40 THE 'derived' fossils. 



Counties where the two seas, northern and southern, gradually 

 approximating, at last surged together and fused into one con- 

 tinuous Upper Neocomian sea. [And this, be it noted, would be 

 almost sure to increase erosion, as there would be currents from 

 the one sea to the other. There is more than one way in which 

 depression means erosion. T. G. B.] 



The Derived Fossils of the Wealden. 



I am not aware of any undoubted remains of Wealden fossils 

 having occurred in the Upware and Brickhill deposits, but in the 

 intermediate area, at Potton, rolled fragments of Endogenites have 

 been found in considerable numbers ; also certain fragments of 

 sandstone in the collection of Mr J. F. Walker are very similar to 

 the Lower Cretaceous Sands (Wealden probably) of Shotover Hill, 

 Oxford. So that, as pointed out some years ago by Professor 

 Morris, it is probable that the Wealden rocks once formed part of 

 the shore line of the Neocomian sea beyond their present limits up 

 to the north of Brickhill ; but we have no reason to believe they 

 ever extended so far as Cambridge. 



It is the opinion of some geologists that the remains of Land 

 Vertebrates at Upware and Potton were derived from the de- 

 nudation of the weald. This theory I cannot support, for some of 

 the specimens are beautifully preserved, being scarcely worn at 

 all ; and the bones are not highly phosphatised, but are com- 

 monly impregnated with iron oxide. Moreover, the shore line and 

 land were close by, and we know that the Iguanodonts continued 

 to live on during Lower Greensand times. Therefore I regard 

 the great majority of these reptile remains of Potton and Upware 

 as having belonged to animals which lived and died during the 

 period of the Iron Sands and Coprolite Nodule beds. Some of 

 the Saurians and Crocodilians are exceptions to this rule, having 

 been washed out of the Kimmeridge Clay, and there are many of 

 them distinguished by being in a more highly phosphatised con- 

 dition, with, occasionally, adherent lum.ps of phosphatic matter. 



