68 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



out by the percolation of water, leaving merely casts in the semi- 

 indurated sand. 



In support of the foregoing arguments, I would further observe 

 that the sand-pipes do not, any more than the gravel-pipes, appear 

 to be confined to any particular zone or belt marking ancient coast- 

 lines. Taking first the Thanet Sands. The outcrop of these beds 

 extends from Ramsgate to Deptford, with a width generally (taking 

 the detached outliers) of five or six miles, and often of ten to 

 twelve. Sand-pipes are common across the breadth as well as 

 along the length of this tract, which can hardly be reduced to the 

 compass of a beach-line. Besides, there is no appearance anywhere 

 of beach-action. The structure of the sands is throughout uniform, 

 and the same in, over, and around the sand-pipes. The same 

 argument applies with greater force to the gravel, which has a still 

 longer and broader extension. Lastly, I can readily conceive small 

 indentations and furrows to be worn by river or sea-shore action *, but 

 I cannot imagine the possibility of hollows, sometimes almost as narrow 

 and as straight as an inverted chimney-shaft, to have been formed by 

 any such operations : the sea-action might form broad saucer or cup- 

 shaped hollows above low-water level, but it could never drill cylin- 

 drical holes 40, 60, or 80 feet deep, for at such a depth they 

 would in all probability be in greater part far below low-water mark, 

 and therefore below a permanent water-level, where tidal water action 

 would necessarily cease, and their excavation by the operation of 

 wave-action would become therefore a physical impossibility. Also 

 such sea-shore hollows would contain within themselves the 

 instruments which had worked them out, and the deeper they were 

 the more rounded and worn would be the pebbles. These again 

 would differ from the material subsequently washed into these 

 cavities, and which material in that case would consist of the first 

 sediment subsequently spread over that spot ; whereas, as a rule, no 

 distinct accumulation of worn pebbles is found at the bottom of the 

 pipes, the contents of which are always similar in lithological structure 

 to the material forming the general superincumbent mass, and are 

 composed, not of successive additions horizontally superimposed and 

 derived only from the same sediment as the layer next upon the 

 chalk, but of all the layers successively superimposed ; and an in- 

 verted cone is thus formed, with a core derived from the higher 

 and later-formed portions of the superincumbent beds. It even 

 sometimes happens, when the pipes are filled with clay, sand, and 

 gravel-drift, that the sides and lower part of the pipe contain per- 

 fectly angular and unworn flints, while the rounded and worn flints 

 form the centre of the pipe — an arrangement the very reverse of 

 what we should expect if the cavities had been drilled by mechanical 

 agency, the agents being the worn pebbles, which should therefore 

 occupy the place here held by angular and unworn flints only. 



I have been thus particular in examining Mr. Trimmer's views, 



* I have seen them on the coast about a foot deep. The gully-holes in many 

 rivers form also a well-known phsenomenon ; these however arise from the infil- 

 tration of water, but belong to the same class of phaenomena as the swallow-holes. 



