form of geological structure best adapted for the construction of Artesian wells. But for this object it is necessary that b should consist of permeable sands, that can absorb and transmit the rain water falling on its surface, and a and c of impermeable strata, through which water cannot pass. The water thus supplied, together with a portion derived from the drainage of the adjacent hills, will then accumulate in b, as in a natural reservoir, and if the quantity drawn off from it be less than the quantity supplied, the reservoir so circumstanced will keep constantly full.* In a well sunk in any part of the country between the outcropping edges of the strata at b and b', the water will rise at the surface with a force proportionate to the depth of the top of the shaft below a straight line drawn from 6 to b', or the distance between the lines y, z and v, L. Let, for example, such an opening be made through the superincumbent beds of clay a at x. It follows that the water would there rise to the level of the line y, z, provided that all the ducts were free, and time were given for its equilibrium to be established.

Practically, however, this height will depend in a great measure upon the distance of the point x from the outcrop of the strata b, upon the mineral character presented by b

*This I believe to be the theory now universally adopted. A curious but very general notion prevailed formerly, which accounted for the phenomena of springs and underground sources, by supposing that the sea penetrated through the strata at its bottom, and was by that means conveyed to any distance under the land, forming a water level almost constant with that of the sea. The extent of filtration caused, it was assumed, its salts to separate from it, and thus rendered the water pure and fresh. That portion of the water which descended to greater depths, where the central heat acted more strongly upon it, being converted into vapour, and rising through the fissures of the rocks, became condensed again on approaching the surface, especially in mountainous districts, and issued in the form of springs. This opinion originated in erroneous views with regard to the permeability of strata at the surface of the earth, it being supposed that rain water never penetrated beyond the depth of a few feet, and that some springs rising on high hills had no higher ground to draw their supplies from. Experiment and observation have proved both these positions to be wrong.

C

throughout that distance, and upon variations of level of the outcrop.

The general conditions of geological structure of the Artesian wells in London and its neighbourhood,* are such as has just been described, a being supposed to represent the London clay, b the beds of sand and mottled clays beneath it, and c the chalk ;-this latter allowing, however, of the passage of water to a considerable extent, and constituting in itself a partially water-bearing deposit (see ¶ 57).

§ 3. Geological Conditions affecting the Value of the Waterbearing Deposits, illustrated in their application to the Lower Tertiary Strata.

18. HAVING premised thus much as to general geological structure, I proceed to the consideration of the less understood conditions which affect the supply of water in the waterbearing strata. From a better acquaintance with the Tertiary strata, and not on account of their relative importance, I purpose confining myself for illustrations in this discussion to the Tertiary deposits exclusively, as the same mode of inquiry will apply with but little modification to any other formation whilst by using them as examples, one of the objects of this inquiry will be forwarded. The main points are

:

First. The extent of the superficial area occupied by the water-bearing deposit.

Second. The lithological character and thickness of the water-bearing deposit, and the extent of its underground

range.

Third. The position of the outcrop of the deposit, whether

* The number of Artesian wells (including both the sand and chalk wells) in and immediately around London is variously estimated at from 300 to 500, if not more.

in valleys or on hills; and whether its outcrop is denuded* or covered with any description of drift.†

Fourth. The general elevation of the country occupied by this outcrop above the levels of the district in which it is proposed to sink Artesian wells.

Fifth. The quantity of rain which falls in the district under consideration, and whether, in addition, it receives any portion of the drainage from adjacent tracts, where the strata are impermeable.

Sixth. The disturbances which may affect the waterbearing strata and break their continuity, whereby the subterranean flow of water would be impeded or prevented.‡

To proceed now to the application of these questions, in the particular instance of the Lower Tertiary Strata.

The reader who does not wish to follow through the amount of detail, which, although necessary as data in evidence, is not essential to the comprehension of the argument, can pass on to p. 93, where the general question is resumed, and afterwards a summary given of comparative results, in which the main facts respecting the waterbearing strata of the Lower Tertiaries, and Upper, and Lower Greensands, are recapitulated. Reference can be made back to the intermediate pages in case of information being required on any particular point.

The question of the Chalk formation, with regard to its value as a water-bearing deposit, forms, however, an exception to this arrangement. It is discussed by itself as a separate and independent inquiry (see § 5).

