416 Notices of Memoirs — The Water- Supply of London. 



absorbed, or percolates tbrongli tbe fissured surface. So close is its 

 texture that the bulk of the rain takes weeks and months to filter 

 down to the level of the water line in the interior of the Chalk 

 hills — a line the depth of which below the surface of the ground 

 may vary from 100 to 300 feet according to the height of the hills. 

 The water thus stored escapes in several ways — some by the streams 

 rising within the Chalk district, — some by springs feeding directly 

 the larger rivers flowing through it, — another portion overflows at 

 the outer escarpment of the Chalk, — and a larger portion issues near 

 its junction with the Tertiary strata. A certain quantity also passes 

 underground, supplies the wells, in the central Tertiary area, and 

 escapes in part at still lower levels at more distant points. 



Where the rise of the bottom of the valleys is more rapid than 

 that of the line of water level, the valleys assume the character so 

 common in Chalk districts, of dry vallej's. Others of these valleys 

 tap the springs in their lower part, whilst the upper part of the 

 same valley is dry. In these cases the head of the stream will often 

 change its position two or three miles higher or lower in the valley, 

 accordingly as the rise and fall of the water level in the hills are 

 influenced by the rainfall. Where the deeper and larger river 

 valleys traverse the Chalk area and intersect the line of water level, 

 these valleys become fringed on the river level with a series of 

 springs, as the Thames in its course from Wallingford to Taplow, 

 the Lea above Broxbourne, the Eavensbourne, the Cray, and the 

 Darenth, and the Thames again from Woolwich to Gravesend. 



The springs along the line of outcrop of the Chalk-marl and Gault 

 being on a higher level than any others, the head of water supplying 

 them is much smaller than that supplying the springs on lower 

 levels within the Chalk area, and consequently with few exceptions 

 these springs are small. They are, however, extremely numerous. 

 Almost every little village under the range of the Chalk downs in 

 Wiltshire, Oxfordshire, and further eastward, has its spring near the 

 foot of the Chalk hills. These collectively would furnish a con- 

 siderable quantity of water, but they are too scattered and wide 

 apart to be available for any general purpose. There are, however, 

 a few large springs amongst them. There is one, for example, at 

 Cherhill, near Calne. This spring never fails, and is said to yield 

 from two to three million gallons of water daily. There are also 

 copious springs near Ellensborough, at Barton-in-the-Clay, near 

 Prince's Eisborough, near Swindon, and at many places along the 

 foot of the North Downs of Kent and Surrey. 



Another and more important class of springs are those which 

 escape along the inner edge of the Chalk along or near the line 

 where it passes under the Tertiary strata, and again where it 

 approaches the sea level. These springs are all placed on relatively 

 low levels, and derive their supplies from the large head of water 

 which extends in the Chalk hills beyond them up to the outcrop of 

 the beds underlying that formation. As the difference of level 

 between these exterior and interior springs varies often from 150 

 to 300 feet, the latter are necessarily much more powerful and 



