106 EXPLANATION OF PLATE 67. 



stratified rocks, terminated abruptly by a cliff on 

 the sea-shore; this figure is intended to illustrate 

 two causes of the production of Springs by descent 

 of water from porous strata at higher levels; the 

 first, producing discharges in valleys of Denudation, 

 along the line of junction of porous with imperme- 

 able strata ; the other, by the interruption offered 

 to descent of water by Faults that intersect the 

 strata. 



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

 permeable stratum a, a>, a sy , resting on an imper- 

 meable bed of Clay b, b', b". Between these two 

 Hills is a Valley of Denudation, B. Towards the 

 head of this Valley the junction of the permeable 

 stratum a, a', with the Clay bed b, 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 down- 

 wards 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 along the sides of the valleys which intersect the 

 junction of the stratum a, a', with the stratum b, b'. 

 See V. I. p. 417.* 



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 water and shelter which these spring- 

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

 planted around the margin of elevated plains. 



