260 



THE GEOLOGIST. 



undisturbed shows this notion not to be tenable 

 referred to passes completely through the pipes. 



besides, the layer of flints 







K f! 

















in 



v r 



ifl ff W Wll] 



^ :L_-~ — : — - -- ^M=- — =-i^==^ 





•Hi 



__i 



Fig. 3. 



-General form of the sand- and gravel-pipes at Grays Tlmrrock, being more or 

 less triangular or funnel-shaped. 



There have been two theories advanced to account for the formation of these 

 singular phenomena. 1. The mechanical theory by Mr. Trimmer, which sup- 

 poses them to be produced by the wearing action of one or more stones, put into 

 rotatory motion by water, at a period when the chalk region in which they occur 

 formed a sea-shore, the waves being the prime moving power, and that the holes 

 thus drilled afterwards filled with gravel or sand. This theory is evidently in- 

 sufficient to explain these very long pipes, on account of the occurrence of flints 

 throughout the whole depth, and not strewn merely at the bottom, as they 

 would be if the pipes had been worked out by mechanical abrasion ; at times 

 flints are found at or near the bottom of sand- and gravel-pipes, but they are not 

 water-worn, and still retain their original shapes, and eveu their calcareous 

 coatings. 



_ The other theory is the chemical, namely, that the pipes have been gradually 

 dissolved to their present shape by the action of water highly charged with car- 

 bonic and other acids, subsequent to the deposition of the sands and gravel 

 above them. 



Suppose slight hollows or depressions to be formed on the surface of the 

 chalk, the acidulated water would collect there, and finding the easier passage 

 downwards, there would soon become fixed water channels, and these small 



Ji! ¥ \ 1 



l'iy. I.— TTpnnl appearance of the surface of the chalk, showing slight degressions and 

 incipient sandpipes. 



