94 



niPEEVIOUS STRATA. 



" When rain falls in sucli a district, after sinking 

 through the surface layer (represented in the dia- 

 gram by a narrow band) it reaches the stratified 



layers beneath. Through these it still further sinks, 

 if they are porous, until it reaches some impervious 

 stratum, which arrests its progress directly down- 

 ward, and compels it to find its way along its upper 

 surface. Thus the rain which falls on the surface 

 represented between (b) and (d) is compelled by the 

 impervious stratum to flow toward (c) ; here it is at 

 once absorbed, but is arrested by the impervious 

 layer (e) ; it is therefore compelled to pass through 

 the porous stratum (c), along the surface of (e), to 

 (rt), where it pours forth in a fountain, or forms a 

 morass or swamp, proportionate in size and extent 

 to the tract of country between (b) and (d), or the 

 quantity of rain which falls upon it. In such a 

 case as is here represented, it will be obvious that 

 the spring may often be at a great distance from the 

 district from which it derives its supply, and this 

 accounts for the fact that drainage-works, on a large 



