1854.] PRESTWICH ON SAND-PIPES. 241 



2. On the Origin of the Sand and Gravel Pipes in the Chalk 

 of the London Tertiary District. By Joseph Prestwich, 

 jun., Esq., F.R.S., F.G.S. 



[The Publication of this paper is postponed-] 



[Abstract.] 



After referring to the observations and researches of earlier writers 

 on these peculiar cavities, the author dwelt upon the theory advocated 

 by Dr. Buckland and Sir C. Lyell, that these cavities were due to 

 the action of water, holding carbonic acid in solution, constantly per- 

 colating through the same cavity, dissolving the chalk and letting 

 down the superincumbent sand and gravel. After pointing out the 

 difficulties in the way of the hypothesis of mechanical water-action, 

 such as the frequently great depth of the pipes (upwards of fifty 

 feet), and the general absence of the rounded pebbles that should 

 have remained in the cavities after having been the immediate 

 agents in the perforation, the author (allowing that irregularities of 

 the surface may have been caused by denuding action) proceeded to 

 point out that the pipes occur wherever a stratum permeable to 

 water overlies the chalk or other calcareous rock to any considerable 

 extent, and suggested that they must have had their origin during 

 a period when the chalk and the superincumbent tertiaries formed 

 an extensive tract of horizontal dry land, previously to the surface 

 assuming its present configuration ; that at these former periods the 

 tertiary sands or the gravel constituted extensive water-bearing 

 strata, whilst, owing to the form of the surface, the water-level in the 

 chalk stood at a height very much less than in the superincumbent 

 beds ; consequently the atmospheric waters, more or less charged with 

 carbonic acid, percolating freely through the superficial sandy beds, 

 rested on the chalk, and by its tendency to find a lower level, gra- 

 dually dissolved passages through the chalk to that lower level at 

 which water would stand in the latter formation. The superincumbent 

 sands or gravels, as the case may be, gradually subsided, more or 

 less conformably, into the deepening cavity caused by the loss of the 

 chalk in the funnel or pipe below. When the chalk and overlying 

 tertiary beds were locally upheaved, shattered, and partially denuded, 

 the newly made valley-courses gave exit, in springs along their sides, 

 both to the water of the lower water-level and the water of the super- 

 ficial sands and gravels ; the sand-pipes becoming almost all deserted 

 as water-channels, except in such local instances, perhaps, as are now 

 seen where cavities are forming in the chalk beneath existing gravels, 

 or where " swallow-holes" in the chalk continue a somewhat analogous 

 action. — Vide supra, p. 223. 



