PROPERTIES OF WATER AND SOURCES OF SUPPLY. 1 7 



cases of wet weather it may be in the form of a 

 convex curve, the convexity being towards the sur-* 

 face, causing what are known as intermittent springs. 

 In carboniferous limestone, which may be regarded 

 as a permeous rock, boulders of clay and unpermeable 

 strata are found. This will also give rise to a spring, 

 because these boulders, occurring in fault in the 

 limestone, constitute a barrage across the natural 

 path of the subterranean water, which immediately 

 rises up the side of the barrage and so to the surface, 

 if sufficient pressure is present on the water behind. 

 These faults are often in parallel lines across the 



(hiterop^l. 



Clay 



FIG. 5. 



limestone right down to low levels, where, of course, 

 owing to the greater pressure, most water is obtained. 

 By far the most important water-bearing strata 

 in this direction, however, is chalk. Chalk has an 

 extraordinary power of holding water, 1 and is to be 

 found over the greater part of the south of England 

 and in most other places. It usually lies below 

 tertiary sands and clays. The inclination of the 

 saturation surface in chalk varies, the variation being 

 usually dependent on the angle at which the stratifica- 

 tion dips. The average dip is about 12 ft. per mile, 

 but may reach 100 ft. in places. 



1 One cubic foot of chalk can hold two gallons of water, or nearly one- 

 third of its bulk. Some chalk rocks which have been subjected to igneous 

 action, as is the case in Co. Antrim, hold but little water. 



2 



