320 Mr. S. V. Wood on some Tubular Cavities in 



the conclusion is strictly true for the strongest charges, as on 

 account of the loss through imperfect insulation these cannot be 

 examined in the manner which we have pursued. 



Before we make the attempt to discover a formula for the 

 calculation of the residue-curve, a few facts must be introduced 

 which may be useful to enable us to form a clear and correct 

 conception with regard to the residue, and thus prepare the way 

 for the hypothesis upon which the formula is to be based. 

 [To be continued.] 



XLVII. On some Tubular Cavities in the Coralline Crag at Sud- 

 bourn and Gedgrave near Orford. By Searles V. Wood, Esq.* 

 [With a Plate.] 



IT has been long observed that the surface of the chalk is more 

 or less perforated with funnel-shaped depressions or " sand- 

 pipes,^^ as they are called in England, while they are known in 

 France under the name of "puits naturels;" and in the year 

 1839, Sir Charles Lyell published a paper in the Philosophical 

 Magazine, in which was brought under notice these sand-pipes, 

 with an account of the manner of their supposed production. 

 These pipes are therein described as varying in magnitude as 

 well as in depth, and are also of various forms and inclina- 

 tions, but are all of an angular or funnel shape, being larger at 

 the upper part and generally terminating in a point : the mode 

 by which it is therein supposed these excavations have been 

 effected is by the operation of a chemical action upon the calca- 

 reous material of the mass of the chalk, the eroding action of 

 acidulated waters, produced by the decomposition of vegetable 

 matter, abstracting the carbonate of lime which had passed off 

 into the atmosphere in the form of carbonic acid gas, and the 

 depression or opening thus produced to have been gradually 

 filled in by the weight of the superincumbent gravel. This 

 hjrpothesis has been called in question by Mr. Trimmer, who in 

 a paper read before the Geological Society in 184'2, and published 

 in their Proceedings, vol. iv. part 1. p. 6, and part 3. p. 48.2, 

 1844, has assigned the production of these depressions to the 

 action of running waters which had ei'oded in furi'ows the sur- 

 face of the chalk, and he considers their funnel-shaped appeai'- 

 ance to be simply a transverse section of these gullies. It is, 

 however, most probable that the surface of the calcareous rocks 

 beneath the superficial gravel has been in various places acted 

 upon both by a chemical agent as well as by mechanical action ; 



* Communicated by the Author ; having been read before the Geological 

 Section of the British Association, at Ipswich, 1851. 



