KIRKBY — SANDI'IPES IN MAGNESIAN LIMESTONE OF DUUIIAM. 335 
water to hold acid in solution, and apply it to a sui-faco capable of 
being eroded by the acid it contains, we should naturally expect — 
granting that the water covered the whole surface — that as the 
hollows were deepened so would the higher portions of the smface 
be lessened in height, so that as the erosion proceeded the relation of 
the inequalities to each other would be pretty nearly preserved in 
their original condition. Mr. Prestwich certainly supposes that in 
the harder limestone the majority of the pipes are founded on cracks 
and fissures, though he is of the opinion that those in the chalk have 
not been thvis assisted, but have been worn out of a solid substance. 
And so far as we may judge from the pipes in the magncsian lime- 
stone, which is perhaps harder than any other limestone in which 
they arc known to occm", they certainly seem to bear out the former 
opinion, for many of them undoubtedly occupy the sites of pre-exist- 
ing breaks in the strata. But if we grant that any could originate 
without such assistance, the question still remains unsolved. And 
that some were not assisted in this way almost seems to be true, and 
some of those in magnesian limestone appear to belong to this class, 
so far as my experience goes. However, we must leave the clearing 
up of this point tor future research ; at present it seems to be a fact 
that some pipes, so far as we know, are formed in rocks that are solid 
and unbroken, and that others exist where cracks or breaks in the 
strata formerly existed. 
These cannot be said to be serious objections to this theory. They 
are certainly difficulties, but such as a better knowledge of the sub- 
ject will most probably remove. They do not affect the piinciple 
involved so much as its application in questions of detail. 
Before concliidiug I may draw attention to the peculiar position of 
the stratum of sand beneath which the pipes only occur. By refeiTing 
again to fig. 1, it will be seen that the sand gradiially thins out as it 
dips, so that the overlying stratum of clay ultimately rests imme- 
diately upon the limestone, and so prevents the water lodged in the 
sand from escaping in the direction which, in its search for the 
lowest level it would naturally take. It also appears to be as 
thoroughly enclosed in lateral directions, at least, though its eastern 
limits have not been reached, it thins out to the west, and is en- 
veloped in the same manner as seen in the transverse section, so that 
it can scarcely be doubted but that this deposit of sand is overlapped 
by clay on all sides, except along its upper edge, where it abuts 
against the gTavel. It consequently follows that though it had the 
power of receiving water collected on the higher grounds, and trans- 
mitted to it by the gravel, yet, on account of the impermeable nature 
of the stratum covering it and overlapping its edges, the Avater it 
received would either have to remain lodged in it, or find a lower 
level by passing through the limestone. Such seems to be exactly 
the circumstances which Mr. Prestwich supposes to have obtained 
duiing the formation of the pipes in the chalk — indeed, which were 
necessary for their formation. It is interesting to know that there is 
so close an agreement in the geological relations of the water-bear- 
