PRESTWICH SAND- AND GRAVEL-PIPES. 



69 



as from his long-continued study of the superficial deposits of the 

 south of England his opinions are necessarily entitled to consider- 

 able weight. In this examination also, as the main features of these 

 sand-pipes have been brought forward, much further description will 

 be obviated. With the general view advanced by Cuvier and 

 Brongniart and by Dr. Buckland, and more critically laid down by 

 Sir Charles Lyell, I fully agree ; and my object now will merely be to 

 adduce some fresh proofs in its favour, and to suggest a general 

 cause for the formation of these peculiar excavations. 



§ 2. Special Phcenomena. 



As the name implies, the pipes are common both under sand and 

 gravel beds. They occur in fact wherever a loose and non-calcareous 

 permeable stratum of any extent overlies the Chalk or some calca- 

 reous rock. They present an infinite variety, but I will confine my- 

 self to the few essential points. 



Some years since I met with an instance of one of these pipes in 

 a chalk-pit near Lower Elmsden, a few miles south-west of Canter- 

 bury, which seemed to me conclusive of their formation by the slow 

 and gradual action of water after the deposition of the superincumbent 

 strata. The following is a section of this sand-pipe. 



Fig. 1. — Section of a Sand-pipe in a Chalk-pit near Lower Elmsden. 



Cl. 



a Thin stony band in the Thanet Sands. 



b Seam of clay and sand with green-coated flints. 



c Chalk. 



It will be observed that this pipe, which is about 1 2 feet deep, is 

 filled with the Thanet Sands, underlaid by the seam of clay and 

 sand (6) with the angular green-coated flints which always occurs at 

 the base of this deposit. It is not often that these sands are solidified, 

 but in this case a thin band (a) is semi-indurated — ^just hard enough 

 to hold together in blocks when broken, but not hard enough to allow 

 of any wear or exposure. This layer of soft stone runs horizontally 

 about 2 feet above the surface of the chalk. When, however, it reaches 

 the sand-pipe, its continuity is interrupted, and it is broken into a 



