332 
TlIK OEOLOGlSr. 
to state the manner in which the evidence offered by the sand-pipes 
I have described bear upon them. 
There are only two theories that require notice — one being what is 
commonly termed the mechanical theory, which was strongly advo- 
cated by Mr. Trimmer, and the other the chemical theory whose most 
important supportei's are Sir Charles Lyell and Mr. J. Prestwich. 
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 surf 
charged with sand, or assisted by stones and pebbles. He quotes 
the peculiar wearing of the rocks of om' coasts, and maintains that 
the agency which formed the basin-like and other shallow cavities in 
tliese 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 suffice to state that tliis 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 
difi'erent periods, through incessant filtration from a higher water- 
bearing stratum in their tendency to reach a lower level, gTadually 
and quietly wore for themselves 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 fissm-es.f 
Against the former theory there are many gi-ave 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. Pi'cstwich have almost, if not quite, demonstrated its insuffi- 
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 vsdth 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 anything of the kind. Indeed it is 
incomprehensible how 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 gTavel-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 affect all rocks of whatever nature or kind, 
just as the surf of a littoral region affects rocks of all descriptions, 
varying its action in degree, and somewhat in mode, as the rocks 
upon which it acts vary in hardness and general structure ? Some 
* Quart. Jour. Geo. Soc, vol. viii., p. 273 ; also vol. xi., p. 62. 
t Quart. Jour. Geo, Soc, vol. xi., p. 80. 
