490 MR. CLEMENT EEID ON THE [Aug. 1 896, 



28. The Eocene Deposits of Dorset. By Clement Reid, Esq.,F.L.S., 

 F.G-.S. (Communicated by permission of the Director-General 

 of H.M. Geological Survey. Eead April 29th, 1896.) 



The new survey of the western end of the Hampshire Basin having 

 necessitated certain modifications in the geological map, it may be 

 useful to lay the principal results before the Society. If the 

 alterations were mere matters of detail, this would scarcely be 

 worth while ; but the more accurate mapping has led to the discovery 

 of a sudden westerly change in the character of the Lower Bagshot 

 Sands, and of a well-marked overlap at their base. The mapping of 

 the Tertiary strata proves also that there is evidence of other periods 

 of earth-movement in southern England, besides those already 

 known, 1 and that we are dealing with one of those regions where 

 folding affects the same area again and again. 



When the Eocene strata are followed westward through Sussex 

 and Hampshire into Dorset, one finds constant local changes in the 

 lithological character of the deposits, though these changes seem all 

 to tend in the same direction. Marine beds become less conspicuous, 

 coarser, more estuarine, and estuarine deposits become truly fluviatile. 

 Thus the London Clay, which exceeds 300 feet in thickness to the 

 east, dwindles to less than 100 feet in Dorset, and becomes more 

 sandy and pebbly, though still apparently of marine origin. The 

 Woolwich and Reading Series — fluvio-marine at Newhaven and 

 Portslade, and slightly so at Lancing — becomes more fluviatile 

 westward, lenticular patches of subangular gravel appearing in it 

 west of Wareham. To what extent the Reading Series rests un- 

 conformably upon the Chalk is difficult to say, though overlap is 

 clearly recognizable in various places. The Lower Bagshot Sands 

 also become coarser and more purely fluviatile westward, the change 

 being a singularly rapid one in the neighbourhood of Dorchester. 

 It is to this change in the Bagshot Sands, and the conclusions to 

 which it leads, that attention will more especially be drawn in 

 the following notes. 



1. Woolwich and Reading Series. 



The extreme variability of these deposits makes it difficult, till 

 a large area has been examined, to master any general tendency in 

 the variations. Wo particular bed seems ever to be traceable more 

 than a short distance, the whole series being made up of a succession 

 of alternating masses of red-mottled clay, loam, sand, and gravel. 

 Plant-beds and marine strata alternate at Newhaven. At Brighton 

 and Portslade the deposits are still of the Woolwich or fluvio-marine 



1 See Reid, 'Pliocene Deposits of North-western Europe,' Nature, 

 toI. xxxiv. (1886) p. 341 ; Reid & Strahan, Mem. Geol. Surv. 'Geology of the 

 Isle of Wight,' 2nd ed. 1889, chap. xiv. ; Reid, 'Pliocene Deposits of Britain ' 

 Mem. Geol. Surv. 1890, pp. 69, 70 ; Strahan, ' On Overthrusts of Tertiary Date 

 in Dorset,' Quart. Journ. Geol. Soc. vol. li. (1895) p 549. 



