268 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



LAUGfST, 



yjOO square miles, in water communication beneath London. If, 

 further, the volume of tlieir masses, with reference only to those 

 beds which are permeable and the area which is elfective, be com- 

 pared, the following is the result in round numbers, each unit 

 representing a mass a mile square and one foot tliick. 



Volume of the permeable portion of the beds beneath the 

 chalk and the London clay ... ... ... 10,500 



Volume of the permeable portion of the upper green- 

 sand 150,000 



The productive and contributing- area of outcrop of the upper 

 fpeensand may be considered to be four times greater, and thu 

 volume of its mass, viewed as a reservoir for water nearly ten 

 times larger than that of the lower tertiary strata. The water in 

 this upjier greensand is everywhere held up by the underlying 

 gault, which consist of a mass of dark grey tenacious and ])er- 

 fectly impermeable clay from 100 to 150 feet thick. 



Below the gault is the lower greensand formation, consisting of 

 a series of beds of loose sands and soft sandstones, with subordi- 

 nate beds of clay, and groups of argillaceous strata ; the sands, 

 however, on the whole, predominate largely. As a mass, it is 

 much more variable in mineral character than the upper green- 

 sand. It also follows a very different rule in its development; the 

 latter thins out as it ranges eastward, whilst exactly tlie revei-se 

 holds good with the former. At Hythe it is, according to Dr. 

 Fitton, 406 feet thick, whereas at Devizes it is only 13 to 20f eet 

 thick ; the decrease, however, is not gradual, for its thickness in 

 Surrey is probably greater even than at Hythe. In Bedfordshire 

 It may be from 250 to 300 feet thick ; it gradually becomes thinner 

 as it ranges to Norfolk on the one side, and to Oxfordshire on the ' 

 other. I 



Without going into further details, it may be observed, that the ' 

 area of outcrop of the lower greensand, with reference to the sur- | 

 face that might contribute to the water supply at London, appears 

 to me not much less than 400 square miles, whilst, in its effective 

 underground range, it is spread over an extent of about 3000 

 square miles, and its effective volume may be represented as a 

 mass of 500,000 square miles, one foot thick. The great excess of i 

 these dimensions over those of the lower tertiaries need scarcely 

 be pointed out. 



The lower greensand is underlied by the Weald clay and the 

 Kimmeridge clay, both of which are of considerable thickness 

 (200 to 400 feet), and perfectly retentive of water. From these 

 facts it tlierefore appears probable that, both with regard to areas 

 for drainage and to capacity for water, the upper and lower green- 

 sands present conditions far more favourable than the lower ter- 

 tiary strata ; for their areas of outcrop are gi-eater, — their litho- 

 logical character and thickness are superior, — the position occu- 

 ])ied by their outcrop is usually more favourable, — their exposed 

 surfaces are generally more free from drift, and they are both, the 

 lower greensands especially, very absorbent. These conditions 

 determined, it remains, however, to ascertain how far they may 

 be rendered inojjerative by disturbances in the strata between their 

 outcrop and a central point at London. The main lines of dis- 

 turbance which have affected this district run nearly east and 

 west ; consequently, as the expansion of the upper greensand and 

 its main receiving surface are to the westward of London, or 

 |)arallel to these lines, the probability of the continuity of the 

 strata being broken between jliddlesex and Wiltshire is less than 

 it would be if they were on a north and south line. 



As far as my own observations go, there are apparently no faults 

 or disturbances of a sufficient power to interrupt the underground 

 ffort- of water from the outcrop of the upper greensand in Wilt- 

 shire, and more especially in Oxfordshire and Berkshire, to London. 

