IV rr.EFACE. 



(ji-eensand forms suggests that in tlie rnaiu the Blackdown Sands 

 lielong to that formation. The strata from bottom to top are 

 mostly sandy, and indeed Lave lithologically an Upper Greensand 

 character, but it may be, though it is impossible to draw sub- 

 divisions on the map, that all three divisions are represented. 



It is well known that of 400 Lower Greensand species of niol- 

 lusca only about 70 pass into the Gault and Upper Greensand. 

 With regard to lithological variations it is also well known that 

 in the eastern half of the escarpment of the North Downs and 

 elsewhere it is impossible satisfactorily to separate the Gault from 

 the Upper Greensand, and the Upper Greensand from the Lower 

 Chalk. They graduate lithologically into each other, the peculiar 

 characters of Greensand and Gault having been locally determined 

 during their deposition by depth of water and distance from 

 land. 



In connexion with this subject I may mention that the so- 

 called Cambridge Upper Greensand lithologically resembles the 

 Chloritic Marl of the south-west of England, which may be said 

 to form the base of the Chalk. 



The Potton beds, called Neocomian, are also somewhat ab- 

 normal in a pal^eontological point of view. The majority of the 

 species of mollusca (22) are known Neocomian forms. Most 

 of the genera of fishes however (Asteracanthus, Lepidotus, Stro- 

 phodus, and Hybodus) and also the two genera of reptiles 

 (Megalosaurus and Plesiosaurus) which are only represented hy 

 rolled and waterworn fragments, have probably been derived 

 from the waste of the Kimmeridge clay on which the Potton 

 beds lie, or even from older Jurassic strata. The Vertebrata 

 therefore cannot be safely classed as of Neocomian age. 



ANDREW C. RAMSAY, 



Director-General. 



