ANCIENT EARTHWORKS ture of this pipe, it was withdrawn and another substituted. This was driven down to the depth of 84 ft. 6 in., when the point was stopped by some hard mass which could not be pene- trated, and the pipe had to be withdrawn, a task which was effected only with the greatest difficulty, though the first pipe had been withdrawn with ease, and the second had been driven down, as nearly as possible, in the track of the first. Then, as the money spent had somewhat exceeded the amount subscribed, it was resolved to withdraw as much of the timber as might be practicable, and to fill up the hole. The Exploration Committee, as a whole, were not able to accept any view as to the causes of this subsidence as conclusive. The present writer, who was a member of the Committee, was permitted, however, to append to the Report some observations pointing out that the clayey beds of the Woolwich series (which were the source of the main difficulty during the exploration), by concentrating the water falling as rain on the surface of Blackheath towards the base of the pebble beds, would prevent it from acting on the chalk beneath. This fact would consequently make any explanation of the subsidence as the result of the natural action of water on the chalk untenable. But that, granting the existence of a shaft ending in a chamber in the chalk, under Blackheath, similar to that at Eltham Park, the result of the action of the water at the base of the Blackheath Pebble Beds on a neglected and disused shaft would be to cause an enlargement there which would ultimately produce a subsidence result- ing in a hole at the surface such as had appeared (see figs. A,B, C, fig. 10). And (to quote the Report) ' Professor Prestwich pronounces, with confidence, that so far as he can judge, the cause of the subsidence is not geological : Mr. Whitaker leans to the same opinion.' ' BlaeKhtortii ftttle Seatty m Sand^Z riyc. Fig. 10. Diagram Sections showing necessary Results of Long Disuse on a Shaft and Chamber at Blackheath. During the present year (1906) a tunnel, in connexion with the Main Drainage works of the London County Council, has been in process of formation in the chalk under Black- heath, and shafts have been sunk at intervals along its course. I noticed that two of these shafts, one north of the road from the south-western corner of Greenwich Park to Morden College, the other on the southern side of the Shooters Hill Road, were in a line with the subsidence which appeared at the surface on 12 April, 1878. Much interesting information was kindly given me by Mr. B. C. Cass, a member of the firm of Messrs. S. Pearson and Sons, the contractors, about the workings under Blackheath. I learned from him that the water found at the base of the pebble beds forming the surface caused no serious difficulty when the shafts were being sunk ; and that, under the spot at which the above-mentioned subsidence occurred, the chalk, at a depth of about 120 ft. from the surface, was found to be broken up and mixed with soil and other material from beds nearer the surface. Though this discovery is evidently important as indicating at this spot a special artificial connexion between the chalk and the surface, it seems at first somewhat disappointing to hear only of a confusion of rocks where a distinct chamber in the chalk, like that at Eltham, might be expected. But it must be remembered that the Eltham shaft and chamber were accidentally discovered after they had • Thus I may claim that the best geological authorities were against a merely geological explanation, and so far, at least, in favour of my view. On the other hand, up to 1881, deneholes had been ignored by everybody but Mr. Spurrell. I feel accordingly that it would not be justifiable, in an account of the deneholes of Kent, to omit the subsidences at Blackheath, and their probable origin. 453
