262 Mr. Lyell on the tubular Cavities Jilled mth Gravel 



which descends into the pipe as far down as where it enters 

 the chalk. 



Fiff. 3. 



iiii;affiti<i:MMitetM'A;al>^iMiii)j.^t^^ 



Upper Portion of a Sand-pipe at Thorpe, near Norwich, from a drawing by 

 J. B. Wigham, Esq, 



" At the junction of the chalk and overlying sand," observes 

 Mr. Wigham, " there occurs at Thorpe, (see fig. 3.) a layer 

 of large flints which have suffered slightly from attrition." 



Origin of the Sand-pipes. — We have now to consider in 

 what manner these cylindrical hollows have been first formed 

 and then filled with gravel and sand. If no pipes but those 

 of the smallest size had occurred, we might hiive imagined 

 that the tap roots of large trees had first pierced the chalk, 

 and then after growing to their full size and decaying had left 

 a vacant space into which loam and gravel fell. But when 

 we reflect on the dimensions of some of the pipes, we at once 

 perceive that more powerful causes must be appealed to. 



On consideration of all the facts above described, we can 

 scarcely hesitate to admit the following conclusions: 1st, That 

 the chalk has been removed by the corroding action of water 

 charged with acid, in which the siliceous nodules being inso- 

 luble, were left in situ in the smaller pipes after the calcareous 

 matrix had been dissolved. 2ndly, It is clear, from the man- 

 ner in which the large detached flints are dispersed through 

 the contents of the widest sand-pipes, that the excavation and 

 filling of the pipe were gradual and contemporaneous pro- 



