82 



SAJSTD-PIPES. 



[Ch. Yll 



But there are other deep hollows of a cylindrical form found in Eng- 

 land, France, and elsewhere, penetrating the white chalk, and filled with 

 sand and gravel, which are not so readily explained. They are some- 

 times called "sand-pipes," or "sand-galls," and "puits naturels," in 

 France. Those represented in the annexed cut were observed by me in 



Fig. 101. 



Sand-pipes in the chalk at Eaton, near Norwich. 



1839, laid open in a large chalk-pit near Norwich. They were of very 

 symmetrical form, the largest more than 12 feet in diameter, and some 

 of them had been traced, by boring, to the depth of more than 60 feet. 

 The smaller ones varied from a few inches to a foot in diameter, and 

 seldom descended more than 12 feet below the surface. Even where 

 three erf them occurred, as at a, fig. 101, very close together, the parting 

 walls of soft white chalk were not broken through. They all taper 

 downwards and end in a point. As a general rule, sand and pebbles 

 occupy the central parts of each pipe, while the sides and bottom are 

 lined with clay. 



Mr. Trimmer, in speaking of appearances of the same kind in the 

 Kentish chalk, attributes the origin of such " sand-galls" to the action 

 of the sea on a beach or shoal, where the waves, charged with shingle 

 and sand, not only wear out longitudinal furrows, such as may be ob- 

 served on the surface of the above-mentioned chalk near Norwich when 

 the incumbent gravel is removed, but also drill deep circular hollows by 

 the rotatory motion imparted to sand and pebbles. Such furrows, as well 

 as vertical cavities, are now formed, he observes, on the coast where the 

 shores are composed of chalk.* 



That the commencement of many of the tubular cavities now under 

 consideration has been due to the cause here assigned, I have little doubt, 

 But such mechanical action could not have hollowed out the whole of 

 the sand-pipes c and d, fig. 101, because several large chalk-flints seen 

 protruding from the walls of the pipes have not been eroded, while sand 

 ; and gravel have penetrated many feet below them. In other cases, as 



* Trimmer, Proceedings of Geol. Soc. voL iv. p. 1, 1842. 



