260 
THE GEOLOGIST. 
undisturbed shows this notion not to be tenable ; besides, the layer of flints 
referred to passes completely through the pipes. 
Fig. 3.— General form of the sand- and gravel-pipes at Grays Thurrock, being more or 
less triangular or funnel-shaped. 
There hare 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- 
sufiicient 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 even their calcareous 
coatings. 
The other theory is the chemical, namely, that the pij)es 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 smaU 
fi I 1 II 
V ^ ( / 
L n 
■ .!!! ^ ^ !!i i 
'i . ^ 
— —J 
Fig. 4.— TJsual appearance of the surface of the chalk, showing shght depressions and 
incipient sandpipes. 
