bearing on theoretical Speculations. 263 



sand and the chalk. On the north, both these formations form 

 chains of steep escarpment separated by deep longitudinal 

 valleys from the central axis and from each other: but on 

 the south, from the general degradation of the sand, the chalk 

 alone forms a regular escarpment. Now most of the main 

 streams of this district have their head waters in the central 

 axis; whence those running northwards into the Thames 

 have to intersect by transverse valleys the two barriers of the 

 Kentish rag hills and of the northern chalk downs, neglect- 

 ing the two intervening longitudinal valleys, into which a dam 

 of less than 100 feet high erected in any of these breaches, 

 which are about 600 feet high, would turn the drainage 

 towards the Straits of Dover. Such are the circumstances of 

 the Wey, the Mole, and the Medway ; the Darent and the 

 Stour rising almost within the limits of the rag hills, indeed, 

 can scarcely be said to break through more than one of these 

 barriers, the chalk. On the south side we have the Arun, 

 the Ader, the Ouse, and the Quckmere, which in like manner 

 break through the single opposing barrier of the chalky South 

 Downs (as the sands do not on this side present a regular 

 escarpment). Now it is I think quite inconceivable that fluvia- 

 tile erosion could possibly have produced such a configura- 

 tion, unless we suppose that the surface originally, when the 

 drainage commenced its work, presented uniform slopes from 

 the central axis to the aestuary of the Thames on the one side, 

 and the sea on the other, the intermediate longitudinal valleys 

 having been then filled up ; and that while the direct drainage 

 excavated the transverse valleys, the lateral drainage exca- 

 vated the longitudinal valleys : in which case I would ask, 

 first, why has the lateral drainage produced so much more 

 considerable effects than the direct drainage? and secondly, 

 how has it happened that the lateral drainage into so many 

 distinct main channels has coincided so as to form one uniform 

 longitudinal valley, instead of ramifications extending from one 

 principal stream without any relation to those of the next 

 principal stream ? While the geologist is studying the valleys, 

 the antiquary will observe throughout this tract the boldest 

 prominences of the escarpments studded with ancient earth- 

 works, which, though placed in the most exposed situations, 

 have resisted the action of atmospherical causes for some 

 twenty centuries : and should the two parties meet under these 

 circumstances, it will be somewhat difficult for the former to 

 persuade the latter that these deep defiles have been worn 

 down by an agency which his own observations naturally lead 

 him to believe to be next to null. 



But it may well be said that the Diluvialist, if he thus assails 



the 



