lxxxii peoceedijS'gs of the geological society, [vol. lxxv, 



riess and in zonal range. In the Cretaceous formations the same 

 tendency is observable ; the expansion of the sandy ' JSIammillatus- 

 Bed' at Folkestone helps partly to fill up the, gap between the 

 Gault and the Lower Greensand ; and at Hythe and Hedhill l the 

 presence of estuarine fossils in the uppermost part of the Weald 

 Clay modifies the sharp transition from marine to freshwater beds 

 noted in the borings. Reverting to the simile of the fan, we may 

 liken the stratigraphical breaks to the ribs, which are strong and 

 close together in the handle of the fan, but thin away to nothing 

 .as they diverge. 



This bald outline of the evidence regarding the Wealden basin 

 will suffice to show that the ruling factor in its history has been 

 a prolonged balanced sinking and contemporaneous infilling with 

 sediments : marine, freshwater, and then again marine. The ques- 

 tion of primary causes and the theory of isostasy are thus brought 

 inevitably to mind, but I shall waive them in favour of a concrete 

 matter which will be easier and perhaps more profitable to discuss, 

 namely, the similarity of the structure in the rocks of the same age 

 in other parts of England. It would have been enlightening also 

 i,o have considered the extension of the Wealden structure over-sea 

 in Northern France ; but this, for the present, I must leave out of 

 account. 2 



1 ' Geology of the Weald ' Mem. Geol. Surv. 1875, p. 98 (Hythe) ; ' Summary 

 of Progress for 1900 ' Mem. Geol. Surv. 1901, p. 117 (Redhill). 



- [Borings in the Boulonnais have proved that the Jurassic dome, which is 

 the prolongation into France of the Wealden Anticline, is based upon a 

 depression of the deep-seated Palaeozoic floor, and that the Jurassic rocks of 

 the dome wedge out southward, as well as northward, beneath the Chalk. 

 The buried trough is shallower here than in Kent, and it probably reaches its 

 eastward termination where the superficial anticline also ends. I find that 

 this structure in the Boulonnais was described incidentally some years ago 

 by J. Gosselet in a paper entitled ' Etude Preliininaire des B-ecents Sondages 

 faits dans le Nord de la France pour la Recherche du Bassin Houiller,' pub- 

 lished in Ann. Soc. Geol. Nord, vol. xxvii (1898) pp. 139-49, of which an 

 English translation by Mr. L. L. Belinfante was published in Trans. Inst. 

 Mining Eng. vol xviii (1899-1900) pp. 317-25. The following passages are 

 quoted from the English translation (op. cit. p. 322) : — 



' Without prejudging the information which may be gained from a study of 

 the Jurassic rocks bored through, it is already evident that the geotectonic 

 anticline of the Boulonnais Jurassic overlies a deep depression of the Palaeozoic 

 surface. This surface indeed lies almost exactly at the same level, 425 to 

 460 feet below ground, at Strouanne, on the north of the Boulonnais, and at 

 Samer, on the south. Between these two extreme points stretches a broad 

 depression corresponding to the Jurassic dome of the Boulonnais : — 





