1022 A TREATISE ON METAMORPHISM. 



movement due to difference in temperature, but probably it is of great 

 consequence only where the heat increment of one column is more than 

 normal on account of mechanical action or of igneous rocks, or both. 

 Mechanical action may squeeze out the water in the openings of rocks or 

 possibly even some of the combined water and thus produce circulation. 

 (See pp. 149, 661-665.) 



Where the rocks are saturated, whatever the cause of the flow of under- 

 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 capillary 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, of deformation, or any other cause, the 

 excess of pressure resulting in movement is behind the current. In propor- 

 tion as the opening approaches a circular form the flow increases because 

 the friction between the moving water and the film of fixed water upon the 

 walls is less per unit volume. The more continuous the openings, the more 

 rapid is the flowage. Flowage increases as the size of the openings increases. 

 In super-capillary openings the ordinary laws of hydrostatics apply, and 

 therefore the flowage may be very rapid. In capillary openings the laws 

 of capillary flow apply, and the movement of water is slow. Where the open- 

 ings are subcapillary, the attraction of the mineral particles extends from wall 

 to wall, the water films are glued to the rocks, and flowage is inappreciable. 

 Flowao-e increases as the amount of openings or the pore space increases. 



So much for the laws controlling the general circulation of aqueous 

 solutions. The combinations of the various factors are so fundamentally 

 different in their effects in the zone of fracture and zone of combined frac- 

 ture and flowage that further statement is necessary in reference to them 



CIRCULATION IN ZONE OF FRACTURE, OR ZONE OF KATAMORPHISM. 



In the zone of fracture a great many openings are of super-capillary 

 and capillary size, and many of them are continuous. It is clear that in 

 this zone the conditions are very different in the belt of weathering above 

 the level of ground water where the rocks are commonly not saturated and 

 in the belt of cementation below the ground water where the rocks are 

 saturated. Each belt requires consideration. 



