WATER-BEARING STRATA. 331 



We have had instances of such years, in 1864 and 1887, 

 when the rainfall was thirty- three per cent, below the 

 average. In the current year, up to the present it is equal 

 to only twenty-nine inches, or nearly twenty-five per cent, 

 below the average. 



Well-sinking Sites. — In sinking wells for large water 

 supplies it is of course necessary to select the site which 

 ^\'ill be most likely to prove favourable to a good yield, and 

 this entails a certain amount of geological knowledge to 

 ensure the success which should attend the operation ; for, 

 although it is pretty generally known in what strata the well 

 should be sunk, it does not always follow that the position 

 of such strata is favourably situated for obtaining water. 



For instance, we should not select the lias formation 

 capping the top of a Carboniferous Limestone hill, where there 

 would be little or no watershed, unless we knew of the 

 existence of water under such strata, indicated by springs, 

 which would most probably arise from some other formation 

 (say New Red Sandstone), from which water was thrown up 

 by the superimposed clay. Nor should we search in New 

 Red Sandstone where its elevation was considerably above 

 the sea level, and on the crest or flank of a hill with a sharp 

 inclination towards a valley drained by natural springs and 

 rivers. Nor again should we seek water in a compact mass of 

 Mountain or Carboniferous Limestone, which is generally of 

 immense thickness (2,000 feet or more in the Bristol district), . 

 and sheds water through large open joints and fissures at a 

 high angle to any depth, although huge quantities of water 

 are stored up in the large caverns so frequently met with in 

 the Mountain Limestone, notably at Cheddar, and in the 

 sinking of the Severn tunnel shaft at Sudbrook, which drew 

 the water from a large tract of Mountain Limestone district 

 between there and Wentwood^. 



