106 EXPLANATION OF PLATE 67. 



the sea-shore; this figure is intended to illustra 

 two causes of the production of Springs by desce 

 of water from porous strata at higher levels; t 

 first, producing discharges in vallies of Denudatio 

 along the line of junction of porous with imperm 

 able strata ; the other, by the interruption offeri 

 to descent of water by Faults that intersect t 

 strata. 



The Hills A, C, are supposed to be formed of 

 permeable stratum a, a\ d\ resting on an impe 

 meable bed of Clay h, h'j b". Between these t 

 Hills is a Valley of Denudation, B. Towards the 

 bead of this Valley the junction of the permeable 

 stratum a, a% with the Clay bed bj b' , produces a 

 spring at the point S. ; here the intersection of 

 these strata by the denudation of the valley affords 

 a perennial issue to the Rain water, which falls 

 upon the adjacent upland plain, and percolating 

 downwards to the bottom of the porous stratum a, a', 

 accumulates therein until it is discharged by nume- 

 rous springs, in positions similar to S, near the head 

 and alono; the sides of the vallies which intersect 

 the junction of the stratum a, a'y with the stratum 

 h,b'. See V. I. p. 559.* 



The Hill C, represents the case of a spring pro- 

 duced by a Fault, H. The Rain that falls upon 

 this Hill between H, and D, descends through the 

 porous stratum a", to the subjacent bed of Clay b" . 



* The term Combe, so common in the names of upland Villages, 

 is usually applied to that unwatered portion of a valley, which forms 

 its continuation beyond, and above the most elevated spring that issues 

 into it ; at this point, or spring head, the valley ends, and the Combe 

 begins. The conveniences of vrater and shelter which these spring- t 

 heads afford, have usually fixed the site of the highest villages that 

 are planted around the margin of elevated plains. 



