832 WATER-BEAEING STRATA. 



These Limestone areas generally store their water in this 

 manner, which finds its way ont through "swallets," or 

 underground channels, or gushes out in large volumes 

 (where it is thrown out by faults, or upthrows, or other 

 dislocations), like the springs at Cheddar and also on the 

 north side of the Mendips at Kickford, near Blagdon (eleven 

 miles south-west of Bristol), where it flows out of the chasm 

 in the almost perpendicular side of the limestone, at the rate 

 of three or four million gallons per day, increasing very 

 quickly and copiously after heavy rainfall. 



Again, at Banwell we have another instance of the outle t 

 or final appearance of one of these swallets, or underground 

 rivers, draining the Mountain Limestone area on the southern 

 and western side of the Mendip range, yielding from three to 

 eight million gallons per day in the driest summer ; the pond 

 at Banwell, which covers about one and a half acres, and is 

 from three to five feet deep, filling in two hours. 



The Chalk Formation. — The chalk formation which sur- 

 rounds London, stretching southwards to Hampshire, Kent, 

 and Sussex, and eastwards taking in a great part of Hertford- 

 shire and the whole of Norfolk and nearly all Suffolk, as 

 well as 23art of the north-east of Lincolnshire and Yorkshire, 

 is a very fruitful source of water supply. It covers an area 

 of 26,600 square miles in England, of which 19,000 square 

 miles are overlaid by impermeable strata, such as clay. 



V/ells are sunk into this formation in the London basin 

 and district adjoining, around Newbury, Wokingham, 

 Leatherhead, and Rickmansworth, and on the outcrop of 

 the chalk of the London formation, and also at Southampton 

 and Portsmouth and numerous other places. 



The London clay superimposing the chalk renders it more 

 dense than it would otherwise be, where it is without such 

 weight, and would consequently leave a freer passage for 



