Ixxvili PKOCEEDLXGS OF THE GEOLOGICAL SOCIETY. [vol. lxxV, 



It is to be noted also that the outcrops of the Lower Greensand 

 .and Wealden Series north and south of Penshurst, show a thick- 

 ness of over 1300 feet of strata, represented by not more than 

 540 feet in the Brabourne Boring. Thus, there is a proved 

 expansion of over 3000 feet south-west of Brabourne, without 

 taking into account the expansion of the Jurassic formations 

 below the Kimmeridge Clay, about which at present we have 

 little or no information. But the fact that the downward slope of 

 the Palaeozoic floor becomes steeper towards the southern end of 

 the section suggests that the expansion of these lower formations 

 also continues. In any case, it is certain that, as the anticline rises 

 towards its crest, the Palaeozoic floor sinks to greater depths 

 beneath it. Even if the Middle Oolites, Lower Oolites, and Lias 

 continue from Brabourne towards the central area without further 

 expansion, it will mean that the Palaeozoic floor between Penshurst 

 and Battle lies at about 3500 feet below sea-level; and the pro- 

 bability that its depth is greater than this is increased by the 

 incoming of a fringe of Trias at Brabourne, for the British Trias 

 is pre-eminently a basin-formation, found usually in deep troughs 

 •of the older rocks, and often thickening rapidly from its marginal 

 belt. The Sub-Wealden Trias, however, up to the present, has- been 

 proved only at Brabourne, so that our knowledge of it is too slight 

 to build upon, and I therefore exclude it from the calculation. 



As to the structure under the southern side of the Weald we 

 have not much definite knowledge, but } T et sufficient to suggest 

 that the conditions are, in the main, similar and complementary to 

 those on the northern side, and that there is here also a reversal of 

 dip between the Cretaceous rocks at the surface and the Jurassic 

 rocks deep underground ; in fact, that the perfect anticline above 

 •covers a perfect syncline below. It fortunately happens that we 

 know the thickness of the Purbeck, Portland, and Kimmeridge 

 formations on the southern flank of the main axis, from the old 

 Sub-Wealden boring and from the recent boring near Battle. 

 The three formations are, in the aggregate, at least 300 feet thinner 

 here than at Penshurst, and stand, moreover, at a somewhat higher 

 level. It is certain, therefore, that somewhere between the two 

 places, the downward plunge and expansion of these formations 

 has been arrested and has given place to an upward curve, accom- 

 panied by attenuation : that is to say that the structure, as a whole, 

 is synclinal. The position of the bottom of the trough has not 

 been exactly ascertained, but may be surmised to lie rather nearer 



