334 



THE GEOLOGIST^ 



walls of the pipes in the magnesian limestone seems to afford ano- 

 ther argument in favour of this theory, for this effect is undoubtedly 

 due to chemical action of some kind ; though some may hold that 

 the decomposition of the surface may have resulted after the forma- 

 tion of the pipe by a very different agent ; and this is certainly 

 possible, though it must be evident at the same time that whatever 

 decomposed the limestone forming the surface of the pipe, had suffi- 

 cient power to be the primary cause of their formation ; for if the 

 application of the decomposing agent effected the results we see in a 

 certain amount of time, it is not to be doubted that by increasing the 

 period of application, so would we increase its results ; so that 

 having here a power competent to originate the phenomena of sand- 

 pipes, it seems more philosophical to credit it with their consumma- 

 tion than to call in the aid of another power whose capabilities even 

 to originate them is almost more than questionable. 



But though the chemical theory of the origin of sand- and gravel- 

 pipes is more satisfactory than the mechanical theory of Mr. 

 Trimmer, or than any conceivable theory of a mechanical nature, and 

 though I have little doubt myself but that they have really originated 

 by chemical action of some kind, yet there are one or two points 

 connected with this theory which seem difficult as yet to explain. 

 For instance, it is not easy to understand how the water contained in 

 the stratum overlying the limestone could be so extra-charged with 

 carbonic acid as to possess erosive power enough to excavate the 

 pipes. It seems plain that more than the usnal quantity of carbonic 

 acid would be required, or sand-pipes would be of more common 

 occurrence where limestone surfaces are exposed to the reach of 

 rain-water ; and if the roots of vegetables supplied tlie extra quantity, 

 as Sir Charles Lyell suggests, I see no reason why they should not be 

 found where limestones he immediately beneath the turf, which I 

 believe is never the case in any district where sand-pipes occm\ It 

 is possible, however, that rain-water may derive from vegetable 

 matter, as in the case of an overlying morass, an addition to its usnal 

 per centage of carbonic acid ; and the fact of the remains of so many 

 small roots being found at a depth of three or four feet in the core 

 of clay of some of the pipes I have described, may, perhaps, be con- 

 sidered as rather indicative of something of the kind. It is also 

 possible that it may derive additions from animal remains imbedded 

 in the overlying alluvium, as Mr. Prestwich suggests ; and the 

 absence of all remains of this kind in the alluvium in question is no 

 proof to the contrary, for this supposition necessarily includes then- 

 destruction in the derivation of their carbonic acid. But still, not- 

 withstanding the possibility of such supplies, I must confess that to 

 me it seems probable that we are still ignorant of the true som-ce of 

 the erosive agent; neither do I see any reason for supposing that 

 carbonic was the only acid emplo^^ed, and that by it alone were the 

 pipes eroded. 



.Vnother difficulty is the special application of the acid or chemical 

 agout to the particular spots occupied by the pipes. If avc suppose 



