102 MR. WM. SHONE ON THE SUBTEREANEAN 



of each rock-basin until submerged and buried under the Blue 

 Clay which divides the Upper and Lower Beds. 



In brief, the stratigraphical history of this most interesting 

 section appears to nie to be as follows : — The Lower Forest-bed 

 flourished upon Glacial Drift very little above the water-level of 

 the district. By subterranean erosion the level was gradually 

 lowered between the points X} and X^, X^ aud_ X*, X' and X% 

 X"^ and Y, until the site of the forest became a peat-morass, which 

 was eventually covered with Blue Clay. The time came when the 

 drainage was interrupted and subterranean erosion ceased ; then 

 the Upper Porest-bed spread out from the points of continuous 

 growth (X^ to X'^) across each rock-basin. It flourished until, the 

 cause of the interrupted drainage was removed, when subterranean 

 erosion again became active and the Upper Forest-bed sank below 

 the water-level, while peat-mosses grew on its site, until in turn 

 they were buried under tidal silt — the rate of subterranean erosion 

 at present being more rapid than the increase of the thickness 

 of the peat by growth. 



The principal characteristic of subterranean erosion is lateral 

 subsidence. Intermittent action follows as a natural consequence. 

 This is especially the case when an impervious bed rests upon a 

 pervious one, as in fig. 2 (p. 98). The liability of subterranean 

 erosion to be interrupted along any inclined plane of underground 

 drainage is, in my opinion, the cause of the bifurcation of the 

 Peat- and Forest-beds ; while the thinning-out of the deposits which 

 separate them into Upper and Lower towards each point of bifurca- 

 tion shows that the subsidence in each case was gradual and lateral. 



Mr. T. Mellard Beade published, in the ' Proceedings of the 

 Geological Society of Liverpool' for 1871-72,^ a number of very 

 valuable sections of the post- Glacial deposits of Lancashire and 

 Cheshire, from which the general tendency of Peat- and Forest- 

 beds to subside laterally is very obvious. I have omitted to 

 mention the numerous theories advanced to account for submerged 

 Peat- and Forest-beds — not, however, from any want of courtesy 

 towards the authors. The views I have attempted to prove are so 

 contrary to current opinion upon this subject as to constitute a 

 new departure, and should therefore be judged (I submit) inde- 

 pendently, according to the facts. 



fLyell, in his ' Principles,' vol. ii., described a submarine forest 

 at Bournemouth, and attributed its position to " the undermining' 

 of the sandy strata on which " it rested. He considered that " the 

 sea, in its progressive encroachments, eventually laid bare, at 

 low water, the foundations of this marshy ground ; in which case 

 much of the sand constituting these foundations might have been 

 washed out by the rapid descent of the fresh water through 

 them at the fall of the tide."^ My experience of the N.W. of 

 England submarine Forest-beds is that they became first sub- 

 merged by subterranean drainage towards the nearest freshwater 



^ Op. cit. p. 36 et seq. and plates ii.-iv. 

 2 Op. cit. 12th ed. (1875) vol. ii. p. 638. 



