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.

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. It will appear from them that the mean results of the whole is very different from any 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.

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.