A HISTORY OF BUCKINGHAMSHIRE 



thick, and contains green-coated nodules of cream-coloured and slightly 

 phosphatic limestone. It has been observed in the Loudwater valley 

 at Wycombe Marsh above High Wycombe, in the Misbourn valley 

 near Amersham, and in the Chess valley near Chesham. 



The mass of the Upper Chalk with its many bands of flint nodules 

 extends over the greater part of the Chalk area in Buckinghamshire, 

 occupying the high grounds above Chesham, Amersham, High Wycombe 

 and Great Marlow, where it is largely covered with gravel, brickearth 

 and clay-with-flints. It is not far below the surface at Eton and Datchet, 

 for it appears above ground at Windsor Castle owing to an anticlinal 

 structure which has locally upraised the Chalk. 



As a whole the Chalk is one of the most uniform of geological 

 formations : its lower portion is argillaceous and an occasional compact 

 and nodular band occurs, but it represents a great and continuous deposit 

 of calcareous mud, slowly accumulated in the deep ocean and due mainly 

 to the decay of calcareous organisms and partly (in its flint bands) to the 

 siliceous matter derived from organisms with siliceous structures. 



Remains of marine saurians and fishes occur, but the more abun- 

 dant fossils are those of mollusca, brachiopoda, echinodermata, and 

 sponges ; and yet despite the absence of any great changes in sedimen- 

 tary condition, such as would be likely to affect the forms of life, there 

 is a gradual change in the assemblages of organic remains in the suc- 

 cessive groups of strata. Owing to the slowness of deposition and 

 uniformity over wide areas in Britain it is convenient to divide the life 

 history into certain zones or assemblages of fossils, characterized by 

 particular genera and species which had a wide distribution in space 

 and a more restricted distribution in time. These zones, though purely 

 zoological, afford useful indices of stratigraphical position, and conven- 

 tional limits are assigned to them in different localities, the order of 

 succession being maintained. 



These zones in Buckinghamshire are as follows : l 



1 W. Whitaker, Quart. Joum. Geol. Soc. xxi. 398, and Geology of London, i. 58 ; C. Barrois, 

 Recberches sur le Terrain Cretace Superieur de fAngkterre et de Flrlande (i 876), p. 150. 



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