part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxxi 



earlier planes, so that by the beginning of the Upper Cretaceous 

 Period the lowest plane, or that at the hase of the Mesozoic sequence, 

 had been bent dovgi at least 5000 feet lower in the central area of 

 the Weald than at its margin under the j)resent Thames valley. 1 

 Before the end of the Upper Cretaceous Period this local subsidence 

 appears to have been at last arrested; and in Tertiary times an 

 opposite tendency set in, as if the basin-like droop of the floor were 

 being stretched or hauled into a natter curve. Through the 

 shallowing of the basin, it followed that the uplift of the infilling 

 sediments was greatest in the deeper part, where they were thickest; 

 and thus, over this part, a superficial bulge was formed, now visible 

 as the Anticline of the Weald. Topley saw that there was some 

 connexion between the differential thickness of the beds and their 

 anticlinal structure, but his explanation of it cannot be sustained. 



The stratigraphical history of the basin, as revealed by the 

 northern borings, presents many points of interest that I must 

 here pass without notice, but there is one particular which it is 

 important to mention. It is with respect to the unconformities 

 and gaps which were found to break the sedimentary sequence at 

 several horizons. As may be gathered from the sections shown, 

 some of these are of great strength ; for example, the Wealden 

 beds lie across the edges of all the Oolitic formations, and pass 

 beyond them on to the Paheozoic floor ; similarly, the Oolites cross 

 the Lias; and the Gault crosses the Wealden and Lower (Jreensand. 

 But in every case where it has been possible to trace these uncon- 

 formities and 'non-sequences ' towards the deeper part of the basin, 

 they have been found to weaken or to die out altogether. The 

 Wealden at Penshurst and Battle fades down imperceptibly into 

 the Purbeck Beds; and these by their estuarine bands afford an 

 easy transition to the underlying Portlandian ; which, in turn, 

 merges gradually into the Kimmeridge Clay, though showing a 

 sharp eroded junction with it at Bra bourne. The obliteration 

 of the breaks lower in the sequence cannot be similarly proved, 

 as the mid- Wealden borings have not gone deep enough, but 

 may be inferred, from the known set ting-in southwards of the 

 Inferior Oolite between the Great Oolite and the Lias, and from 

 the expansion in the same direction of the Lias itself, both in thick- 



1 These conditions have bees discussed by Sir Aubrey Strahan in his 



Presidential Address for 1913 (op. cit.) and illustrated by a contoured sketch- 



map (PI. C) ; also by Dr. A. M. Davies, Q. J. G. S. vol. lxix (1913) pp. :'>■', 1 38 j 



and Mr. H. A. Baker, Geol. Mag. dec. 6, vol. iv (1917) pp. 398 403, and :. 12-50. 



V( 1,. l.WV. f 



