XC PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [vol. lxxv„ 



The approximation of the three totals must, of course, be more 

 or less fortuitous, seeing that the items represent only our present 

 knowledge, and may have to be altered, particularly in the 

 Wealden section, if borings should ever be made through the 

 deeper parts of the basins. But, in a broad way, the correspond- 

 ence of the figures must imply an almost equal amount of 

 differential subsidence in the three areas between the beginning and 

 the ending of the Jurassic Period. 



It has been generally recognized that the middle of England 

 was covered by a shallow sea during the ac cumulation of the 

 Oolites ; and an obvious deduction to be drawn from the facts 

 which I have summarized is, that through all the Jurassic Period 

 there existed a steadily deepening trough which swept in a bold, 

 curve from north to south, bulging towards the west, with land not 

 far distant on both sides ; and that the history of the development 

 and obliteration of this trough has been nearly the same in all 

 parts of its course. The infilling and sinking have gone on for a 

 very long time in apparent association, as if due to compensative 

 adjustment ; but, in all cases, there has been a final recovery which 

 was greatest where the deposits were thickest. This recovery 

 implies a shallowing of the trough, and may have been due to a 

 stretching of the floor with consequent reduction of the curvature. 



In tracing out these structures in the Jurassic and Cretaceous, 

 rocks, I have been tempted to look for them- in the older forma- 

 tions also, particularly as Topley, following Hull, found some 

 similarity of structure in the Triassic rocks as well as in certain 

 areas of Carboniferous strata. A good case could indeed be made 

 out for the principle of the infilled basin and the differential 

 recovery in nearly every English stratified formation, but the 

 evidence is generally imperfect, and not worth pressing. I will 

 content myself, therefore, with touching briefly upon a few salient 

 points of resemblance. 



The Trias of the Midlands, as Hull and Topley observed, thins. 

 rapidly in a south-easterly direction, and we have learnt now from 

 borings that it wedges out in this direction sooner than the Juras- 

 sic rocks. This condition prevails probably as far north as the 

 Wash, but may be modified farther north, as a deep boring 

 at Market Weighton l in South Yorkshire has recently revealed 

 an unexpected thickness of Trias and Permian in a quarter where 



1 Described by Dr. W. Gibson in ' Summary of Progress for 1917 ' Appen- 

 dix I, Mem. Geol. Surv. 1918, pp. 42-45. 



