lowest, that the springs from these sands are likely to be the strongest and most numerous.

*

When the Lower Greensand contains subordinate beds of clay, the line s, s′, x, s′′, will necessarily be interrupted, and the spring system of the district will become more complicated; but the same rules will still apply to each separate group of strata.

86. The following are a few of the places where sections showing the structure of the Lower Greensand may be seen :

mon.

Upper Division.

The sides of the lane commencing at the London Road, three furlongs northward from Limpsfield Church (Kent), and running through Laurel Grove to Limpsfield ComPits near Betchworth (Surrey),—one, five furlongs on the Dorking Road, -another, one furlong on the Reigate Road. Sides of the lane leading from the Punchbowl Inn (three-quarters of a mile from Dorking, on the Reigate Road), southward to Chart Park. Sides of the lane leading from Tittings Farm near Chilworth (Surrey), southward to Halfpenny Farm. (This section shows both the upper and lower divisions—it is an almost perfect and very interesting section of the whole series of strata composing this formation). Pits immediately east of Silsoe, Beds.

Lower Division.

Pits on the common immediately south of Limpsfield. Cutting at the junction of the Brighton and Dover Railways. Road cutting over Tilburstow Hill, immediately south of Godstone, and the pits on the east of the road at the top of the same hill. Pits at Cold-harbour on the top of Leith Hill. Road cuttings on the high road from Woburn to Newport Pagnel,—the first two miles out of Woburn.

General Sections.

Railway section from the Leighton Buzzard station (Bucks) northward for two miles beyond it. Railway section from Biggleswade (Beds) northward to Sandy. Part of the cuttings on the railway between Reigate and Guildford (the other cuttings on this line are through the Upper Greensand and the Gault). Railway sections adjoining Farnham.

Beside these particular localities, good sections are exhibited very commonly on the sides of the lanes and roads throughout a large portion of these districts.

As in and adjoining the valley of the Mole where it enters the Greensand at Dorking, that of the Wey south of Guildford, and of the Dart at Riverhead.

H

§ 9. The Weald Clay and the Kimmeridge Clay.

87. THE Lower Greensand is, throughout, incumbent upon one or other of these formations; which, as they consist of impervious strata of clay usually several hundred feet thick, effectually prevent the downward escape of the water, accumulated in the overlying beds.

88. The following table shows at one view the extent in square miles of the superficial areas of the several formations described in the foregoing pages, with reference both to the divisional arrangement that has been adopted and to the entire surface occupied by each deposit within the boundary of the map.

These, however, are to be considered only as approximate

measurements.

IV. GENERAL INFERENCES.

§ 10. On the Relation between the actual and the effective superficial Areas of the several Water-bearing Deposits.

89. It has been convenient in the preceding pages to divide the whole of the area, occupied by the Lower Tertiary and Lower Cretaceous strata on the Map, into four divisions, and to consider each group separately in stratigraphical order. It cannot fail to have been observed that the position of outcrop, and the lithological character of the several formations, are liable to such variation, that the proportion of the rainfall that can pass into the strata becomes materially modified, and depends upon other conditions than those of superficial area solely. We will now endeavour to attach some definite value to these interferences, and to eliminate the portions of the surface which may represent the really effective areas; although these must be viewed only as very general approximations made merely to meet, in some measure, the necessities of the case, and to indicate the course which should be taken. Having, however, thus obtained a series of numerical results connected with the dimensions of the different strata, we can endeavour to determine their relative importance as sources of water-supply. It will be desirable also to ascertain whether these strata receive any portion of the water gathered on the adjacent impermeable deposits.

90. It has been shown that in the districts east of the meridian of Deptford,—including north Kent, Essex, Suffolk, Cambridgeshire, and part of Herts,-neither the Lower Tertiary strata, nor the Upper Greensand appear to contribute, except

in a very small degree, to the supply in the metropolitan district (32, 40, 45, 68).* This is important, for of the total area of the first of these formations, viz. 354 square miles, nearly 259 are, in consequence, at once excluded from our calculations; while only about 10 out of the 173 square miles of the second are lost. Our attention may, therefore, be confined to the portions of those formations situated west of that line, and whose relative superficial areas are:—

From this list it appears that in the district east of the meridian of Deptford the exposed surface of the Upper Greensand is two-fifths, and that of the Lower Greensand more than four times, larger than that of the Lower Tertiary strata. But although these numbers give the dimensions of the areas of the water-bearing strata, they do not represent their exact and relative values, to which we must endeavour to approximate. All the detail bearing upon this part of the subject has been given in the preceding pages. We have now only to recapitulate the general results.

91. Tertiary Strata.-On the south side of this portion of the Tertiary district these strata form a narrow zone of outcrop from Croydon to Hungerford, and on the north, a broader zone from Hungerford to Ware. But it has been shown (¶ 23, 28, 46, 48), that, owing to the large development of the mottled clays in this group,-at points not more distant on the one side of this district than Leatherhead, or, at all events, Guildford, and, on the other side, than Maidenhead, -these Lower Tertiary strata frequently become almost impermeable; so that their outcrop, beyond a line connecting those places, is probably almost inoperative, in the supply of water to the beds beneath London. This would exclude,

*The case is different with respect to the Lower Greensand (¶ 93).

as ineffective, the whole of the superficial area westward of that line, amounting to twenty-nine square miles; or we will take twenty-eight square miles, as it is just possible that some small permeable beds may keep up a partial communication between these different portions of the Tertiary area. There 'remain eighteen miles of southern outcrop, the larger portion of which is probably effective; still, as the mottled clays occupy part of the zone between Guildford and Ewell, as a covering of drift is not uncommon, and as near Deptford the strata are a good deal broken by faults, I would further reduce the eighteen square miles to twelve.

Of the northern outcrop, we might take two square miles from Bray to Slough, and about three in the valley of the Lea in the neighbourhood of Hoddesdon and Broxbourne,— of Bow and Stratford; but even these portions of it are so much covered by drift that these five miles may be reduced to three of effective area. In all the intermediate tract the outcrop is so unfavourably placed, the mottled clays are so largely developed, and the surface is so generally covered by a drift of gravel, clay, and brick earth, that considerable allowances must be made for these causes. The value to be attached to them is of course in a great measure conjectural, but I am satisfied that the deduction on their account ought to be very large,-that of this portion of the outcrop we cannot take more than seven square miles as a contributing surface to the London district. The full extent of this northwestern area is fifty square miles, which would thus be reduced to ten. This area may appear small, but it will, I think, be found sufficient, when we consider the supply of water in the south-western division, the effective area of which does not much exceed this size.

I would also take as a possible source of supply two square miles of the Essex or north-eastern division.

92. Upper Greensand.-From the more uniform lithological