546 Herbert A. Baker — Unconformity betiveen 



north-east while deposition of Portlandian sediments was in progress 

 in the sonth-west. This state of affairs persisted into Purbeckian 

 times, and was probably accentuated by differential movement. 

 Direct posthumous uplift along the axis may have occurred in post- 

 Portlandian times, but it seems likely that we have in East Kent, on 

 the western flank of the ancient ridge, an illustration of the operation 

 of the principle of isostasy, a sagging of the area of deposition in the 

 south-west, under the weight of the great accumulation of Jurassic 

 strata, being compensated by concomitant uplift in the north-east. 

 Differential movement of this character was apparently taking place 

 in Purbeckian times, for although the Purbeck appears never to 

 have extended far to the north-east, yet it transgresses on to the 

 Kirnmeridgian. 



Passing on now to consider the character of the Cretaceous rocks 

 which overstep the successive members of the Jurassic until they 

 rest upon the Palaeozoic floor, the writer ventures to remark that it 

 would probably be difficult to find a happier illustration of the utility 

 of constructing maps of isopachyte systems of strata than that 

 afforded by the Wealden of East Kent. The isopachytes are found 

 to throw a very interesting light upon the question of the conditions 

 under which these strata were deposited (see Map 2). 



It is immediately apparent that the isopachytes are of a character 

 very different from that possessed by those of a marine formation. 

 They are clearly the lines of an estuarine deposit, and indicate 

 plainly that the Wealden of East Kent was deposited by a river 

 flowing from the north-east. In the view of the writer this river 

 was consequent upon the easterly Charnian ridge. South of 

 Woodnesborough it appears to have divided into two streams which 

 turned south-east and flowed as subsequent or strike streams into the 

 Wealden lake. The southerly slope of the land being now 

 exceedingly gentle in the East Kent area, in consequence of much 

 levelling up having been effected by the steady accumulation of great 

 wedge-like masses of Mesozoics to the south-westward, local con- 

 ditions became significant in deciding the course of the streams when 

 the influence of the primary ridge began to die out with distance.. 

 The course of one stream appears to have been determined to some 

 extent by the Snowdown disturbance, and it appears to have selected 

 the Oxford Clay outcrop for some distance in preference to the- 

 harder Corallian limestones. Similarly, the more westerly stream 

 appears to have selected the outcrop of the Kimmeridge Clay, and 

 the erosion of the later Jurassics from the Dover area was doubtless 

 due to its agency. 



It is interesting to consider the nature of the "Wealden sediments 

 in the light of the information afforded by the map. Mr. Lamplugh, 

 in the Survey memoir, 1 has expressed the opinion that the Wealden 

 section in the Dover shafts offers strong indication of the inflowing 

 of a river from the north-east. The present map shows the correct- 

 ness of this view. He expresses surprise, however, at the seeming 

 lack of Mesozoic material among the Wealden sediments, in view of 

 the fact that such a river must have crossed an area of ancient 

 1 Lamplugh, loc. cit. 



