lx'XXVl PEOCEEDIXGS 0E THE QEOLOGICAL SOCIETY, [vol. lxxv,. 



That the South-Midland and Wealden areas have had a very- 

 similar tectonic history during Jurassic times becomes still more 

 apparent on comparison of their sediments. These, while differing 

 in many points of detail, present the same general characters and r 

 in most parts of the sequence, are remarkably alike in composition. 

 They are almost wholly of marine origin, and, in the South. 

 Midlands as in the Weald, they include at several horizons. wide- 

 spread deposits of original horizontality, which by their present 

 divergence prove a constantly-renewed differential sinking of the 

 western trough and an equivalent wedge-like accumulation of 

 sediments in the depression. Nearly to the close of Jurassic times 

 the two areas continued to maintain a close correspondence in 

 respect to sea-level; but then there came a change and a difference,, 

 probably due to the setting-in of the western uplift, by which most 

 of the Midland and Western tract appears to have been raised 

 above the inundation of the great Wealden lake or estuary, and 

 was not laden like the southern basin with a huge mass of fresh- 

 water deposits. With the idea of compensative adjustment in mind, 

 it is tempting to speculate on the possibility that the more elevated 

 position of the Jurassic rocks in the South Midland area at the 

 present day may be due to the original lack of this extra load, and 

 to the consequent earlier removal of whatever Cretaceous cover- 

 it may have had. 



The section may be taken as typical for the Jurassic rocks of 

 the middle of England at least as far northward as the Wash, and 

 probably up to the Humber. Everywhere in this belt the sedi- 

 ments as a whole have their greatest thickness and fullest sequence 

 in the west, and become attenuated and interrupted by gaps and 

 unconformities eastwards. Information as to their arrangement 

 after they disappear beneath the Chalk is at present scanty, but all 

 of it goes to show that they soon wedge out upon a rising slope of 

 Palaeozoic rocks continuous with that proved in the South Midlands. 

 Deep borings at Culford, Lowestoft, Harwich, and other places l 

 have demonstrated that they are wanting under the greater part of 

 Essex and Suffolk, with the implication that the same condition 

 extends well into Norfolk. In fact, wherever the boring-test has 

 been applied in the eastern counties, it has proved that the Jurassic 

 wedge does not extend far in under the Chalk, and that the condi- 



1 For a convenient list and summary of deep borings in the south and east 

 of England, see Appendix to Sir Aubrey Strahan s Presidential Address for 

 1913, Q. J. G..S. vol. lxix, pp. lxxxiv-xci. 



