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Rh 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 :