524 STRATIGRAPHICAL GEOLOGY 



that these parts were the scene of prolonged volcanic activity in 

 Tertiary time, and that they were doubtless then raised to a higher 

 relative level than they had previously occupied, for the north- 

 west part of the plateau does not rise to more than 3000 feet. 



The facts which have chiefly to be considered are the characters 

 of the Upper Cretaceous deposits which surround the Central 

 Plateau. Those on the north and north-east sides are chalks 

 which have the character of deep-sea deposits, being fine foramini- 

 feral oozes in which Textularia and Globigerina are the prevalent 

 forms. On the north-west the Senonian is largely a Bryozoan 

 deposit formed in no great depth of water, and the Campanian is 

 a siliceous rock analogous to " gaize," and mainly composed of 

 sponge spicules ; this may have been accumulated in water of 200 

 to 300 fathoms (about 1500 feet). On the south-east side in 

 Aquitaine we find the Senonian composed of marly limestones, 

 which are often sandy and include banks of Hippurite and 

 Eadiolite limestone, deposits which are not of deep-water origin. 

 The Campanian of the same area, however, consists for the most 

 part of fine white chalky limestones in alternating solid and 

 crumbly beds with layers of bkck flints. M. de Grossouvre 

 describes this material as similar to chalk, and as "denoting a 

 deposit formed at a much greater distance from shores, and in 

 much deeper water than the sediments " of the Senonian. 31 



The successive stages of the series probably passed transgressively 

 one beyond the other on to the flanks of the Central Plateau, and if 

 the average depth of water under which the Campanian sediments 

 was deposited was only 400 fathoms (i.e. 2400 feet), it is clear that 

 large parts of the plateau must have been submerged ; how 

 much would require a special study to determine. 



From the Campanian subsidence there was a gradual recovery, 

 accompanied by a certain amount of lateral pressure which produced 

 a warping or upward flexure of parts of the mass of deposit, so 

 that a series of broad anticlines and synclines were formed, and 

 the summits of the anticlines were planed off by the currents 

 and waves of the shallowing seas. Hence, as we shall see, even 

 the earliest Eocene deposits are more or less unconformable to the 

 underlying Cretaceous strata. 



REFERENCES 



1 W. Whitaker, Quart. Journ. Geol. Soc. vol. xlii. p. 26 (1886). 



2 See "Geology of the Weald," by W. Topley, Mem. Geol. Survey (1875). 



3 See "Geology of the Isle of Purbeck," by A. Strahan, Mem. Geol. Survey 

 (1898). 



