sur- 

 face are generally described under the term capillarity. However, it is 

 evident that under the term thus used are included three very different 

 things. The principles involved in the flow of water through capillary 

 tubes are very different from those which control the free surface of ground 

 water in capillary tubes, and these laws again are different from those 

 which control the creep of water along the walls of openings. 



vegetation — The roots of plants absorb ground water and transport it to 

 the surface. The absorption of water by plant roots causes a relative 

 deficiency of water. This deficiency is remedied by the movement of 

 water from other places toward the roots by the forces already considered. 

 But the influence of roots upon the flow of ground water mainly concerns 

 the belt of weathering. The subject is therefore later considered. (See 

 pp. 417, 422-423.) 



General statements. — In conclusion, it may be said that the immediate cause 

 of movements of ground water are five — gravity, heat, mechanical action, 

 molecular attraction, and vegetation. 



So far as the forces are concerned, the vertical component of the' move- 

 ments of ground water is of far the greatest importance. 



But whatever the cause of the flow of ground water, the direction of 

 movement is from places of greater pressure to places of less pressure. A 

 current going in any direction is evidence of an excess of pressure in the 

 rear of the current. Thus water which enters by seepage or through capil- 

 lary tubes into a larger opening, such as a fissure, must be under greater 

 pressure than the column of water into which it makes its way. Whether 

 the motive force in the movement of the water be difference in gravitative 

 stress or temperature, or any other cause, the excess of pressure resulting 

 in movement is behind the current. 



