1852.] PRESTWICH ON THE THANET SANDS. 257 



It therefore appears that the chalk was extensively denuded before, 

 or at the commencement of, the deposition of the oldest tertiaries, 

 and that this denudation was stronger towards the south than the 

 north ; consequently the chalk, to ha\'e been brought within the 

 action of these denuding forces, must have had its surface more ex- 

 posed in one direction than in the other, and have undergone, even 

 at this early period, in order to have become so exposed, an elevation 

 to the south of the tertiary area and about parallel with the escarp- 

 ment of the North Downs. The phsenomena in the Hampshire ter- 

 tiary district seem to be very similar ; although, from the complete 

 denudation of the South Downs, tertiary outliers, like those on the 

 North Downs, are wanting, still there are indications of the lower 

 tertiary beds having spread over the chalk. It follows therefore that 

 if the ratio of decrease in the thickness of the chalk were continued 

 from Hertfordshire, then beneath the London tertiaries, and across 

 the North Downs, to the Wealden area (before its denudation), it is 

 probable that, before reaching the centre {x) of the latter district, the 

 chalk had either thinned out altogether or else existed merely as a thin 

 crust ; and consequently that an elevation of the lower cretaceous and 

 Wealden series, intermediate between the London and Hampshire 

 tertiary districts, or in fact in the position of the present Weald, had 

 already taken place before any of the tertiary beds were deposited. 



It may be objected that the elevation of this central Wealden mass 

 already existed before the chalk was deposited, — that it was a shoal 

 in the old chalk sea, — and even that the cretaceous series formed 

 originally but a thin covering over this part of the old sea-bed, and only 

 attained their full development as the water became deeper towards 

 the north. If this however were the case, there would be an over- 

 lapping in some part of the series, and we should further have the 

 lower cretaceous beds (1 & 2, fig. 7) wrapping round this shoal with 

 a thickness gradually increasing as we receded from it into deeper 

 water, so that on the edge of the shoal itself the lower beds would be 

 necessarily very thin or wanting, whilst we should have in the upper 

 beds of the chalk near this centre («) indications of a littoral zone. 



But we find both the Gault and Upper Greensand on the whole as 

 well developed in Kent and Sussex {y, fig. 8) as in Cambridgeshire and 

 Bedfordshire (x) . Nevertheless there is a change in the Lower Chalk 

 — the Clunch of the latter counties is evidently a far thicker and more 

 important rock than the Lower Chalk of the former counties. It is 

 therefore not improbable that there was a slow and quiet movement 

 of elevation in Kent, or of depression in Cambridgeshire, and whose 

 commencement might date from changes that took place at the end 



known, covered by 100 to 200 feet of tertiary strata. From Croydon and Epsom, 

 towards Merstham and Dorking, there is evidence indicating that the chalk is pro- 

 bably not more than 400 to 500 feet thick, whilst on the summit of the North 

 Downs overlooking Godstone, Reigate, and Dorking there are outliers of the lower 

 tertiary beds, which reach to the very edge of the escarpment overlooking the 

 greensand district ; and from raeasm*ement of the chalk between the upper green- 

 sand and the base of these tertiaries, I find that at Dorking and Reigate the chalk 

 is not above 400 to 450 feet tliick, whilst above Godstone it does not exceed a 

 thickness of 2»^^ feet. 



