80 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



deepening, so much so that it was necessary every few years to fill 

 them up. I have however some doubts on this subject*. 



In the case, again, of the swallow-holes described in a previous 

 paper f, we witnessed a somewhat similar action of the escape of water 

 from a higher level to a lower one in the Chalk. In such cases, how- 

 ever, the more free and rapid flow of water has probably worn a 

 continuous passage in the mass of the Chalk. These funnel-shaped 

 holes also are caused by the dissolving power of water holding car- 

 bonic acid in solution, and not by mechanical agencies, for the feeding 

 stream is usually unimportant, transports but little sediment, and 

 filters quietly through the small quantity of sand and gravel that 

 remains undisturbed at the bottom of these excavations. 



In conclusion, therefore, I view these sand- and gravel-pipes in the 

 chalk and other soft calcareous strata J as extinct natural water-con- 

 duits, which the waters, at different periods, through incessant filtra- 

 tion from a higher water-bearing stratum in their tendency to reach 

 a lower level, gradually and quietly wore for themselves by their 

 solvent action alone ; the size of the pipes mainly depending both 

 upon the length of time the operation continued, and also upon the 

 extent of difference of level between the two water-surfaces. 



EXPLANATION OF PLATE VI. 



The two Sections (1 and 2) are from actual survey across parts of 

 the neighbourhood of London, where the sequence of the phsenomena 

 over a length of country, easily accessible (and at the same time 

 offering some very attractive scenery), presents the required conditions 

 of showing the relation of the pipes to the main masses of sand and 

 gravel, and also the independence of the whole to the existing con- 

 figuration of the surface. The heights are taken approximately with 

 the aneroid barometer and may not be perfectly exact, but I believe 

 them to be sufficiently so for the purpose in view. 



The scale of heights to distances is inevitably greatly exaggerated, 

 in consequence of the diminutiveness of the subordinate features to be 

 noticed. The scale of distances is the same as that of the Ordnance 



* It has since been suggested to me that these depressions may be caused by 

 old chalk-holes filled up with rubbish over which the grass has grown, but the 

 decay of which leads to a constant falling in of the surface. This is likely enough to 

 occur, and it would be difficult without opening some of these hollows to determine 

 the point. The record of such places having been chalk-holes is of course likely 

 soon to be lost, although at the same time one would have expected the frequent 

 excavations of such holes would have caused the resulting appearances to be better 

 known. Might not, however, the circumstance of decaying vegetable or animal 

 matter being accumulated in the excavations tend to the evolution of a larger 

 proportion of carbonic acid, which, taken up by the water draining from the 

 adjacent surface, would tend to set in operation the action we have been alluding to ? 

 It is, I think, quite possible that both the natural and artificial causes may pro- 

 duce the same result. 



t Quart. Joum. Geol. Soc. vol. x. p. 222. 



X The pipes in the harder limestones, which, where these same conditions pre- 

 vailed, would necessarily operate a similar water-wear, are more likely to have re- 

 sulted from this wear having been directed into given channels by pre-existing 

 cracks and fissures. Some gravel-pipes in the Ragstone at Maidstone aflford excel- 

 lent illustrations of such results. 



