432 



NATURE 



[Mar. 28, 1872 



and beyond ; whereas, with the exception of Kilburn, hardly a 

 house was to lie met with a few years since between Paddington 

 and Edgeware, or between Marylebone and Ilendon ; and not 

 many even between the New Road and Highgate and Ilamp- 

 steacL As a marked case of the excluding effects of a large 

 tract of impermeable strata close to a great city, I may mention 

 the denuded London-clay district extending from a mile north of 

 Acton, Ealing, and Hanwell, to Stanmore, Pinner and Icken- 

 ham, near Uxbridge. With the exception of Harrow {which 

 stands on an outlier of the Bagshot Sands), and Perivale, and 

 Greenford {on outliers of gravel), there are only the small villages 

 of Northholt and Greenford Green. In the earlier edition of 

 the Ordnance Maps, there was a tract of ten square miles north 

 and westward of Harrow within which there were only four 

 houses. Vet the ground is all cultivated and productive. But 

 immediately eastward of this area, and ranging thence to the 

 valley of the Lea, the ground rises higher, and most of the 

 London-clay hills a e capped by gravel of an older age than that 

 of the London valley, and belonging to the boulder-clay series. 

 On these we have the old settlements of Ilendon, Stanmore, 

 Finchley, Barnet, Totteridge, Whetstone, Southgate, and others. 



There is yet another very common source of well-water supply 

 from beds of gravel directing population to low sites in valleys, 

 which is this. Everywhere on the banks of the Thames and its 

 tributaries there is a lower-lying bed of valley-gravel or of rubble 

 on, and often passing beneath, the level of the river. This bed is 

 supplied with water both by rain falling on it, by springs thrown 

 out from the adjacent hills or by the drainage from those hills 

 and in places by infiltration from the river, when, from any 

 cause, the line of water in the gravel falls below that of the 

 adjacent river ; while, on the other hand, the surplus land- 

 supplies find their way direct and unseen, from the bed of gravel 

 to the river. A great part of London south of the Thames, West- 

 minster, Battersea, and a number of towns up the Thames, as 

 Hammersmith, Brentford, Eton, Maidenhead, and others, to- 

 gether with Newbury and several villages on the Kennet, also 

 the towns of Ware and Hertford on the Lea, have this shallow 

 well-supply. A great many towns and numberless villages along 

 most of our river-valleys all through England, and on whatever 

 formation situated, are dependent on this superficial source of 

 supply, a supply much more permanent than the other shallow 

 well-supplies, in consequence of the outside aid from springs and 

 rivers. It is, however, only in case of exceedingly dry seasons 

 or of excessive pumping, that the supply requires to be supple- 

 mented by the river-waters. As, in ground of this description, 

 the land-water is generally dammed back by the stream, the 

 level of the water in the wells, which are always shallow, varies 

 with the level of the water m the streams, rising and falling more 

 or less with them. 



A few of the higher London-clay hills in the neighbourhood of 

 London are also capped by outliers of the Bagshot Sands, as, for 

 example, Harrow, Harapstead, and Highgate, all of which are 

 sites of old habitations. The sands at these places attain a thick- 

 ness of from 30 to So feet, are very permeable, and afford a suffi- 

 cient water-supply by means of wells to alimited population. A num- 

 ber of well-known small springs are thrown out at the contact of the 

 sands and the clay on the slopes just below and around the 

 summit both of Highgate and Hampstead Hills. In some 

 instances, owing to the presence of iron in the sands, they are 

 slightly chalybeate. When the Baghshot Sands, further west- 

 ward of London, attain their fuller development of from 300 to 

 400 feet, tlie depth to the water-level at their base becomes so 

 great ttiat the upper porous beds are left high and dry, and form 

 uncultivated wastes, such as Bagshot Heath, Frimley Heath, 

 and others ; but on the outside of this area, where the sands 

 become thinner, and the water-level more within reach, we fiad 

 a number of villages, such as Englefield Green, Sunninghill, 

 Brackwell, Wokingham, Alderstone, Esher, Weybrijge, 

 Woking, &c. There are also some thin subordinate beds of 

 clay in the middle of the series which hold up a sufficient quan- 

 tity of water for small local supplies, and give rise to small 

 streams in the valleys of the Black water and of Chobham. The 

 running nature of portions of these sands, and the presence of 

 beds of ferruginous and green sands, often interfere much with 

 the construction of deep wells, and the quantity of the well- 

 water ; and, externally, the mix-^d clay-and-sand character 

 of the upper beds of the London clay fails to give any 

 good retaining-line for the water, which therefore rarely issues 

 as springs, but oozes out from the general surface of the inter- 

 mediate spongy mass. 



