THE NEW AGRICULTURE. 87 



some impervious rock like clay, underlies another layer of a porous 

 material, like sand. The rain which falls on the surface of the 

 ground, and sinks through the upper bed, will be arrested by the 

 lower one, and made either to gather there, or find its escape along 

 the surface of that lower bed. If a hollow or valley should have 

 its bottom below the level of the line along which the water flows, 

 springs will gush out along the sides of the valley. The line of es- 

 cape may be either the junction between two different kinds of 

 rock, or some of the numerous joints already referred to. What- 

 ever it be, the water cannot help flowing onward and downward, as 

 long as there is any passage along which it can find its way; and 

 the rocks underneath are so full of cracks, that it has no difficulty 

 in doing so. 



" But it must happen that a great deal of the underground 

 water descends far below the level of the valleys, and even below 

 the level of the sea. And yet, though it should descend for several 

 miles, it comes at last to the surface again. To realize clearly how 

 this takes place, let us follow a particular drop of water from the 

 time it sinks into the earth as rain, to the time when, after a long 

 journey up and down in the bowels of the earth, it once more 

 reaches the surface. It soaks through the soil together with other 

 drops, and joins some feeble trickle, or some other ample flow of 

 water, which works its way throiigh crevices and tunnels of the 

 rocks. It sinks in this way to perhaps a depth of several thousand 

 feet until it reaches some strata through which it cannot readily 

 make further way. Unable to work its way downward, the pent- 

 up water must try to find escape in some other direction. By the 

 pressure from above it is driven through othor cracks and pass- 

 ages, winding up and down until at last it comes to the surface 

 again. It breaks out there as a gushing spring. 



" Rain is water nearly in a state of purity. After journeying up 



