GROUND WATER 141 



or mode, of packing. Thus, if a quantity of shot be poured into a 

 glass, and the quantity of water required to till tlie pores be 

 measured, it will be found that the porosity varies greatly if different 

 methods be used in filling the glass, each producing a different 

 arrangement, but that the same values may be obtained with small 

 shot as with large. (Slichter-37 :<?/.) Slichter has determined that 

 the minimum porosity of a mass of spheres, packed in the most com- 

 pact manner, is 25.95 P^^" cent, of the whole space occupied by the 

 spheres, while the maximum porosity is 47.64 per cent, of the whole 

 mass. ( Slichter-38 -.203. ) 



The pores between the grains are both larger in diameter and 

 shorter in length for a packing of spheres having a large porosity 

 than they are for a packing of low porosity. As already noted, 

 slight variations in the shape of the grains make little difference in 

 the porosity of the mass, but variations in size within the mass 

 make a great difiference. Thus a mass of sand of uniform-sized 

 grains will have a greater porosity than one in which the grains 

 vary greatly. Many rocks of originally great porosity have suffered 

 a considerable reduction in this respect by the filling of the pores 

 by secondarily introduced mineral matter. 



The Water Table. 



The level beneath which the soil is completely saturated is called 

 the level of groundivater, or the water table or ivater plane. This 

 is also the summit plane of the true groundwater, that between it 

 and the surface of the lithosphere constituting the subsurface water. 

 This latter plays an important part in the influence it has on the soil 

 and on plant growth. 



The depth of the water table varies greatly both regionally and 

 seasonally. Where the rainfall is heavy it usually lies only a few 

 feet below the surface, but in arid regions its depth may be several 

 hundred feet. In general the surface of the water table corre- 

 sponds to the surface of the land, but the irregularities are less pro- 

 nounced (Fig. 24.) Where the water table lies deep, it can be 

 reached only by deep wells, and springs are wanting. Where it co- 

 incides with the surface of the ground, the region is a swamp or 

 marsh. 



Depth and Quantity of Ground Water. 



According to \'an Hise (42:595; 43:/(S'o), at a depth of about 

 6 miles (about 10,000 meters) below the surface, the rock pressure 



