64 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



In the preceding remarks I have confined myself to a defence of 

 my own conclusions. The only objection which I shall advance at 

 present against the theory of Mr. Prestwich is, that while it supposes 

 the cavities to have been filled by the subsidence of strata horizontally 

 deposited before the formation of the pipes, some of his sections show 

 a band of clay containing green flints at the base of those strata, which 

 is continuous between the pipes and on the sides of the cavities. If, 

 therefore, its position within the cavity is the result of subsidence, 

 and not an original condition of deposit, that portion which originally 

 extended over the mouth of the cavity now covers an area within it 

 much more extensive than any over which it could have been spread 

 without such a degree of stretching as would have been inconsistent 

 with the continuity of the band, and of which it exhibits no evidence. 



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. 



[This paper was read January 18, 1854*.] 



[Plate VI.] 



§ 1. Introduction. 



The peculiar funnel- and shaft-shaped cavities, so common in many 

 places on the surface of calcareous rocks, sometimes only a few feet 

 but not unfrequently many yards deep, and filled with sand, gravel, 

 and loam belonging to deposits of various subsequent dates, form a 

 very remarkable geological feature, especially in Chalk districts. 

 They have long been the object of occasional attention, and various 

 suggestions have been made to account for their origin. The ex- 

 planations, however, have mostly had reference only to the mode in 

 which these cavities could have been excavated, and have not often 

 embraced any general view of the conditions which led to the wide- 

 spread operation of the required causes. The Chalk district around 

 London is peculiarly favourable for the study of this phaenomenon ; 

 and my object in the following pages is to endeavour to determine 

 the general law which has led to a result so common here, and so 

 universal in almost all calcareous districts. 



Cuvier and Brongniartf make frequent mention of sand-pipes {puits 

 naturels) in the Chalk and the Calcaire grossier, ascribe their origin 

 to the percolation of water, and show that they belonged to different 

 geological periods. Of the exact mode of operation, or the causes 

 which produced the phsenomena, they offered no explanation. 



These general views, as guiding the line of argument, have, with 

 few exceptions, been usually received by geologists. Amongst others 

 M. Passyl, in describing the Chalk district of Normandy, alluded to 



* For the other communications read at this Evening Meeting, see Quart. 

 Journ. Geol. Soc. vol. x. p. 231 et seq. 



t Description geognostique des Environs de Paris, edit. 1822, p. 76, 134, 141. 

 % Desc. geogn. du Dept. de la Seine inferieure, p. 139. 