* The action which has operated the excavation of the valleys, and of which the result is termed "denudation," has, in the country around London, in some places left the surface entirely bare; as, for example, in the case of the London clay at Primrose Hill, or of the chalk on the south Downs around Brighton, or, still more strikingly, the hills around Tunbridge Wells. In these instances, the clay, chalk, and sandstones come close to the surface, and are merely coated with a few inches of loose earth.

+ Drift is a term applied to superficial accumulations of gravel, clay, sand, or brickearth, which are usually, but not always unstratified, and dispersed with much irregularity over the face of almost every country. Thus the London clay at Clapham Common, Kensington, and Hyde Park, is covered by a drift of gravel; at West Drayton by one of gravel and brick-earth; and the chalk on the hills around Ware is covered by other forms of drift in the shape of clays, gravel, and sand.

Mr. E. W. Brayley has directed my attention to a passage in the Life of William Smith, by Prof. J. Phillips, giving an account (p. 80-86) of some obervations of Mr. Smith on the causes regulating a supply of water to a well

19. With regard to the first question, it is evident that a series of permeable strata encased between two impermeable formations can receive a supply of water at those points only where they crop out and are exposed on the surface of the land. The primary conditions affecting the result depend upon the fall of rain in the district where the outcrop takes place; the quantity of rain-water which any permeable strata can gather, being in proportion to the magnitude of their superficial areas. If the mean annual fall in any district amounts to twenty-four inches, then each square mile will receive a daily average of 950,947 gallons of rain water. It is, therefore, a matter of essential importance to ascertain, with as much accuracy as possible, the extent of exposed surface of any water-bearing deposit so as to determine the maximum quantity of rain-water it is capable of receiving.

In deposits of so loose and friable a character as those of which the country around London consists, it is difficult accurately to trace the outcrop of any stratum. The gentle slopes of the hills, the absence of hard rocks, the high cultivation, and luxuriant vegetation, combining to conceal the geological features of the district.*

One side of the district occupied by the Lower Tertiary strata is bounded by the chalk, and this line of boundary, from the distinct mineral character of the rock, there is no difficulty in determining. On the other side is the London

clay, and there the line of division is generally very obscure, in Wiltshire. They are of much interest, as showing at that early period (1816) in Geology the application to this subject of many of the considerations mentioned above.

*This difficulty of determining the geological structure of the country by sections at the surface, had led me for some years past to collect all sections of wells, and had thus supplied me with much of the data connected with the Tertiary strata required for this inquiry. The late Dr. James Mitchell, a most zealous geologist, and who for many years diligently studied the country around London, had paid, however, far more attention to this particular subject of wells, and I am indebted to him for much valuable information upon it. His notes also have been obligingly placed in my hands by his nephew, Mr. J. Templeton, of Exeter.

and has led to far too great* an extension of the tract occupied by the Lower Tertiary strata. (See Map.)

20. The surface formed by the outcropping of any deposit in a country of hill and valley is necessarily extremely irregular, and it would be difficult to measure, in the ordinary way, the area of such a district; I have therefore used another method, which seems to give results sufficiently correct for our present purpose. I find by it that the area occupied by the outcrop of the beds between the London clay and the chalk, is equal to about 354 square miles. This includes the whole district from Hungerford in Berkshire to the Reculvers on the coast of Kent on the one side, and from Hungerford to Woodbridge in Suffolk on the other, embracing all the outcrop connected in unbroken line with the central mass of London clay, but excluding all outliers or partly detached portions (excepting those in Kent) the underground drainage from which is intercepted by want of continuity in the strata.

21. The second question relates to the effect which the mineral character of the formation will have upon the quantity of water which it may hold and transmit. If the strata consist of sand, water will pass through them with facility, and they will also hold a considerable proportion between the interstices of their component grains; whereas a bed of pure clay will not allow of the passage of water. These are the two extremes of the case; the intermixture of these materials in the same bed will of course, according to their relative propor

* More than double.

+ A plan borrowed from the geographers—that of cutting out from a map on paper of uniform thickness and on a large scale (one inch to the mile), and weighing the superficial area of each deposit. Knowing the weight of a square of 100 miles, cut out of the same paper, it is easy to estimate roughly the area in square miles of any other surface, whatever may be its figure. The experiments were repeated several times, and mean results are given. The weights were taken to a tenth of a grain.

See Table, p. 114.