tions, modify the transmission of water. I find by experiment that a siliceous sand of ordinary character will hold on an average rather more than one-third of its bulk of water, or from two to two and a half gallons in one cubic foot. In strata so composed, the water may be termed " free," as it passes easily in all directions, and, under the pressure of a column of water, is comparatively but little impeded by capillary attraction. These are the conditions of a true permeable stratum. Where the strata are more compact and solid, as in sandstone, limestone, and oolite, although all such rocks imbibe more or less water, yet the water so absorbed does not pass freely through the mass, but is held in the pores of the rock by capillary attraction, and parted with but very slowly; so that in such deposits water can be freely transmitted only in the planes of bedding and in fissures.

If the water-bearing deposit be of uniform lithological character over a large area, then the proposition is reduced to its simplest form; but when, as in the deposit between the London clay and the chalk, the strata consist of variable mineral ingredients, it becomes essential to estimate the extent of these variations; for very different conclusions might be drawn from an inspection of the Lower Tertiary strata at different localities.

22. In the fine section exposed in the cliffs between Herne Bay and the Reculvers, a considerable mass of fossiliferous sands is seen to rise from beneath the London clay.

The annexed diagram represents a view of a portion of this cliff a mile and a half east of Herne Bay, and continued downwards, by estimation below the surface of the ground, to the chalk.

In this section there is evidently a very large proportion of sand, and consequently a large capacity for water.

Again at Upnor, near Rochester, the sands marked b3, in Fig. 3, are as much as sixty to eighty feet thick, and

continue so to Gravesend, Purfleet, and Erith.* In the first of these places they may be seen capping Windmill Hill; in the second, forming the hill (now nearly removed) on which the experimental light-house is built; and in the third in the large ballast pits on the banks of the river. The average

thickness of these sands in this district may be about fifty to sixty feet. In their range from east to west, the beds 62 become more clayey and less permeable, and b1 very thin. As we approach London, the thickness of 63 also diminishes.

*For descriptions of several sections in this district see a paper by Mr. J. Morris, in Proc. Geol. Soc. Vol. II. p. 450.

+ This section, as well as those in Figs. 4 to 7, is upon a vertical scale of fifty feet to the inch.

In the ballast pits at the west end of Woolwich,* this sand bed is not more than thirty-five feet thick, and as it passes under London it becomes still thinner.

23. The annexed diagram (Fig. 4) represents a general or average section of the strata on which London stands.†

The great increase in the proportion of the argillaceous strata, and the decrease of the beds of sand, in the Lower Tertiary

* For detailed sections of the Woolwich and Lewisham pits see a Paper by Dr. Buckland, in the Transactions of the Geological Society, Vol. IV. p. 276; also, Philipps and Connybeare's Outlines of the Geology of England, p. 47.

For some valuable details on this subject, see the work recently published by Mr. R. Mylne, “On the strata beneath London," in which the well-sections, and the levels of the strata, are given with great accuracy.

There is generally a small quantity of water found in this bed in parts of the neighbourhood of London. Owing, however, to the constant presence of green and ferruginous sands, traces of vegetable matter 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 63, which is the great waterbearing 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.

strata, is here very apparent, and from this point westward to Hungerford, clays decidedly predominate; while at the same time the series presents such rapid variations, even on the same level, and at short distances, that no two sections are alike.

24. On the southern boundary of the Tertiary district from Croydon to Leatherhead, the sands, 63,* maintain a thickness of twenty to forty feet, whilst the associated beds of clay are of inferior importance.

Crossing over to the northern boundary of the Tertiary district, we will take another section (Fig. 5), representing the usual features of the deposit in that part of the country. It is from a cutting at the brickfield, west of the small village of Hedgerley, six miles northward of Windsor.

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

* I am not sure that these sands can geologically be identified with those b3 on the same level further eastward. As, however, they occupy the same relative position as 63 does at Woolwich, they may for our present purpose be considered as their equivalent.

a pit, only a third of a mile westward of the one at Hedgerley, is annexed, in Fig. 6.

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.*

26. Many other sections might be given; but as they do not present any exception to the general rule, we will pass to one at the western extremity of the Tertiary district at Pebble Hill, near Hungerford (Fig. 7). Here again the mottled clays are in considerable force, sands forming the smaller part of the series.

These sections (Nos. 3 to 7) give a fair average representation of the strata generally between the London clay and the chalk.

* For details of this Section, see Dr. Buckland's paper in the 4th vol. Trans. Geol. Soc. Several sections, illustrative to some extent of the Lower Tertiary strata, are given in a paper of the author's, published in the Quart. Journ. Geol. Society for August, 1850: there, however, the argument is connected with the organic remains of the strata.