SPRINGS AND MARSHES. 95 



scale, sometimes materially lessen the supply of 

 water at places remote from the scene of operations. 

 " In the instance already given, the water forming 

 the spring is represented as gaining access to the 

 porous stratum at a point where it crops out from 

 beneath an impervious one, and as passing along to 

 its point of discharge at a considerable depth, and 

 under several layers of various character. Some- 

 times, in an undulating country, large tracts may 

 rest immediately upon some highly porous stratum, 

 as from (b) to (c) in the following diagram, render- 

 ing the necessity of draining less apparent, while 

 the country from (a) to (h) and from (c) to (d) may 

 be full of springs and marshes. This arises partly 

 from the fact that the rain which falls in these 

 latter districts is unable to find a way of escape, and 

 partly because the natural drainage of the more 

 porous soils adjoining is discharged upon it. 



" Again : the rocks lying under the surface are 

 sometimes so full of fissures, that although they 

 themselves are impervious to water, yet so com- 

 pletely do they carry off rain, that in some parts of 

 the county of Durham [England] the sinking of 

 wells is rendered useless, and the farmers find it 



