Reports & Proceedings — Geological Society of London, 235 



The unconformity of the Eocene xipon the Chalk is discussed, and 

 reasons are given for regarding the layer of green-coated flints at the 

 bottom of the Thanet Beds in the area as a true basal conglomerate. 

 The Eocene deposits overstep the Chalk zones from the zone of Ostrea 

 lunata to that of Micraster cor-anguimim, and also transgress the Chalk 

 surface-contours ; owing to the greater dip of the base of the Eocene, 

 the latter also bevels off the zone 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 hypsometrical map of the Chalk 

 surface in the London liasin 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-east- 

 wards, 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 analysis (obtained by 

 elutriation) throw light on the conditions of disposition, and permit of 

 the adoption of an exact terminologv. The visible Thanet Beds are 

 of two facies : («) 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 Reading Beds are described, and 

 it is shown that the Pebble beds belong lithologically and petro- 

 logically to the Reading 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 occasional small, angular, 

 colourless garnets, also by abundant staurolite, tourmaline, and 

 kyanite), but differences occur which allow of the beds being easily 

 distinguished one from the other. There is remarkable constancy in 

 mineral composition over wide areas, and in deposits lithologically 



