PRESTWICH — SAND- AND GRAVEL-PIPES. 79 



The difference of the two water-levels assumed above (Diag. A & B, 

 PL VI.) is not, I should mention, a merely hypothetical case. On 

 the Chalk hills of Kent and Surrey the water-line in the Chalk often 

 lies at a depth of from 300 to 400 feet, whilst in the small outliers of 

 Tertiary sands and in the gravel on these hills a small body of 

 water, where the mass of strata is large enough and other con- 

 ditions favourable, is held up by these beds at a depth of 10, 20, or 

 30 feet only. 



As the gravel is generally without any such partially impermeable 

 seam at its base as occurs in the Tertiary sands, the underlying 

 chalk surface seems to have been liable to be attacked by the acidu- 

 lated waters in a greater number of places, and to present a larger 

 proportion of pipes and indentations than when overlaid by the 

 sands. In the case both of the sands and the gravel, I presume 

 the strata to be fully charged with water accumulating and lodging 

 in them, and not merely giving a temporary passage to an occasional 

 rain-fall. This is an essential condition. A seam of clay is occasionally 

 met with at the base of the gravel, and a thin layer of tough clay 

 is of common occurrence on the sides and at the base of the pipes. 

 It has the appearance of having been washed out of the superincum- 

 bent gravel and stopped by the chalk, as in the case of an ordinary filter. 

 It is less apparent in the pits under the sands which are cleaner. 



With regard to an objection which has sometimes been raised to 

 this view of the formation of sand- and gravel-pipes, viz. that the 

 water does not hold a sufficient quantity of carbonic acid to operate 

 on so large a scale, it must not be overlooked that it is not only 

 the quantity obtained by the rain directly from the atmosphere 

 and from the ground, but also the additional and constantly forming 

 supply that is evolved by the action of the air, likewise held in so- 

 lution by rain-water *, upon the remains of organic matter, whether 

 vegetable or animal, dispersed in however small quantities throughout 

 the strata, and tending to the renewal of the carbonic acid removed, or 

 rather, if the expression may be used, rendered latent, by coming 

 into contact and combining with any carbonate of lime occurring 

 perchance in these otherwise generally non-calcareous water-bearing 

 strata. The length of time during which the operation continued I 

 also suppose to have been exceedingly great. 



Although these old channels (the pipes) have ceased to act, similar 

 phaenomena to that which produced them may still be occasionally 

 observed, or rather it is possible that some existing phsenomena may 

 be referred to the same agency. Mr. Strickland pointed out this 

 fact to Sir Charles Lyell in the neighbourhood of Henley ; and I 

 have had my attention directed to a spot of ground in Rickmans- 

 worth Park, where a bed of gravel caps a chalk hill overlooking the 

 valley of the Colne. Circular indentations are there formed on 

 the surface of the ground, which I was informed kept constantly 



* Rain-water absorbs air in the proportion of about ^th of its volume, but the 

 oxygen is always in excess to the proportion in the atmosphere. The water from 

 the Artesian well of Crenelle, after having passed 100 miles underground, still 

 contains small proportions of air, carbonic acid, and organic matter. 



