DIPERVIOUS STRATA. 93 



found some bed of rock or clay impervious to water. 

 The tendency of the rain which falls upon the earth 

 is to sink directly downward; but, turned aside 

 by the many obstacles referred to, it often passes 

 obliquely, or almost horizontally through the soil. 

 The drop which falls upon the hill-side sinks per- 

 haps a few inches, when it meets with a bed of clay, 

 upon which it glides along for many days, until at 

 last it is borne out to the surface, to be drunk up 

 by the sun on some far-off slope. Another, falling 

 upon the sandy plain, sinks at once to the water- 

 line which rests on clay beneath, and, slowly creep- 

 ing along, helps to form a swamp or bog in the 

 valley. Sometimes the rain which falls upon the 

 high land is collected together by fissures in the 

 rocks, or by seams or ruptures in the impervious 

 strata below the surface, and finds its vent in a 

 gushing spring upon the hill-side." 



When rain falls on a tract of country, part of it 

 flows over the surface, and makes its appearance in 

 the numerous natural and artificial courses which 

 may exist, while another portion is absorbed by the 

 soil and the porous strata which lie under it. Let 

 the following diagram represent such a tract of 

 country, and let the portion (e) represent clay, or 

 other impervious strata, while the remaining por- 

 tions exhibit those layers of gravel, sand, or chalk 

 which permit a free passage for the water.^ 



1 Cyclopedia of Agriculture, by Girdwood. 



