416 A TREATISE ON METAMORPHISM. 



However, it does not follow that the water present in. the belt of 

 weathering at any one time is as great as the amount in the belt of cemen- 

 tation. Indeed, the amount of water in the belt of weathering is only a 

 small fraction of that in the belt of cementation. The explanation of this 

 is that the water contributed to the belt of weathering remains in the 

 belt only a short time — a few minutes, a few hours, a few days, or possibly 

 a few years; while much of the water of the belt of cementation remains in 

 that belt for many years. In the case of the deeper and longer circulations 

 of the latter belt, as shown on pages 585-586, much of the water must 

 have remained underground for centuries. Therefore, while a much larger 

 percentage of the water of precipitation takes part in the circulation of the 

 belt of weathering than in the belt of cementation, at any given time the 

 amount of water at work in this belt is much less than that in the belt of 

 cementation. 



THE CIRCFLATIOX. 



It has already been seen (pp. 146-152) that the chief factors which 

 control the movement of the water in the belt of weathering are (1) 

 gravity, (2) mechanical movement, (3) molecular attraction, and (4) plant 

 roots. The special action of these factors in the belt of weathering needs 

 consideration. 



The force of gravity is vertical and downward, and therefore, so far as 

 this force is concerned, there is a continual tendency for water to be drawn 

 from above the surface of the ground into and directly through the belt of 

 weathering to the belt of cementation. Where the different parts of the 

 water interfere, as they do in the belt of cementation, there may be super- 

 imposed on this vertical movement very important lateral movements, as 

 seen on pages 572-576. But the interference due to hydrostatic pressure 

 comes only when the rocks are saturated. In general the rocks of the belt 

 of weathering are not saturated; and hence the lateral movements in this 

 belt due directly to gravity are unimportant, except locally and for short 

 periods of time. 



In the belt of weathering the effect of mechanical movement controlled 

 by gravity is to steadily make the soils more compact, and thus to decrease 

 the size of the openings and so to promote capillary action. Also, to some 

 extent, mechanical consolidation tends to squeeze out the moisture and so 

 to force it to the surface or to the belt of cementation. 



