KTRKBT — SANDPIPES IN MAGNESIAN LIMESTONE OF DURHAM. 335 



water to hold acid in solution, and apply it to a surface capable of 

 being eroded by tbe 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 surface 

 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 thus assisted, but have been worn out of a solid substance. 

 And so far as we may judge from the pipes in the magnesian lime- 

 stone, which is perhaps harder than any other limestone in which 

 they are known to occur, 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 for 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 concluding I may draw attention to the peculiar position of 

 the stratum of sand beneath which the pipes only occur. By referring 

 again to fig. 1, it will be seen that the sand gradually 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 gravel. 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 water 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 sujDposes to have obtained 

 during 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- 



