and Sand called " Sand-pipes" in the Chalk near Nm^wich. 263 



cesses. For had the tubes, some of which are from 50 to 60 

 feet deep, and seven yards or more wide, been hollowed out 

 of the chalk before the introduction of any foreign matter 

 from above, great heaps of unrounded flints must have fallen 

 to the bottom, derived from all those intersected layers of 

 flint which formed part of the chalk above. We have seen in 

 the smaller pipes, where the flints are still in situ, that the 

 sands and gravel have penetrated many feet, and often yards, 

 below them ; so that if these cavities had been further extended 

 in width and depth, the large flints would have been loosened 

 from their matrix, and would have sunk down upon gravel 

 and other matter already introduced, and which had reached 

 a lower level. Srdly. As a corollary of the above proposi- 

 tions we must hold that the strata of the Norwich crag had 

 been already deposited upon the chalk before the excavation 

 of the sand-pipes, and this is further confirmed by the manner 

 in which the layers of loose gravel of the pipe d, fig. 1, and 

 the dark sand with casts of shells, e, fig. 3, have sunk into 

 the pipe. 



Having then adopted these opinions, and rejected all sud- 

 den and violent agency, whether for the erosion or filling of 

 the cavities, it only remains for us to inquire how waters 

 charged with acid may most naturally be conceived to have 

 produced such hollows. If some of the largest pipes of which 

 the bottom has not been yet reached, be prolonged indefinitely 

 downwards and connected with deep fissures, we may sup- 

 pose that springs charged with carbonic acid rose up at some 

 former period through the chalk and crag while these were 

 still submerged, as we know that in many parts of the bed of 

 the sea such springs do break forth. In proportion as the 

 chalk was corroded, the incumbent substances would sub- 

 side into the hollow thus formed, and the water would freely 

 percolate the matter thus intruding itself, dissolving any cal- 

 careous ingredients which may be associated with it, and still 

 continuing to widen the tube by corroding its walls. 



But this hypothesis will not account for the form of the 

 greater number of the sand-pipes, as some, even of those 

 which exceed fifty feet in depth, have been found to diminish 

 gradually downwards to a point. It is therefore more pro- 

 bable that such pipes are due to rain-water, which becoming 

 impregnated with carbonic acid derived from the atmosphere 

 and vegetable soil, has descended into pits or furrows which 

 may have existed on the surface of the chalk. Such water, 

 after dissolving a portion of the chalk, may readily have passed 

 out of the cavities which it gradually eroded, and penetrating 

 downwards might break out again in other places in the form 



