section, Fig. 7, representing the usual features of the deposit in the northern part of the Tertiary district. It is from a cutting at a brickfield west of the small village of Hedgerley, 6 miles northward of Windsor.

Here we see a large development of the mottled clays, and but little sand.

Fig. 8.

A somewhat similar section is exhibited at Oak End, near Chalfont St. Giles. But to show how rapidly this series changes its character, the section of a pit only a third of a mile westward of the one at Hedgerley is given in Fig. 8.

In this latter section the mottled clays have nearly disappeared, and are replaced by beds of sand with thin seams of mottled clays. At Twyford, near Reading, and at Old Basing, near Basingstoke, the mottled clays again occupy, as at Hedgerley, nearly the whole space between the London clays and the chalk. Near Reading a good section of these beds was exhibited in the Sonning cutting of the Great Western Railway; they consisted chiefly of mottled clays. At the Katsgrove pits, Reading, the beds are more sandy. Referring back to Fig. 6, it may be noticed that there is generally a small quantity of water found in the bed marked 1, in parts of the neighbourhood of London. Owing, however, to the constant presence of green and ferruginous sands, traces of vegetable matters and remains of fossil shells, the water is usually indifferent and chalybeate. The well-diggers term this a slow spring. They well express the difference by saying that the water creeps up from this stratum, whereas that it bursts up from the lower sands 3, which is the great water-bearing stratum. In the irregular sand beds interstratified with the mottled clays between these two strata water is also found, but not in any large quantity.

Fig. 9 is a section at the western extremity of the Tertiary district at Pebble Hill, near Hungerford. Here again the mottled clays are in considerable force, sands forming the smaller part of the series.

The following lists exhibit the aggregate thickness of all the beds of sand occurring between the London clay and the chalk at various localities in the Tertiary district.

any

It will appear from them that the mean results of the whole is very different from of those obtained in separate divisions of the country. The mean thickness of the deposit throughout the whole Tertiary area may be taken at 62 feet, of which 36 feet consist of sands and 26 feet of clays; but as only a portion of this district contributes to the water supply of London, it will facilitate our inquiry if we divide it into two parts, the one westward of and including London, and the other eastward of it, introducing also some further subdivisions into each.

Fig. 9.

The mean of the three columns in two western sections gives a thickness to this formation of 57 feet, of which only 19 feet are sand and permeable to water, and the remaining 38 feet consist of impermeable clays, affording no supply of water.

The area, both at the surface and underground, over which they extend is about 1086 square miles.

Average

26 29

Average

18 51

Average

13 34

The average total thickness of the eastern district deduced from the nine sections we have taken gives 68 feet, of which 53 feet are sands and 15 feet clays. The larger area, 1849 square miles, over which the eastern portion of the Tertiary series extends, and the greater volume of the water-bearing beds, constitute important differences in favour of this district; and if there had been no geological disturbances to interfere with the continuous character of the strata, we might have looked to this quarter for a large supply of water to the Artesian wells of London.

From these tables it will be readily perceived that the strata of which the water-bearing deposits are composed are very variable in their relative thickness. They consist, in fact, of alternating beds of clay and sand, in proportions constantly changing. In one place, as at Hedgerley, the aggregate beds of sand may be 5 feet thick, and the clays 45 feet; whilst at another, as at Leatherhead, the sands may be 35, and the clays 20 feet thick, and some such variation is observable in every locality. But although we may thus in some measure judge of the capacity of these beds for water, this method fails to show whether the communication from one part of the area to another is free, or impeded by causes connected with mineral character. Now as we know that these beds not only vary in their thickness, but that they also frequently thin out, and sometimes pass one into another, it may happen that a very large development of clay at any one place may altogether stop the transit of the water in that locality. Thus in Fig. 10 the beds of sand at y allow of the free passage of water, but at x, where clays occupy the whole thickness, it cannot pass; the obstruction which this cause may offer to the underground flow of water can only be determined by experience. It must not, however, be supposed that such a variation in the strata is permanent or general along any given line. It is always local, some of the beds of clay commonly thinning out after a certain horizontal range, so that, although

Fig. 10.

y

the water may be impeded or retarded in a direct course, it most probably can, in part or altogether, pass round by some point where the strata have not undergone the same alteration.

POSITION AND GENERAL CONDITIONS OF THE OUTCROP. This involves some considerations to which an exact value cannot at present be given, yet which require notice, as they to a great extent determine the proportion of water which can pass from the surface into the mass of the water-bearing strata. In the first place, when the outcrop of these strata occurs in a valley, as represented in Fig. 11, it is evident that b may not

Fig. 11.

only retain all the water which might fall on its surface, but also would receive a proportion of that draining off from the strata of a and c. This form of the surface generally prevails wherever the water-bearing strata are softer and less coherent than the strata above and below them.

It may be observed in the Lower Tertiary series at Sutton, Carshalton, and Croydon, where a small and shallow valley, excavated in these sands and mottled clays, ranges parallel with the chalk hills.

It is apparent again between Epsom and Leatherhead, and also in some places between Guildford and Farnham, as well as between Odiham and Kingsclere. The Southampton Railway crosses this small valley on an embankment at Old Basing.

This may be considered as the prevailing, but not exclusive, form of structure from Croydon to near Hungerford. The advantage, however, to be gained from it in point of watersupply is much limited by the rather high angle at which the strata are inclined, as well as by their small development, which greatly restrict the breadth of the surface occupied by the outcrop. It rarely exceeds a quarter of a mile, and is