412 A TKEAT1SE ON METAMORPHISM. 



account of the slow movement a greater head is necessary in order that 

 equilibrium shall obtain in the sea of ground water between additions 

 from precipitation and subtractions by flowage. Second, the level of the 

 ground water is also dependent upon capillarity. The effect of capillarity 

 is to raise the water surface in the small openings of the rocks above its 

 natural level as found in wells. The amount of this rise is inversely 

 as the diameter of the openings This amount, as shown on page 151, 

 varies from nothing in the largest capillary tubes to 166 meters in 

 circular tubes of limited size between capillary and subcapillary openings, 

 and to 83 meters in similar sheet openings. In circular capillary openings 

 between 1 mm. and 0.001 mm. in diameter the rise varies from 3.32 cm. to 

 33. 2 meters, and in similar sheet openings one-half of this amount. A 

 comparison of the above numbers shows that in openings limiting those of 

 capillary and subcapillary size the rise is five times these larger amounts. 

 When it is remembered that in large classes of sediments, such as fine sands 

 and clays, the openings approach subcapillary size, or even are of subcapillary 

 size, it is evident that the amount which the level of ground water may be 

 raised as a result of capillarity may be a very important factor in determining 

 its position. 



While the level of ground water in rocks in which the openings are 

 capillary or subcapillary may be greatly affected by capillarity, in the 

 supercapillary and larger capillary openings this is not an appreciable 

 factor. It follows that there may be great changes in the level of ground 

 water in passing from places where the openings are supercapillary to where 

 they are capillary, and from places where the openings are capillary to 

 where they are subcapillary. 



Illustrations of these principles are found in all countries. In fractured 

 limestone regions containing caves the level of ground water may follow 

 approximately that of the drainage of the district, and thus there be a great 

 difference between the topography of the surface and that of the ground- 

 water level. Where a region is covered with a thick mantle of fine material, 

 as drift, the topography of the ground water may very closely follow that 

 of the surface. A good illustration of an open limestone area is furnished 

 by the Grand Canyon region of Colorado. In this district the topography 

 is much accentuated and the rocks are largely limestone, commonly much 



