332 



THE GEOLOGIST. 



to state the manner in which the evidence oiFered by the sand-pipes 

 I have described bear npon them. 



There are only two theories that require notice— one being what is 

 commonly termed the mechanical theory, which v/as strongly advo- 

 cated by Mr. Trimmer, and the other the chemical theory whose most 

 important supporters are Sir Charles Lyell and Mr. J. Prestwdch. 



The former doctrine, as propounded by Mr. Trimmer, supposes the 

 pipes to have been formed on coast lines and between tide marks, or 

 at least within the area of broken water, by the action of the snrf 

 charged with sand, or assisted by stones and pebbles. He qnotes 

 the i^eculiar wearing of the rocks of our coasts, and maintains that 

 the agency which formed the basin-like and other shallow cavities in 

 these rocks, is the same that excavated the sand- and gravel-pipes of 

 the chalk and other limestones.* 



According to the other theory the pipes have been eroded by the 

 chemical action of carbonic acid held in solution by water. Without 

 going into details, it may sufiB.ce to state that this theory, as elabo- 

 rated by Mr. Prestwich, supposes the sand- and gravel-pipes of the 

 chalk to be "extinct natural water- conduits, which the waters at 

 different periods, through incessant filtration from a higher water- 

 bearing stratum in their tendency to reach a lower level, gradually 

 and quietly wore for them^selves by their solvent action alone." But 

 it is also the opinion of this geologist that in the harder limestones 

 many of the pipes may have been formed on the sites of pre-existing 

 cracks and fissures. f 



Against the former theory there are many grave objections, and in 

 my opinion, it most certainly fails to account for the origin of the 

 sand-pipes in the magnesian limestone. And Sir Charles Lyell and 

 Mr. Prestwich have almost, if not quite, demonstrated its insuflS- 

 ciency to account for the phenomena observed in those in the chalk. 

 It was the opinion of Mr. Trimmer that pipes in the act of formation 

 were to be found in the rocks of modern beaches, but I think with 

 Mr. Prestwich that in this opinion he was mistaken. At least, I 

 know of nothing analogous to them on the Dm^ham coast, and I have 

 conversed with those who have been longer acquainted with it than 

 myself, and who have also examined the sand-pipes, and they are 

 likewise equally ignorant of any tiling of the kind. Indeed it is 

 incomprehensible liow the action of the surf, howsoever assisted or 

 directed, could drill holes so deep and yet so narrow. One objection 

 of great force is that sand- and gravel-pipes are invariably found in 

 calcareous strata. Now how is this to be explained on the supposi- 

 tion of their origin being mechanical ? For, supposing this had been 

 the case, why do they not afifect all rocks of whatever natui^e or kind, 

 just as the surf of a littoral region afiects rocks of all descriptions, 

 vaiying its action in degree, and somewhat in mode, as the rocks 

 upon which it acts vary in hardness and general structm-e ? Some 



* Qiuxrt. Jtmv. Geo. Soc, vol. viii., p. 273 ; also vol. xi., p. 62. 

 t Quart. Jour. Geo. Soc., vol. xi., p. 80. 



