3 i2 SCIENCE PROGRESS 



(the North and South Downs) which everywhere present a steep face (escarp- 

 ment) towards the interior, and, with one brief exception, a much more gentle 

 slope towards the exterior. Precisely similar features are shown by another 

 series of hills, running roughly parallel with the Downs, in the Lower Greensand 

 formation, but the range is not nearly so continuous, and only attains to any 

 conspicuous elevation in the north-western corner of the area. The principal 

 rivers for the most part pursue a "longitudinal" course, parallel with the escarp- 

 ments ; but only one of them escapes directly to the sea ; all the others sooner or 

 later join "transverse" streams which pass through the escarpments, in narrow 

 valleys, towards the exterior ; and this is the more remarkable because, in the 

 majority of cases, these rivers rise near the axis of the Wealden Area in ground 

 decidedly lower than the range of Downs which they afterwards traverse. In 

 no case does any important stream enter the area from the outside. 



What explanation does science offer of these remarkable features ? 



Up to the middle of last century it was commonly believed that the whole 

 region, in very much its present form, had comparatively recently been invaded 

 by the sea, and that the chalk escarpments were old sea-cliffs ; but this view, 

 which offered no reasonable explanation of the transverse rivers, has long been 

 abandoned, and no one now doubts that the existing sculpture of the surface is 

 almost entirely due to subaerial agencies — especially rain and rivers. Opinions 

 differ, however, as to the initial stages of this process. 



Of course the strata, which now slope away to the north and south on either 

 side of an approximately east and west axis (roughly corresponding with the main 

 watershed), were originally horizontal, and {pace Major Marriott) there is no 

 doubt that all the beds from the Hastings Sands to the Chalk (which is as far as 

 we need carry our inquiries) spread over the whole of the district, and for many 

 miles beyond it. At the close of the Cretaceous period there seems to have been 

 some upheaval and some denudation, which probably, in places, exposed the 

 Lower Chalk, and possibly even some Lower Cretaceous beds ; but it is unlikely 

 that the latter were extensively uncovered, since there is practically no Lower 

 Greensand chert in the Eocene deposits. 



But it was not until the close of the Eocene (at earliest) that those great earth- 

 movements occurred which heaved up the whole Wealden Area (including the 

 Bas Boulonnais in north-eastern France) into one wide, but comparatively flat 

 arch or dome ; and it is at this point that opinions first seriously diverge. It is 

 agreed that the whole crown of the dome, amounting to thousands of feet in 

 thickness, has been removed ; but the exact mode of removal is open to question. 

 Some hold that the present river system began at this stage, and that the removal 

 has been effected entirely by subaerial denudation. Others believe that the sea 

 played an important part and wore off the whole summit of the dome to a " plain of 

 marine denudation " ; and that it was on this plain, upheaved and slightly arched 

 in, probably, Pliocene times, that our existing rivers originated. The ultimate 

 result, in all its main features, would probably be the same in either case ; and it 

 is only by careful inquiry into detail that we can hope to determine the relative 

 merits of the two hypotheses. Such an inquiry would be impossible in a paper 

 like this, but a brief outline of the main features of evolution may be attempted, 

 and for this purpose it will be convenient to take the second, or " Planation 

 Hypothesis," as we may call it, first. 



Fig. i shows, in a diagrammatic form, and with an exaggerated vertical scale 

 (i) the dome postulated by the first hypothesis ; (2) the marine plain of the second 

 hypothesis, uplifted and slightly curved ; (3) the present surface, with hill-ranges 



