182 NEOZOIC TIME. [CHA.P. X. 



coming in from the north-west, as Dr. A. G-eikie briefly sug- 

 gests ; 1 that such a river must have traversed the north- 

 western plains on their upheaval from the Jurassic sea was 

 pointed out in the last chapter, and it was supposed that a 

 lake then existed on the site of the Irish Sea, and received 

 the detritus brought down by this stream. It was sug- 

 gested that it continued a south-easterly course over the 

 Jurassic clays, and discharged a large volume of water, but 

 a small amount of sediment, into the Purbeck gulf. In 

 Wealden time it may have carried a larger quantity of 

 sediment, though we can hardly attribute the sands of the 

 Lower Wealden to the influx of such a river, for after so 

 long a course over a low-lying clayey country its current 

 would not be likely to carry anything coarser than mud, 

 unless it was joined by powerful tributaries from Wales. 



We may therefore conclude that the principal rivers 

 flowing into the Wealden lake came from the west and 

 north-west, draining the large tract of country which then 

 connected Brittany and Cornwall with Ireland. Other 

 streams of smaller volume, but carrying much detritus, 

 flowed in on the north and north-east. To the position of 

 the outlet we are guided chiefly by the disposition of the 

 overlying beds, and the tract over which they are most 

 deeply accumulated being on a line from the Isle of Wight 

 through the Paris basin, we may consequently assume that 

 the excurrent river ran from a south-eastern prolongation 

 of the lake, and, passing across France, fell into a gulf of 

 the Neocomian sea that stretched northward as far as 

 Vassy-sur-Marne. 



Next let us endeavour to estimate the area over which 

 the Wealden deposits originally extended, which is of 

 course nearly the same thing as estimating the size of the 

 lake itself. We know that these strata extend from the 

 centre of Dorset to the Boulonnais, a distance of nearly 

 1 " Textbook of Geology," first edition, p. 817. 



