part 1] an\ i \ i:i;v\ky address of the president. Ixkxv 



continued thinning and partly by erosional planing- down, as in the 

 northern flank of the Weald. There is. however, an important 



difference in the attitude of the two sections, which obscures the 

 comparison. In the Wealden section the .Jurassic rocks are wholly 

 below sea-level, and are so disposed that the plane of their 

 uppermost portion makes the nearest approach to horizontalitv; 

 while in the South- Midland section the western end is so far 

 uplifted that the whole sequence is brought above sea-level, and the 

 plane of approximate horizontalitv falls in the middle of the 

 Lias. The whole pile is thus tilted towards the east, and has had 

 its high-lying western portion cut away by denudation. We have 

 strong reason to believe that the Jurassic strata, along with the 

 Trias, originally ended off westwards in old shore-lines against the 

 Palaeozoic massif on the Welsh border, and the formations presum- 

 ably became attenuated towards this margin as well as towards the 

 east. The trough thus outlined is like that of the Weald, but 

 from its tilted position and the reduction of its infilling strata by 

 denudation, we cannot now determine whether the likeness was 

 further marked by the former existence of a superficial anticline, 

 •although there is a suggestion in the general structure that this may 

 have been the case. It is certain, at any rate, that a concealed 

 syncline, with dips in one part counter to those at the surface, can 

 be recognized beneath the pile, showing distinctly in the buried 

 Liassic rocks and still better in the Trias. The reversal of dip in 

 the deeper formations was indeed pointed out by Topley (Q. J. Gr. S. 

 vol. xxx, 1874, p. 189) in the following terms : — 



• We infer, for instance, because of what we know of the dip of the Great 

 Oolite in Gloucestershire and Oxfordshire, that the beds have been upheaved 

 towards the west and north-west, which has resulted in a dip to the east and 

 south-east, or towards the London Basin. But if, over this area, denudation 

 had gone much further than it has done, and all the Oolites down to the 

 higher part of the Lower Lias had been swept away, we should observe that 

 the beds along this line were approximately flat : and we should infer that 

 there had been no upheaval here, or that the net results of such movements 

 as had taken place had been to leave the beds in a horizontal position. If the 

 whole of the Lias had been denuded, we should infer a slight westerly dip or 

 from the London Basin. If denudation had gone far into the Tri.':s. sweeping 

 away the whole of the higher Secondary rocks, we might have inferred a 

 considerable westerly dip.' 



To this account we can now add the evidence of the Calvert 

 Boring, which has proved a definite westward dip in the Lower Lias 

 itself ; otherwise the statement requires no modification. 



