180 NEOZOIC TIME. [CHAP. X. 



some portions of the Purbeck series, a strong resemblance 

 to the Tertiary lacustrine beds of central France." 



From the nature and fossil contents of the lower and 

 middle beds, he infers that the waters were at first shallow 

 and frequently disturbed by the currents of inflowing rivers ; 

 " the finely laminated strata of the Upper Wealden, on the 

 contrary, are such as belong to deeper waters, and rarely, if 

 ever, show traces of disturbance." These considerations 

 lead us to conclude that during the earlier part of the 

 Wealden epoch the country was still rising, and that the 

 Wealden rivers then attained their maximum velocity and 

 carrying power ; for a time perhaps the land was stationary, 

 but subsidence quickly ensued, diminishing the river- 

 action, increasing the area of the Wealden lake, and adding 

 depth to its central waters. 



The history of the Purbeck- Wealden episode may there- 

 fore be summed up as follows : The Purbeck series was 

 formed during upheaval in lagoons that were gradually 

 cut off from the retreating sea ; the Lower Wealden beds 

 were formed in a freshwater lake on the site of the old 

 lagoons, and the level of its waters was maintained by the 

 influx of one or more powerful rivers ; the Upper Wealden 

 was formed during a subsidence which widened the area of 

 the lake, and eventually depressed it beneath the waters of 

 the returning sea. 



The next point which calls for consideration is the 

 drainage system of the Wealden land, and whether it is 

 possible to ascertain the direction from which the principal 

 river or rivers ran into the lake. It must be remembered 

 that in passing from the Purbeck to the Wealden we find 

 evidence of rapidly increasing fluviatile action, as if some 

 large river had suddenly invaded the lake, or as if rivers 

 which had previously carried little but matter in solution 

 now became loaded with a quantity of detritus in suspen- 

 sion. Possibly both operations really took place. It has 



