8 GENERAL ACCOUNT OF THE DEPOSITS. 



abundant scattering of large grains of quartz, ironstone, etc.; and 

 many irregular nodules of coarse irony sandstone, as big as potatoes, 

 are also found throughout its thickness. We have then no hesita- 

 tion in referring this bed to the Lower Greensand — a conclusion 

 which is further supported by its perfect transition into the 'silt' 

 bed below. 



Also this clay bed yielded specimens of beautifully fibrous fossil 

 wood, reminding one of the Lower Greensand wood of Shanklin, 

 Isle of Wight. 



The fossils of the Upware and Brickhill Neocomians are pre- 

 served, for the most part, in calcite, the mineral being, in some of 

 the zoological groups distinctly crystalline ; some few organic struc- 

 tures have been replaced by limonite, and fossil wood occurs in the 

 usual silicified condition. Of other fossils only the inside moulds 

 and external casts are known, these being formed of ferruginous 

 sandstone, limonite, and phosphate of lime. 



The ' Derived ' Fossils. 



It is of the first importance at once to divide the fossils of these 

 rocks into two great groups. All those which are mineralised in 

 phosphate of lime, together with many of those in limonite are 

 ' derived ' fossils. They are the remains of organisms which never 

 belonged to the Neocomian period itself, any more than a fossil in 

 flint now washed out from the Dover chalk and buried in the 

 sandy shore belongs to our present epoch. 



This separation of the fossils into two groups is, as a rule, as 

 perfectly easy as it is a conspicuous necessity; for the derived 

 fossils besides being characterised by the materials in which they 

 are preserved, belong mostly to Jurassic species, and they occur 

 usually as internal moulds which have been mutilated by attri- 

 tion as they were rolled into pebbles during long years of wear and 

 tear upon the ancient sea beach 1 2 . 



1 Mr J. F. Walker first pointed out that at Potton there are two great groups of 

 fossil remains, -which are in this place (a) the indigenous fauna, preserved in 

 Oxide of Iron, (b) the derived fossils, preserved in phosphate of lime. 



2 Mr H. G. Seely has given it as his opinion that all these fossils were natives of 

 the bed in which they are found. Annals and Mag. Nat. Hist. 1866. 