 With regard to the lower greensand, the case is different. Its 

 contributing surfaces lie northward and southward of London, ami 

 it is traversed longitudinally by some faults of considerable mag- 

 nitude. The nu)st imiiortant one runs east and west, imme- 

 diately beyond the outcrop of the upper greensand and the gault 

 at the base of the North Downs, and is of a force sufficient at 

 places to shift the whole thickness of the lower greensand out of 

 its true position. If this fault were continuous, and its effects 

 Cijual from any point near Westcrham to Farnliam, then it is pro- 

 bable that the drainage of nearly the entire zone of exposed sur- 

 face of the lower greensand — here two to five miles broad — would 

 lie intercepted by it. But lines of disturbance are rarely main- 

 tained in equal power through any great length of range; they 

 are, as it were, intermittent ; therefore, although the continuity 

 of the lower greensand might be broken at one or more places, yet 

 at other places it might be, and no doubt is maintained. For in a 



formation of so variable and arenaceous character as this, it is not 

 necessary that each stratum should preserve its continuity. If the 

 mass is displaced to the extent of 100 to 200 feet, provided strata 

 of a loose sandy nature are brought into juxtaposition on the oppo- 

 site sides of the fault, that will be sufficient to keep up tlie circu- 

 lation of water from one side of the disturbance to the other and 

 the waters dammed back in those parts where the disturbance is 

 greater, and the continuity completely destroyed, will pass round 

 through such ])oints of communication. Nevertheless, there is no 

 doubt that the water-value of strata so affected must be lessened, 

 and for this allowance has been made. On the zone of outcrop to 

 the north of Lomlon there seems to be but few disturbances, or, at 

 all events, not any of sufficient magnitude to produce great inter- 

 ference, except westward beyond Abingdon, where the lower green- 

 sand is entirely cut off by a fault. The previous calculations there- 

 foi'e only refer to the districts from Wallingford and Biggleswade, 

 and from Farnham to Maidstone. But it is to be observed also 

 that the lines of disturbance which so materially affect the compa- 

 ratively thin and not deep-seated beds of the lower tertiary strata 

 would be of little consequence in the deep-seated greensands, 

 where the water-level is so far above the level of the disturbed 

 strata. It is therefore, I think, probable that the upper and lower 

 greensands constitute two important zones of water-bearing strata 

 underneath London, and it next becomes a question to determine 

 at what depth they may be met with. 



The thickness of the chalk has been very variously estimated, 

 but there are good geological grounds for presuming that the 

 chalk underneath London is not above 600 to 700 feet thick; if, 

 therefore, a point be taken where the tertiary strata are not above 

 200 feet thick, it is probable that the upper greensand would be 

 reached at a depth not exceeding at a maximum 1000 feet, and 

 the lower greensand at 1200 feet. Supposing this to be the case, 

 then, as the outcrop of the upper greensand above Trinity high- 

 watermark at Londonis 360 feet southward at Merstham, and 135 ft. 

 northward beyond Hitchin ; and as, with few exceptions, it con- 

 tinues to maintain a high and increasing level from these points 

 westward to Calne and Devizes, where it reaches an elevation of 

 about 450 feet ; it follows that a supply of water from this source 

 at London would probably rise to a height of from 100 to 150 feet 

 above the level of the Thames at London. The outcrop of the 

 lower greensand varies from 100 to 300 feet above the same level ; 

 and, as the distance of its contributing areas of outcrop from 

 London is much less than those of the upper greensand, it is pro- 

 bable tliat its waters might rise to a height of 80 to 100 feet, or 

 more, above the level of tlie Thames. 



In conclusion, although the supply of water obtained from the 

 lower tertiary strata .at and around London is confessedly inade- 

 quate to the supj)ly of a large town, yet it is, as a local supply, in 

 many cases, of considerable value. If, therefore, with an area of 

 oitcrop of such limited extent, and a capacity of such moderate 

 volume, the lower tertiary strata nevertheless are of not inconsi- 

 derable value as regards their water supply, then it is prolable 

 that, with dimensions in every respect so mucli greater, and under 

 conditions genei'ally so much more favourable, the upper green- 

 sand, and the lower greensand more especially, must possess a 

 water value very considerably greater; and there appears to me to 

 be no reason why, in the case of the upper greensand, the downs 

 and valleys of Wiltshire and the ]dains of Oxfordshire and Berk- 

 shire, should not contribute their contingent to a supply of water 

 at London, or why even a very much larger supply, amounting 

 possibly to as much as 50,000,000 of gallons daily, if needed, should 

 not be furnished by the lower greensand of the hills of Kent, 

 Surrey, Buckinghamshire, and Bedfordshire. The first souixe of 

 supply would be doubtless purer and better ; but the latter would 

 be more abundant, and more generally available for all ordinary 

 purposes requiring large supplies, while, fnun its being naturally 

 at a high pressure, it might be applicable, not only to sanitaj-y 

 improvements, but also to the ornament and convenience of the 

 metropolis. 



Monsieur D'Archiac, who has paid great attention to the subject 

 of the water-bearing strata of the tertiary and cretaceous series of 

 France, confirms, as the result of his experience, the rule laid 

 down upon perfectly independent grounds by the Abbe Paramelle, 

 in his 'Observations on Springs,' viz., "That tlie underground 

 currents of water follow the same law as those which flow on the 

 surface." This is a natural consequence of the physical structure 

 which determines the water-sheds of a country. Applied to the 

 case before us, it would corroborate the views advanced in this 

 jiajier; for as the 'Ihames and its tributaries effect the surface- 

 drainage of the tertiary and cretaceous districts around London, 



