RELATIONS BETWEEN SOLUTION AND DEPOSITION. 615 



journey in a formation, even if but a small fraction of that in the case men- 

 tioned, should not become saturated with the compounds with which it is 

 in contact. 



The journey of the water in the belt of cementation involves a com- 

 ponent parallel to the surface and a vertical component. As a result of the 

 component of the movement of ground water parallel to the surface there 

 is a tendency for material to be taken into solution and to be abstracted 

 by the water. For a given depth the pressure and the temperature are, on 

 the average, the same. The longer this course the nearer will the water 

 approach to saturation with the compounds with which it is in contact, 

 because of the time factor. If the journey be long, the state of saturation 

 may be attained at a comparatively early stage, after which the additions 

 and subtractions of material may be presumed, on the average, to neutral- 

 ize each other, although in a given instance the total amount of material 

 held in solution would vary greatly, depending upon the character of the 

 rocks with which the water is in contact. 



Under normal conditions, so far as the vertical component is concerned 

 there is steadily increasing pressure and temperature during the downward 

 movement, and steadily decreasing pressure and temperature during the 

 upward movement. If the places of entrance and issue were at the same 

 level, these two factors might be considered to neutralize each other, but 

 the descending column is necessarily the longer; therefore the balance of 

 the two processes is in favor of solution rather than deposition. Also, unless 

 the water has become saturated during its descent solution will continue 

 during its ascent until saturation is attained, and if the vertical component 

 be short, this may not occur before the entire ascent has been made. 



Finally, the temperature of the entering and issuing water is of great 

 consequence in the matter of relative quantities of dissolved and deposited 

 materials. Even if the temperature were precisely the same at the enter- 

 ing and issuing points this would favor solution, for increase in temperature 

 as the water descends very greatly accelerates the process of solution, and 

 as the water ascends precipitation takes place only when saturation is 

 reached, and then only to an extent sufficient to prevent supersaturation. 



But into this matter of temperature another very important factor 

 enters. It has been shown on pages 589-592 that, on the average, the 

 temperature of the issuing water is higher than that of the entering water. 



