20 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Ten samples, at twenty foot intervals, were analysed for their organic-shelled 

 microplankton content. The microplankton content was quite rich and fairly well 

 preserved. Cysts of dinoflagellates were predominant over acritarch remains, and 

 spores and pollen were relatively rare. 



c. The London Clay 



By G. L. WILLIAMS & C. DOWNIE 



Dinoflagellate cysts (hystrichospheres) were first briefly recorded from the London 

 Clay by E. W. Wetherell (1892). No further work was done on them for over 60 

 years. They first came to the attention of one of us (CD.) in 1958, when Murray 

 Hughes of the Geological Survey sent for identification a number of species picked 

 out from washed foraminifera preparations which had come from London Clay at 

 Isleworth, Middlesex. Mr. D. Curry (1958 : 56) had however previously exhibited 

 hystrichospheres from the Eocene at a meeting of the Geologists' Association in 

 November 1957. Subsequently Eager & Sarjeant (1963) recorded Hystricho- 

 sphaeridium similarly obtained from Berkshire. Macko (1963) figured a variety of 

 forms from the London Clay without identifying any of them conclusively. 



The first systematic work on the dinoflagellates of the formation was begun in i960 

 in Sheffield by G. L. Williams. His thesis (1963) dealt with all the planktonic 

 dinoflagellates and acritarchs in the samples examined. Many of these were 

 derived from older formations and are being described elsewhere ; the indigenous 

 acritarchs and dinoflagellates will be listed here, but the acritarchs will not be 

 described. Dinoflagellates belonging to Hystrichosphaera, Haplosphaeridium, Clei- 

 stosphaeridium, Hystrichosphaeridium and allied genera are treated by Davey & 

 Williams in Sections IV and V of the present work. 



Stratigraphy. The London Clay is confined to two areas in Southern England, 

 the London and Hampshire Basins respectively. They formed one continuous 

 basin of deposition during Eocene times but subsequent orogenic movements 

 and erosion have given rise to the intervening Wealden Dome separating the two 

 main outcrops of the London Clay. The term basin when speaking of the two areas 

 is therefore used in a structural sense referring to the present day conditions only. 

 There is close similarity between the lithology of the London Clay of the western 

 margin of the London Basin and of the Hampshire Basin where the " typical " stiff 

 blue grey clay is increasingly replaced by arenaceous beds, loamy and sandy bands 

 being common. 



To the east of the London Basin, the blue clay reaches its greatest thickness, at 

 Sheppey, where it is estimated to be over 500 feet thick with little change in lithology 

 throughout. Collecting was undertaken at three places, Studland Bay in Dorset, 

 Whitecliff Bay in the Isle of Wight, both in the Hampshire Basin, and Sheppey in the 

 London Basin. In addition, Professor H. L. Hawkins kindly provided samples from 

 borehole cores from the Enborne Valley, lying in the west of the London Basin 

 where the lithology shows striking similarities to that of the Hampshire Basin. 



