lOO NATURAL SCIENCE. August. 



gap between these and the nearest of the truly marine Westletons 

 is a great one : the most north-easterly of the patches in the 

 London area is at Highbeech, at the height of 370 feet ; the nearest 

 of the Suffolk and northern Essex series is 30 miles distant at 

 Braintree, where the gravels are at the level of 240 feet. There 

 seems, moreover, a considerable difference in the gravels of the two 

 areas : those of the northern Essex and Suffolk group are probably 

 truly marine, whereas, in the case of those of the London district, 

 there seems no evidence to refer them to the same agency, except 

 their gradual rise when followed to the south-west. The acceptance 

 of the correlation and original continuity of the two sets of gravels 

 in these two distant areas is essential to Professor Prestwich's 

 hypothesis. It is almost impossible actually to disprove this, but the 

 evidence in its favour is at present very limited. 



The view that the whole of the South-East of England has been 

 submerged below the sea in Pleistocene times is also one that has 

 not found much favour. Messrs. Monckton and Herries, for example, 

 certainly do not accept any submergence of the district in Essex 

 around Brentwood and Warley since Eocene times (no. 3, pp. 22, 23 ; 

 nos. 4 and 5). It may be objected that the patches of sand on the 

 North Downs of Kent and Surrey, which are referred to the base 

 of the Pliocene series, prove a subsidence at this period ; but as far 

 as I am aware, the only one of the patches certainly marine is that 

 in the neighbourhood of Lenham, east of Maidstone. This may 

 indicate merely a gulf running from the Pliocene sea up the valley 

 of the Swale. There is no proof whatever for the Pliocene age or 

 marine origin of the sand patches elsewhere on the North Downs, 

 such as that at Coulsdon. 



Nevertheless, in spite of the general hesitation expressed, it 

 seems impossible to get any one piece of direct evidence that is 

 final against Professor Prestwich's conclusions. His theory is 

 founded mainly on a series of inferences rather than direct proof, and 

 inferential evidence is all we have against it. Dr. Hicks's (6) demon- 

 stration that the Hendon Chalky Boulder Clay mantles the surface and 

 slopes of Pre-Glacial hills and valleys, and Mr. T. V. Holmes's 

 discovery of Boulder Clay at Upminster (8), showing that at least 

 two-thirds of the Thames Valley had been eroded before the Chalky 

 Boulder Clay was deposited, are neither of them final ; for Professor 

 Prestwich fully admits a considerable amount of erosion before the 

 Chalky Boulder Clay, but later than his early Pleistocene Westleton 

 Shingle. Both the cases described by Dr. Hicks and Mr. Holmes 

 are to the south of the Westleton line, and far from the great Chiltern 

 escarpment, on which the point in dispute must turn. 



There are, however, two areas which give strong a priori evidence 

 against the truth of Professor Prestwich's theory. These are the 

 Chiltern Hills and the Downs of western Berkshire, the structure 

 of which we will now proceed to consider. 



