| Measurement of Sections Westward of London. | ||||
| On or near the Southern Boundary of the Tertiary District. | ||||
| Sand. | Clay. | |||
| ft. | ft. | |||
| Streatham | 30 | 25 | ||
| Mitcham | 47 | 34 | ||
| Croydon | 35 | ? | 20 | ? |
| Epsom | 31 | 23 | ||
| Fetcham | 35 | 20 | ||
| Guildford | 10 | ? | 40 | |
| Chinham, near Basingstoke | 20 | ? | 30 | |
| Itchingswell, near Kingsclere | 22 | 34 | ||
| Highclere | 24 | 27 | ||
| Pebble Hill, near Hungerford | 9 | 39 | ||
| Average | 26 | 29 | ||
| On a Central Line in the Tertiary District. | ||||||
| Sand. | Clay. | |||||
| Sand. | Clay. | ft. | ft. | |||
| London: | ft. | ft. | ||||
| Millbank | 49 | 40 | ||||
| Trafalgar Square | 49 | 30 | ||||
| Tottenham Court Road | 35 | 30 | ||||
| Pentonville | 34 | 44 | 46 | 39 | ||
| Barclay’s Brewery | 55 | 42 | ||||
| Lombard Street | 53 | 35 | ||||
| The Mint | 49 | 38 | ||||
| Whitechapel | 45 | 50 | ||||
| Garrett, near Wandsworth | 20 | 52 | ||||
| Isleworth | 17 | 70 | ||||
| Twickenham | 7 | 50 | ||||
| Chobham | 3 | 45 | ||||
| Average | 18 | 51 | ||||
| On or near the Northern Boundary of the Tertiary District. | ||
| Sand. | Clay. | |
| ft. | ft. | |
| Hatfield | 23 | 2 |
| Watford | 25 | 10 |
| Pinner | 12 | 32 |
| Oak End, Chalfont St. Giles | 3 | 40 |
| Hedgerley, near Slough | 5 | 45 |
| Starveall „ „ | 13 | 20 |
| Twyford | 5 | 60 |
| Sonning, near Reading | 12 | 54 |
| Reading | 16 | 33 |
| Newbury | 20 | 36 |
| Pebble Hill | 9 | 39 |
| 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.
Fig. 10.
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 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.