and Sand called " Sand-pipes" in the Chalk near Norwich. 265 



cut away by denudation, and other beds afterwards superim- 

 posed. 



As to the sandy clay found at the bottom and round the 

 exterior of the pipes, there can be little doubt, whatever hy- 

 pothesis we adopt, that this is due to rain-water which in its 

 passage through the gravel and loam has become charged 

 with fine particles of mud and iron, and has parted with these 

 particles at the point where it was absorbed bj' the surround- 

 ing chalk. A very minute quantity of the same mud enfiltered 

 into the contiguous chalk itself, has discoloured the rock and 

 rendered it impure as before described. The moistened state 

 of the chalk for a distance of several feet from each pipe, also 

 shows that this cause is still in operation. The layer of ochreous 

 clay extending upwards beyond the pipe, and intervening 

 between the chalk and overlying gravel or sand, may in like 

 manner be ascribed to the absorption of muddy water by the 

 porous chalk, a vacant space being gradually prepared for 

 the deposition of the mud by the corrosion of the limestone 

 by the acidulous water. 



It is scarcely necessary to state that the gradual under- 

 mining of the pipes and the successive subsidence of small 

 masses, is an hypothesis which accords well with the fact be- 

 fore alluded to, p. 259, and J] fig. 1, that beds, of loose gravel 

 and sand once horizontal, now bend into the orifice of some 

 tubes in a vertical direction. Had the entiie pipe been filled 

 at once, this arrangement would have been destroyed, and 

 accordingly no such stratification remains in those materials 

 which have descended by repeated movements to considerable 

 depths in large pipes. The grains of the sandstone contain- 

 ing casts of shells which at Thorpe form the dark-coloured 

 bed df fig. 3, which enters the pipe on both sides for many 

 yards, must have been loose and incoherent when they first 

 assumed their present position, and must have been afterwards 

 consolidated within the pipe. 



Assuming then that the sand-pipes of Norfolk are due to 

 atmospheric waters, it follows that chalk covered by crag had 

 emerged from the sea before the formation of the pipes. How 

 then shall we explain those cases where chalk not covered by 

 gravel or crag is traversed by large and deep sand-pipes? as 

 at Heigham, fig. 2, and other neighbouring localities. We 

 may answer that jiqueous denudation has removed large por- 

 tions of a deposit once overlying the chalk, and which sup- 

 plied, in the manner already described, the contents of the 

 sand-pipes. We may also suppose that this same denudation 

 has obliterated all traces of superficial pits and hollows like 

 those above noticed as having been recently formed at Henley. 



