74 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



§ 4. Theoretical Considerations, and Conclusion. 



On reference to the Sections 1 and 2 (PI. VI.), which give the pre- 

 sent configuration of the surface and the actual distribution along 

 certain lines, in the one case of the sand-pipes connected with the 

 Tertiary strata, and in the other of the gravel-pipes connected with 

 the drift, it will be seen by the dotted lines that all these pipes are 

 related to the sands and gravel before the continuity of the latter was 

 interrupted and when they extended in unbroken beds over the face 

 of the land ; that, in fact, the present valleys have all evidently been 

 excavated since the formation of these pipes, which consequently 

 belong to an older and anterior state of things. If, therefore, we 

 restore that state, it will appear that over the areas under examina- 

 tion, the Tertiary sands at one period and the gravel at another 

 formed extensive and nearly level beds stretching, without apparent 

 break, over large tracts of country. 



Now, as these deposits are extremely permeable, they would under 

 these conditions naturally constitute water-bearing strata, reposing 

 in either case on the thick mass of the Chalk. The water in both 

 these masses of water-bearing strata would also naturally descend to 

 the lowest level and there remain until it found vent. 



The Chalk, although it absorbs water in large quantities and with 

 great facility, is not properly a freely permeable deposit. When satu- 

 rated it will in fact hold up water, which then only passes through it 

 with excessive slowness if not assisted by cracks and fissures. Now it is 

 a well-known fact that the rain-water, which filtrates through the 

 mass of any permeable deposit or percolates through the fissures of 

 any impermeable one, tends to descend in both cases to a certain level, 

 dependent on the one side upon the nearest sea-level (if one be near), 

 and on the other on the level of the adjacent valleys; and that a slightly 

 cur^^ed line drawn between these points will always give the height 

 at which the water in the intermediate hills stands with certain limited 

 variations throughout the year *. This surface is called the line of 

 water-level. Of course when these water-bearing strata are over- 

 laid by an impermeable deposit supporting another permeable bed 

 at a height rising above this main and lower level, this second de- 

 posit will form an exception to the general law and present a higher 

 and independent water-level of its own. As, therefore, during this 

 former period, the mass of the Lower Tertiary sands would hold and 

 transmit water with great facility, that water would everywhere press 

 on the chalk, and tend to penetrate into it through cracks and fis- 

 sures, or to permeate into its general mass with extreme slowness at 

 the points of least resistance, a slowness in some measure assisted by 

 the thin seam of sandy clay with flints at the base of the sands. If, 

 under these circumstances, the mass of the chalk stood much above 

 the sea-level on one side, and the mean level of any large inland 

 plains on the other, then the water which escaped from the sands 

 would pass downwards through the chalk, and form a second water- 



* I have treated these questions at some length in " The Inquiry on the Water- 

 bearing Strata of London ;" Chapter on the Chalk, p. 57. 