The 70 to 100 feet of sands and pebble-beds belonging to the 

 lower tertiary strata under the London clay, and overlying the 

 chalk, are also very permeable, and being intercalated with some 

 beds of retentive clay, they give rise to one or two levels of 

 water, affording wherever these strata form the surface, as at 

 Blacklieath, Bexley, Chiselhurst, and Bromley, a moderate 

 water-supply to shallow wells. Where these sands dip under 

 the London clay, and only present a narrow belt on the surface, 

 a small valley is commonly formed into which the London-clay 

 hills drain on the one side, and on the other the chalk dammed 

 back by the Tertiary strata throws out its springs, and the sands 

 are thus kept charged with water up to a short depth from the 

 surface. As instances of the many places whose sites have been 

 determined by these favourable circumstances, I may name Croy- 

 don, Beddington, Carshalton, Sutton, Clieam, Ewell, the villages 

 between Epsom, Ashstead, and Leatherhead, to Guildford, and 

 again between Old Basing and Kingsclere. 



But besides furnishing a supply by ordinary wells to a number 

 of villages on their line of outcrop, the Lower Tertiary sands have 

 of late years contributed to the metropolitan supply, as well as 

 to the supply of those adjacent districts where the surface is 

 formed of tenacious clay, and water is scarce, by means of 

 artesian wells. For along the line of country just named, and 

 along a more irregular belt on the north of London, these sands 

 pass beneath the London clay, so that the water they receive 

 from rain and springs on the surface, passes underground, where 

 it is prevented from rising by the impermeable superincumbent 

 clay ; consequently, as there is no outlet for the water below 

 ground, these sand-beds are filled with water along their whole 

 underground range, between their outcrop in Surrey and that in 

 Hertfordshire. 



I need not dwell here upon the constructionsof Artesian wells, 

 which have been explained by Hericart de Thury, Arago, 

 Degousce and Laurent, Bumell, Hughes, myself, and others, 

 beyond offering a few explanatory remarks on this particular case, 

 which we shall also have to bring to bear upon the origin of 

 springs. 



The surface of the ground at the outcrop, just referred to, of 

 the Lower Tertiary sands is about 100 ft. above the level of the 

 Thames, whilst ur.ler London the sands are at a depth of from 

 too ft. to 220 ft. below that level, thus forming the shell of a 

 basin from 200 ft. to 300 ft. deep, the centre of which is filled 

 with a depressed mass of impermeable clay. There is, however, 

 a notch in the lip of the basin, where it is traversed by the 

 Thames, at Deptford and Greenwich, which is at a lower level of 

 100 ft. than the rest of the rim. Below this level, as there is no 

 escape for the water, the strata are naturally perpetually water- 

 logged ; and if any water is withdrawn from one part, it is, 

 owing to the permeability of the strata, at once replaced from 

 adjacent parts of the same strata. Early in the present century, 

 bore-holes were made through the overlying London clay to 

 the sands at depths of from So ft. to 140 ft., and the water from 

 these deep-seated springs rose at once to a height of several feet 

 above the level of the Thames, where it tended to maintain 

 itself, and thus form, in the lower-lying districts, permanent 

 natural fountains. But the e.ase and facility with which this 

 abundant supply was obtained, led to the construction of so great 

 a number of such wells that a time soon came when the annual 

 rain outfall no longer sufficed to meet the demand, or, rather, it 

 could not be transmitted fast enough to the central area of ab- 

 straction to replace the out-draught. The consequence was that, 

 after some years, the water ceased to overflow, and the line of 

 water-level has gradually sunk at London, until it now stands 

 some 70 ft. or 80 ft. beneath the surface level. This, however, 

 is not the case at a distance from London ; and in many parts of 

 Middlesex, and more especially in Essex, where Artesian wells 

 are common, they have been found of very great service. 



In order to supply the deficiency thus caused in the Lower 

 Tertiary sand, most of the Artesian wells in London have of 

 late years been carried down into the underlying chalk, which 

 also extends beneath London at depths of from 150 ft. to aSoft. 

 Both formations are permeable, but in different ways. On both 

 the rainfall is at once absorbed, but the transmission of it is 

 effected in different ways. Through the sands it filters at 

 once ; but not so with the chalk. A cubic foot of the latter 

 will hold two gallons of water by mere capillary attraction ; 

 but it parts with this with difficulty. Still in time it finds 

 its way through the body of the chalk, aided by the innumer- 

 able joints, fissures, and lines of flints by which this forma- 

 tion is traversed ; and, when once under the line of satura- 



