412 Geological Society : — 



at the bottom of the Thanet Beds in the area as a true basal con- 

 glomerate. The Eocene deposits overstep the Chalk zones from 

 the zone of Ostrea lunata to that of Micraster cor-anguinum, and 

 also transgress the Chalk surface-contours ; owing to the greater 

 dip of the base of the Eocene, the latter also bevels off the zones 

 in South-Eastern Suffolk. 



Evidence is adduced to show that the London Clay overlaps the 

 Lower London Tertiaries. and rests directly upon the Chalk in 

 Norfolk. The Reading Beds also overlap the Thanet Beds in the 

 western part of the area. A bypsometrical map of the Chalk- 

 surface in the London Basin is presented, and a minimum estimate 

 of the unconformity, in terms of thickness of Chalk removed, is 

 given for the northern part of the basin. 



Isopachytes for the London Clay and Lower London Tertiaries 

 (the north-western feather-edge of each deposit being the zero 

 isopachyte) have been plotted, and since the beds thicken on each 

 side of a central area in South-Eastern Suffolk, where the sub- 

 Eocene Chalk-surface gradient is least, the curves are seen to be 

 convex to a central axis. The Chalk zones, Chalk surface-contours, 

 Lower London Tertiaries, and London Clay successively change 

 strike as they are traced north-eastwards. The instability over 

 the axis, which may be of Charnian trend, took the form of an 

 earth-ripple forced north-eastwards, throughout Eocene times, 

 towards that part of East Anglia where the Palaeozoic platform 

 rises. The unconformity of the Pliocene deposits upon the Eocene 

 and the Chalk is shown to be significant in this connexion. 



Stratigraphical details of the various divisions and descriptions 

 of new sections are given. Tables of mechanical analyses (obtained 

 by elutriation) throw light on the conditions of deposition, and 

 permit of the adoption of an exact terminology. The visible 

 Thanet Beds are of two faeies : (a) the Ipswich or Eastern type, 

 with predominant green clays, and (b) the Sudbury or Western 

 type, characterized by the prevalence of light-coloured sands. 



The variations in lithology of the Beading Beds are described, 

 and it is shown that the Pebble-Beds belong lithologically and 

 petrologically to the Beading Beds, but that their scanty fauna 

 is a London Clay one. The fauna of the London Clay is also 

 enumerated, and the author is of the opinion that the bed was 

 deposited under isostatic conditions accompanying a sagging on 

 each side of the central axis mentioned above. 



The distribution of the sarsens in the area is plotted out on a 

 map, and their petrology is considered; it is concluded that, in 

 this district, they are derived from the sands of the Reading Beds. 



The mineral constitution of the various divisions of the Eocene 

 Beds is discussed in detail. The mineral assemblage is seen to 

 conform to one type throughout (characterized by abundant small, 

 angular, colourless garnets, also by staurolite, tourmaline, and 

 k} r anite), but differences occur which allow of the beds being easily 

 distinguished one from the other. There is remarkable constancv in 



