Correspondence — Rev. 0. Fisher. 45 



SUBSIDENCES AT BLACKHEATH. 

 Sir, — A frieud has just sent me the report of the committee for 

 the exploration of the subsidences of Blackheath.'^ The description 

 of the locality by Mr. Holmes is very clear ; and the arguments for 

 an arohseological explanation of the phenomenon well put. There 

 is one point, however, that is not mentioned. I would ask, are these 

 occurrences a new feature at Blackheath ? It appears that one oc- 

 curred in April, 1878, and two in November, 1880. It is hardly 

 probable that three should have occurred within so short an interval 

 from causes which, if they be those suggested by Mr. Holmes, must 

 at any rate have been in operation for at least centuries ; and no 

 such event have happened before. 



I have not examined this locality, but Mr. Holmes' excellent 

 diagram renders that hardly necessary. « 



I have given some attention to phenomena of this kind. In 1858 

 I examined, and described in the Journal of the Geol. Soc.,^ a most 

 remarkable but too little known collection of natural conical pits ou 

 Affpuddle and Piddletown Heaths in Dorsetshire. The stratification 

 is there very similar to that at Blackheath, but the depth to the chalk 

 surface probably less. The pits may be counted, I suppose, by hun- 

 dreds. The larger ones are marked by small circles on the Ordnance 

 Map. It is evident that the sinking of the soil is in constant pro- 

 gress, for there are recent shallow step-like depressions round the 

 edges of many of the pits, and I have been assured by old residents 

 in the neighbourhood, that a well-shaped pit once suddenly apj)eared, 

 and its sides afterwards fell in. There is, then, evidently a constant 

 cause at work, and it^ result continually manifested. Is there any 

 evidence of this being the case at Blackheath? Apparently not. 

 The cases, therefore, are probably not analogous ; and this would be 

 sufficiently accounted for by the greater depth of the chalk at Black- 

 heath, and the nearness of the water-level in it to its upper surface ; 

 so that " pipes " would not be produced in the chalk beneath the 

 Tertiaries. 



But I have also described another instance of a natural pit formed 

 quite recently, where, as far as I know, none had ever been formed 

 before ; and in other respects having points of similarity to those at 

 Blackheath. This occurred in 1861 at Lexden, near Colchester. It is 

 described in this Magazine.^ The points of similarity appear to be ; 

 (1) The novelty of the occurrence; (2) The depth of the chalk 

 below the surface ; (3) The chalk being covered by an impervious 

 stratum (London clay) ; (4) The surface of the chalk being below 

 the water-level of the district ; (5) The recent establishment of 

 works in the neighbourhood, pumping water from the chalk. 



By sending these remarks I do not presume to controvert the 

 arch^ological view taken of the case, for I have no special knowledge 



1 Blackheath: Edgar Neve ; 1881. 



2 Vol. XV. p. 187, 1859. It need hardly be said that the views there given on 

 denudation are out of date. 



■' Vol. II. No. 9, p. 101. (The diagram being inverted is corrected at the end of 

 No. 10, facing p. 192.) 



