22 PHYSIOGRAPHY. [chap. 



in most cases, to allow water to drain more or less freely 

 through them. The particles of which the rock is made up 

 are themselves impermeable, but they are so built together that 

 little spaces, or interstices, are generally left between the 

 individual particles, and the result is the formation of a 

 rock which, hard as it may be, presents a texture approxi- 

 mately like that of a sponge. The water trickles between 

 the particles of such a rock, and thus gradually soaks 

 through its mass. Close in grain as the rock may appear 

 to the eye, it is nevertheless capable, in most cases, of 

 absorbing water; and, hence, stone when freshly taken from 

 the quarry usually holds moisture, known to the workman 

 as " quarry water." Even when a rock offers too close a 

 texture to admit moisture freely, it commonly happens that 

 it is more or less fissured ; and the water which falls upon 

 the rock then dribbles through the little cracks, and thus 

 gains ready access to subterranean channels, much in the 

 same way as it would if the rock were of open texture. 



After a good deal of rain has fallen upon a porous rock, 

 its pores become choked with water, and the rock at last 

 gets saturated, like a piece of sugar which has been dipped 

 for a few moments into a cup of tea. If more rain now 

 fall upon the rock, it can no longer be sucked in and 

 retained, but will flow off the damp surface, just as it would 

 from the surface of an impermeable rock. 



Suppose a layer of a porous substance to rest upon a bed 

 of comparatively impervious rock, and it is easy enough to 

 see what, under such circumstances,' will become of the rain 

 which falls upon the surface. Let Fig. 6 illustrate such a 

 case. Here the dotted part of the figure ABC D,. repre- 

 sents a permeable rock, say beds of sand, whilst the lower 

 shaded portion, C D E F, indicates an impermeable rock, 

 say a stiff clay. It is supposed, in a figure such as this, that 



