IxXVi PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [March I9I3, 



The objects, however, which I had more especially in mind in 

 preparing this map were an illustration of the magnitude of the 

 post-Oligocene movements, and of their effect upon the platform. 

 The deepest hollow, as I have already mentioned, occurs in the 

 Hampshire Basin, where the base of the Gault is estimated to lie 

 more than 2500 feet below Ordnance datum. In the London 

 Basin the greatest depth attained may amount to 1500 feet, in a 

 small area 4 or 5 miles north of the Hog's Back. A depth of 

 1419 feet was proved in a deep boring close by. Between the two 

 basins rises the anticline of the AVeald. The ground in the 

 Wealden area reaches an elevation of 803 feet above the sea at its 

 highest point. I assume that the aggregate thickness of the part 

 of the Wealden formation and the Lower Greensand which has 

 been removed by denudation must have exceeded 700 feet, and that 

 the elevation of the base of the Gault in the crown of the arch may 

 be safely put at not less than 1500 feet. It follows that the 

 difference in level in the base of the Gault caused by the post- 

 Oligocene movements is not less than 3000 feet as between the 

 Wealden Anticline and the London Basin, and not less than 4000 

 feet as between that anticline and the Hampshire Basin. These 

 are large figures, and in face of them we are compelled to assume 

 that the platform cannot have escaped warping through the post- 

 Oligocene movements, even though it may have stood firm against 

 the more trivial folding and faulting suffered by the Secondary 

 rocks. 



Having now gained some conception of the magnitude of the 

 latest movements, we may endeavour to ascertain what the form 

 of the platform would be if those effects were eliminated : in other 

 words, to ascertain what its form was at the commencement of 

 Upper Cretaceous time. Lor this purpose we combine the two 

 maps (Pis. A & B), and by their aid correct the elevation or 

 depression undergone at every known point on the platform. 



PI. B shows what correction is necessary at any spot to 

 bring the base of the Gault to horizontality at present sea-level. 

 The same correction made in the depth to the platform at the 

 corresponding points on PI. A will restore those points to the rela- 

 tive levels which they held when the Gault was deposited. Thus all 

 the points where the Gault rests directly upon the platform will 

 be indicated by the figure 0. At the Richmond boring, where the 

 base of the Gault lies at 1122 feet below Ordnance datum, the 



