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



It is known that tlie Lower Greensands exist at Eeigate and are 

 about 450 feet thick, and that they occur again in Bedfordshire with 

 a thickness of about 200 feet. In both cases they dip towards 

 London, disappearing beneath the Gault. "We know that they do 

 not exist under London (Kentish Town). It follows, therefore, that 

 in the one case they cease at some point between Eeigate or 

 Merstham and London, and in the other between Baldock and 

 London. As at both ends they are of considerable thickness, and 

 the Gault is continuous, it is certain that the Greensands must range 

 from these outcrops some way towards London, probably thinning 

 off gradually against the flanks of the underground ridge of old 

 rocks. So far as they continue, so far will they form a valuable and 

 copious water-bearing bed, the water from which would overflow at 

 the levels lower than that of their outcrop. The extent of their 

 underground range could only be determined by boring. It might 

 be as far as Croydon, or even still nearer to London. The same 

 would happen to the north of London, but the distance there is 

 greater, the beds are not so thick, and the conditions generally are 

 less favourable. The great purity of the water from the Grenelle 

 and other artesian wells in the Lower Greensand is well known, and 

 there is reason to suppose that the quality of the water obtained 

 from the same formation in the vicinity of London would prove 

 equally good. The excessive length of filtration would at all events 

 ensure freedom from organic impurities. 



The quality of the water flowing from the Lower Greensands is 

 excellent for all domestic purposes, being bright and limpid, of a 

 degree of hardness varying only from about Z° to 9° of Clark's test, 

 and generally very free from organic matter. 



The springs in this formation are not very numerous, owing to 

 the prevalence of sandy beds, but in some of the more stony beds 

 there are some fine springs, as for example ; — 



1. The springs in Bradbourne Park and at Eiverhead near Sevenoaks. 



2. Several springs near Dorking. 



3. Spring at Weston Street. 



4. Spring at Moorhead Park near Farnham. 



To the south of London a great thickness of Weald Clay separates 

 the Lower Greensand from the Hastings Sand of the Weald of Kent 

 and Sussex ; but although these beds are thick they are very local, 

 only partially permeable, and are of no avail. 



The strata which next succeed are only developed in the north and 

 north-west of the London basin. 



The Portland Stone and Sands are from 35 to 50 feet thick, and 

 are, in some places, of local importance, but none of the springs 

 would be available for distant purposes. 



The Kimmeridge Clay is impermeable and from 250 to 300 feet 

 thick. 



The Coral Bag and Calcareous Grit are from 20 to 50 feet thick 

 and give rise but to a few small springs. These beds thin off as 

 they range to the east and south-east, so that in Buckinghamshire, 

 and probably underground in Berkshire, the Kimmeridge and Oxford 

 clays come into contact, and the Coral Eag ceases to exist. 



