56 



and snoAv commenced to work on the Wealden district 

 within the Chalk ring; and doubtless at first many streams 

 made their way from the centre, Southward and North- 

 ward to the sea. These rivulets, at first numerous, would 

 ultimately, from peculiarities in the structure and differing 

 hardness of the strata over which they had to make their 

 way, be obliged to unite into larger rivers to overcome 

 obstacles which singly they could not pass ; as each 

 rivulet left its course to unite with others, the old channel 

 would be left as a slight gap in the ridge of rock it 

 had partially worn, while the larger stream would be 

 strengthened and have its channel enlarged in an in- 

 creasing ratio. These streams again, some giving up and 

 some uniting, would ultimately greatly increase the number 

 of useless channels, while diminishing the number of 

 active ones, until the whole number once great, ultimately 

 became small; and this will, if nothing occurs to the 

 contrary, become smaller. 



The main or transverse streams which drained from 

 the central portion of the Weald, were here and there 

 forced to deviate from the direct line North and South, by 

 the nature of the ground. 



If a vertical section taken along the crest of the North 

 Downs be prepared, the gaps made by the present rivers 

 and also some made by rivers which from some cause have 

 ceased to flow will be apparent. A similar section along 

 the crest of the Lower Greensand escarpment will demon- 

 strate some variations from the direct line spoken of. 

 [see pi. ii.) If a vertical section of the South Downs be 

 prepared like that of the North, similar gaps will be seen. 



In the North and South Downs it was noticed long 

 ago that these exits of the rivers from the Weald occasion- 

 ally coincided, i.e.., that they were almost due N. and S. of 

 each other, and it will also be found that if the larger gaps 

 without rivers or passes, be included in the examination, 

 the coincidence will be still more remarkable, river with 

 river, and gap with gap. 



