THE TRAVELS OF THE UNDERGROUND WATER 181 



Wherever in descending from the surface an impervious layer, 

 such as clay, is encountered, the further downward progress of the 

 water is arrested. Now conducted in a lateral direction it issues 

 at the surface as a spring at the line of emergence of the upper sur- 

 face of the impervious layer (Fig. 189). 



FIG. 189. Diagram to show how an impervious layer conducts the descending 

 water in a lateral direction to issue in surface springs. 



The trunk channels of descending water. While within the 

 unconsolidated rock materials near the surface of the earth, it is 

 clear that water can circulate in proportion as the materials are 

 porous and so relatively pervious. As the pore spaces become 

 minute and capillary, the difficulty of permeation through the 

 materials becomes very great. Thus in the noncoherent rocks 

 it is the coarse gravel and the layers of sand which serve as the 

 underground channels, while the fine clays have the effect of an 

 impervious wall upon the circulating waters. In coarse sand as 

 much as a third of the volume of the material is pore space for the 

 absorption and transmission of water. Even under these favor- 

 able conditions the movement of the water is exceedingly slow 

 and usually less than a fifth of a mile a year. 



Within the hard rocks it is the sandstones which have the largest 

 pore spaces, but in 

 nearly all consolidated 

 rocks there are addi- 

 tional spaces along 

 certain of the bedding 

 planes, the joint open- 

 ings (Fig. 190), and 

 the crushed zones of 

 displacement, so that 

 these parting planes 

 become the trunk 



channels SO to Speak -^ IG> 190. Sketch map of the Oucane de Chabrieres 



f , i y .. near Chorges in the High Alps, to illustrate the cor- 



CUiaimg rogion of limestone along two series of vertical joints 



Water. It is along (after Martel). 



