Notices of Memoirs — The Water- Supply of London. 417 



permanent, and will continue to run long after most of the others 

 have become dry. Among the more copious and remarkable springs 

 of this class are those at Chadwell near Ware, Otter's Pool near 

 "Watford, Froxfield near Hungerford, Beddington and Carshalton, 

 Orpington, Grays Thurrock, Springhead near Gravesend, Ospringe 

 near Faversham, besides a number of smaller ones. The origin and 

 source of supply of some of these springs are indicated in the 

 sections. 



In the neighbourhood of London the wells in the Chalk form an 

 important auxiliary source of water-supply, and they might, no 

 doubt, be considerably increased in Kent without interfering with 

 the springs in the valleys above London, as the store from which 

 those wells draw their supplies overflows in numberless springs 

 along the Thames below London at levels where they are not 

 generally'- available. 



The lower beds of the Chalk are so argillaceous as often to hold 

 up the water and to lose their ordinary permeable character. 



The Upper Greensand forms so much a part of the Lower Chalk, 

 and is so slightly developed near London, that we have grouped it 

 with the Chalk. It is only in Wiltshire that it acquires an 

 importance entitling it to be considered apart. Under London it 

 becomes also so argillaceous as to lose its Avater-bearing character. 



The Upper and Lower Greensands are separated by 100 to 200 

 feet of impermeable clay, known as the Gault. The numerous 

 small streams rising at the foot of the Chalk hills have their source 

 generally in springs thrown out by the Chalk-marl, or the Gault. 



The Lower Greensands form a mass of siliceous sandy strata from 

 200 to 500 feet thick, and with an available area of above 500 

 square miles. Cropping out both to the north and south of London 

 conformably to the Chalk, which is known to pass below the Tertiary 

 strata under London, it was supposed that the Lower Greensands 

 were also continuous below London, in the same way as the Lower 

 Greensands of the plains of Champagne pass under Paris at the 

 depth of 1,800 feet. The experience, however, obtained at Kentish 

 Town, at the deep well sunk through the Chalk a few years since by 

 the Hampstead Company, showed that although the Tertiary strata, 

 the Chalk, and the Gault followed in regular order, a change took 

 place at the base of the Gault, and instead of the Lower Greensands, 

 a series of red and grey sandstones were met with. These were 

 bored into for a thickness of 188 feet, without passing through them, 

 and the work was abandoned. No organic remains were discovered 

 to indicate the age of these sandstones, and the hand-specimens 

 were insufficient to determine the question. They may have be- 

 longed to some member of the New Eed Sandstone, or possibly to 

 the Old Eed. In any case they seem to form part of an under- 

 ground ridge of old Secondaiy or Palasozoic rocks which, ranging 

 from Belgium, pass under the Chalk at Calais and Harwich, at both 

 which places they have been met with, and probably extend under 

 London in the direction of Somersetshire. The width of this belt 

 can only be determined by experiment. 



VOL. VI. — NO. LXIII. 27 



