Vol. 69.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxvii 



depth to the platform must be diminished by that amount, and will 

 be indicated by — 9S instead of —1220 feet ; while in the Calvert 

 and Bletchley borings, where the Gault no longer exists, but 

 where its base is estimated to have been 700 feet above sea-leveL 

 the respective depths to the platform must be increased by that 

 amount, and become —853 and —859 feet instead of —153 and 

 — 159 feet. The map forming PL C shows the result of applying 

 this correction to every point within, or close to, the area occupied 

 by the Gault, at which the platform has been proved. It therefore 

 reproduces the form which the platform would possess if the base 

 of the Gault Avere a true plane at sea-level. In other words, it 

 eliminates the effects of the post-Oligocene movements. 



On comparing PI. C with PI. A we are able to realize that the 

 form of the platform was much altered by those movements. 



In the central portion of the map a tract of high elevation 

 clearly defines itself under London, and reappears north-eastwards 

 near Harwich. 



The contouring between Harwich and Lowestoft now ranges 

 north-north-eastwards along the slopes of that tract, instead of 

 northwards. 



The ridge apparently proved by the Calvert and Bletchley 

 borings is no longer in evidence. Those places now appear as 

 being situated on the north-western slopes of the elevated tract ; 

 nor is there any reason to doubt that the same slope extends south- 

 westwards towards Burford, where I have inserted an estimate of 

 — 1300 to —1600 feet, on the supposition that the base of the Gault,. 

 if it still existed, would be between 500 and 800 feet above the sea. 

 Thus the existence of a ridge in the Palaeozoic platform is not in 

 itself sufficient to prove the continuation of an ancient axis, such as 

 that of Charnwood. On the other hand, the fact that Cambrian 

 rocks constitute the platform at Calvert, and the probability . that 

 Charnian rocks were reached at Bletchley are highly significant of an, 

 old line of upheaval. 



On the south side of the elevated tract the features of the platform 

 increase in boldness. The upland ends in a great declivity which 

 is well-defined from Dover past Penshurst, and has a gradient of 

 upwards of 1 in 26. That its descent continues to a depth of more 

 than 4000 feet is proved at Penshurst, but how much farther it may 

 go is not known. Assuming, however, that 1500 feet is not an 

 exaggerated estimate for the elevation of the base of the Gault in the 

 crown of the Wealden arch, and taking this in connexion with the 



