the same which fractures at Tilburstow Hill near Godstone, and then ranges at or near the outskirt of the Lower Greensand, in the direction generally of Maidstone. If this be its course, this disturbance, which is the most important one in the district, would pass outside, and not affect the area of the Lower Greensand between Dorking, Reigate, Godstone, and Westerham,-the zone of greatest consequence for the supply of water to this deposit beneath London. There are, however, several lesser lines of disturbance traversing this district, but they are of minor importance. From Farnham to Guildford, and Albury, the Greensands are tilted up at a high angle of 40° to 60°; but to the eastward of the latter place the angle is always much less, rarely exceeding 20°, and more generally falling to 5° or 10°.

81. Passing to the northern outcrop of the Lower Greensand, we there find its continuity distinctly broken for a considerable distance in its range through Berkshire, and cut off from contributing in any degree to the supply of water in the central districts. This is owing, probably, to a great fault, extending, in the valley parallel to the chalk escarpment, from the west-south-west of Swindon to near Abingdon: the disturbance being of such a magnitude as to remove the lower greensand, bringing up the Kimmeridge clay to the place which it should occupy on the surface.

In its range from this point to Cambridgeshire, this deposit may be traversed by other lines of disturbance, but south of Bury Hill. This line of disturbance is laid down on the Map, but I was not at all aware of the extent of the elevation, until its importance was pointed out to me by Mr. Barclay. Therefore this promontory of the Weald is not shown; it is such that it excludes a great part, and would exclude the whole, of the Leith Hill district from forming part of the effective area, if this axis of disturbance, which Mr. Austen had before ascertained to be extremely conspicuous at Chilworth, should prove to be continuous in its effects between Dorking and Guildford. But this is doubtful.

I find however that Mr. Hopkins, in his Paper on the Theory of the Wealden, in describing the range of this line of disturbance, has been the first to notice the occurrence of the Wealden at Bury Hill (Geol. Trans. Vol. VII. p. 19, 1845).

I do not think that the continuity of the strata is sufficiently broken to keep back the waters received at its out

If the Lower Greensand were a thin group of strata, a small fault might cause a complete break; but in a deposit several hundred feet thick, and composed in greater part of thick beds of sand irregularly recurring, unless the extent of vertical disturbance should everywhere exceed the thickness of the formation, a certain amount of communication would be kept up between the opposite sides of the fault whenever the disjointed edges of any of the numerous beds of sand came into juxtaposition, especially where the depth of the strata caused the pressure of the water to be great.

The reader, who has passed over the physical details commencing at p. 19, may here resume the more general description.

82. On the Springs in the Lower Greensand.-An attempt has been made to illustrate by a diagram the distribution of the free water in the mass of the chalk (p. 72). The object of fig. 18 is to exhibit the same phenomenon in the Lower Greensand, and to show the position which this formation holds relatively to the Upper Greensand, the Gault, and the Wealden. This transverse section is intended also to embrace the leading physical features of the greensand district on the south of London, and to give a general representation of the country between the North Downs, and the weald of Kent and Surrey, a distance of from three to six miles.

The greater part of this tract is occupied by the Lower

* I suspect that there is a considerable disturbance in this formation in the neighbourhood of Aylesbury. It may be a prolongation of the disturbance noticed near Swindon, and deserves attention, as affecting in some measure the present question.

Greensand, whose breadth varies according to the curvature of the strata,-being narrow when the curve takes a rapid bend upwards, and wider where it is prolonged, or forms a series of rolls.

The edges of this formation are bounded on the north by the Gault at s, and on the south by the Wealden at s"; they are both impermeable strata, and present water-tight surfaces to the sand between them, so that any water which might find its way below the margins of these deposits could not escape again, but would follow the subterranean course of the intermediate strata. In the process of time, and by the constant operation of the rain-fall, the large underground mass of the Lower Greensand has been filled to its edges with water, and any further addition causes it to run over. It is this overflowing which gives rise to the springs of the district. Their magnitude will depend upon the breadth and "massif" of the Lower Greensand, and upon the difference of level of the Gault and Wealden.

83. We will now assume that the Lower Greensand consists throughout of sands of uniform texture. Supposing that no rain had fallen for such a time that the waters con

tained in it were in a state of equilibrium: they would then stand at the level v, L, and all the springs would cease to flow. But when the mass of strata above v L is large this can rarely happen, on account of the fall of rain taking place from time to time throughout the year, and the texture of the strata opposing a certain resistance to its passage, which impediment to its transit is sufficient to diffuse, uniformly over a long period, the delivery of the water that it receives irregularly at shorter intervals.

The water first percolates downwards through the sands until it reaches the line of water-level, and then flows horizontally towards, and tends to escape at, the point of lowest surface-level s. (We are now supposing that this is the only vent.) The successive rain-falls keep adding to its volume, until the resistance presented by the lithological structure of the mass is balanced by the weight of the head of water accumulated above the level s, L; the flow at s then becomes constant, and the mean daily delivery will be an average of the total quantity of rain infiltered during a month, a year, or even a longer period, according to the size of the mass in which the water is stored.

Where, however, there are more than one point of issue, if the marginal edges of the two series of impermeable strata at s and s" are on the same level, the water flows both ways in nearly equal force; but when, as in fig. 18, the Wealden at s" rise above the Gault at s, the water tends to accumulate in the Lower Greensand f, until it reaches a line connecting s and s". This line will not be straight, but will present a curve varying constantly in its form according to the distance between s and s", the resistance opposed to the passage of the water, and the variation in the rain-fall. This would not so much affect the main spring at s as the minor ones at s' and s", for when the curve s, s', x, s′′ reaches, at x, a level higher than the point s" the water above the line pro

longed horizontally from s" will tend to find its level and escape at s", notwithstanding the rise of the strata in that direction. This spring would be the first to cease to flow in dry weather. A further fall of the water-level to the line sz, would next affect the spring at s', whilst that at s might, still maintain its perennial character, so long as any water sufficient to overcome the resistance of the strata remained above the level of v, L.

84. The sands being of comparatively uniform texture and resisting power, water can pass through them freely in all directions; and that which is stored in this Lower Greensand will tend rather to overflow, not at any particular points only, but along the whole length of the rim of the Gault,—to ooze out in fact along the entire line of the valleys at s and s', the transverse section of which is shown in fig. 18, except where a local depression in the lip of the Gault diverts a greater flow and issue of water than usual to that point. In consequence of this lithological character the springs in arenaceous strata are generally not so large as in the chalk or similar deposits, where the water is confined to and issues through a few large channels.

85. Some conception may be formed of the permanence which these springs may exhibit when it is considered that the fall of the water-level, s, s', x, s", of one foot over an area of one square mile, probably sets free not less than 50,000,000 gallons of water. But the water having to move laterally, and entirely through small channels (the interstices of the sands), and not having the advantage of the pressure which is exerted at greater depths, its motion is slow; and as it overflows the marginal edges of the strata generally,—and also transudes and escapes at the surface wherever subordinate beds of clay occur,-a large quantity, not being in bulk sufficient to form permanent springs, must be lost by evaporation. It is where the transverse depressions in the zone of outcrop are