446 Evans — On Cavities in Old River Gravel. 



in the Chalk are due to the percolation of water charged with the 

 carbonic acid derived from the decomposition of vegetable matter, 

 which dissolves the calcareous portion of the Chalk, leaving any 

 argillaceous particles behind, with which (and with the superin- 

 cumbent gravel, sand, or clay, which descends as the calcareous 

 matter beneath is removed,) the pipes are usually filled. The 

 ari'angement of the matter enclosed in the pipes and of the bands of 

 flints through which they sometimes pass, proves the correctness of 

 Mr. Prestwich's views. The general action of the carbonated water 

 on calcareous strata is evinced by similar pipes occurring in Oolitic 

 beds, such for instance as the celebrated natural wells near Poictiers 

 and in the Coralline Crag of Suffolk, which in some places, as in a 

 pit not far from the Eailway Station at Aldeburgh, is absolutely 

 riddled with such pipes. And not only have such pipes been formed 

 in this manner, but the whole upper surface of the Chalk, where 

 covered by beds of gravel, appears in many instances to have been 

 so eroded that the beds which would seem to have been originally 

 laid out horizontally on an approximately smooth base, now rest 

 upon a rough and irregular surface full of peaks and hollows. 

 Where, as is sometimes the case, the gravel in pits has been cleared 

 away completely, so as to leave the upper surface of the Chalk bare, 

 a sort of model of a mountainous country is presented to our eyes. 



We have seen that in nearly all cases the pipes that have been 

 thus gradually eroded, have been filled by the superincumbent beds 

 gradually following down, but we can readily conceive instances in 

 which some one or other of the upper beds might be so tenacious as 

 not to subside into the hollow beneath until a large superficial area 

 was left unsupported. The result in such a case would be a cavern 

 of greater or less magnitude, from the bottom of which proceeded a 

 pipe passing through calcareous rock, and filled with the remains of 

 the less tenacious beds underlying the more unyielding bed, which 

 would form the ceiling of the cavern. These conditions were ful- 

 filled in the cavity in the Broom Hill Pit, where the tenacious bed 

 was the sand with the compact argillaceous band at its base, the in- 

 coherent beds which filled the pipe were the ochreous gravel and 

 sands, and the pipe was eroded through the calcareous gravel, and 

 probably into the Chalk itself. Since then, in fact within the last 

 few days, I have had another opportunity of visiting the pits at 

 Santon Downham, of there inspecting one of the caves, which, how- 

 ever, it was not possible thoroughly to investigate. Only the upper 

 part of the cavity was visible, of a regular vaulted form, the sides 

 being formed of coarse incoherent gravel, while the roof or ceiling 

 consisted of a much finer gravel, with a more coherent matrix 

 of red sand. The bottom of the cavity was at the time filled with 

 coarse gravel fallen in from the sides, and there was no opportunity 

 of observing whether there was any sandpipe at the bottom, though 

 no doubt such was the case. In the wall of gravel close by, there 

 were two sandpipes in which the upper beds had followed down in 

 the usual manner. 



With regard to the supposed almost exclusive occurrence of the 



