1866.] HUGHES THANET SAND AND CHALK. 403 



green bed with flints at the base of the Thanet Sand. The junction 

 may be seen in a chalk-pit in the brick-field north of Moor Street. 

 But there are local circumstances by which this may be explained : — 



1st. There is a bed of clay in the Thanet Sand, as may be seen in 

 the road-cutting leading to the brick-field ; and 



2nd. There is other evidence of a line of upheaval running nearly 

 east and west through this part of the district, which, in conjunction 

 with the bed of clay above mentioned, would have the eff'ect of 

 throwing the drainage off" to the north and south, and therefore 

 partly protecting the surface of the chalk here from the action of the 

 carbonated water. 



Similar reasoning may be applied to explain the local phenomena 

 of the more westerly part of the Tertiary basin — as, for instance, at 

 Reading and Newbury, where the surface of the chalk is bored by 

 Lithodomi to a considerable depth. Here the impervious beds of clay 

 in the overlying Reading series have probably contributed most to- 

 wards protecting not only the oysters of the lowest part of the Ter- 

 tiary beds, but also the surface of the chalk from the action of the 

 carbonated water. It is known that the clays of the Woolwich and 

 Reading series allow very little water to pass through them*. 



An examination of the junction of the Sandgate Beds and the 

 Kentish Rag leads to a similar conclusion. In the large quarries on 

 the north-west of Maidstone, at the base of the Sandgate Beds, there 

 is a greensand, generally on rubbly Kentish Rag. The thickness of 

 these intermediate beds varies. It is greater in hollows and depres- 

 sions, where, we have reason to believe, a larger amount of the Rag 

 has been removed. 



In the brick-fields close to the town of Maidstone the brick-earth 

 occurs in long furrows of the nature of pipes, in the Kentish Rag. 

 The whole of these furrows are lined with greensand and rubbly lime- 

 stone, similar to that found at the base of the Sandgate Bedsf. 



Now, unless we allow that the greensand has been derived from 

 the decomposition of the Rag after the deposition of the brick-earth, 

 and that the rubbly limestone below it is the same in process of de- 

 composition, we have only the improbable alternative left that the 

 irregular denudation which removed the Sandgate Beds previously to 

 the deposition of the brick-earth left the same thin bed at the surface 

 over this large area. 



There are several interesting questions raised by this view of the 

 origin of such beds. What do they represent ? Do they represent 

 the periods during which the surface of the limestone has been above 

 the sea-level ? for, except in some peculiar cases, if it were under 

 the sea, and the water on its surface were at rest, i. e. not constantly 

 renewed by fresh infiltration, it would soon become saturated with 

 carbonate of lime, and could remove no more. 



How much limestone has been removed since the deposition of the 

 overlying beds ? The thickness of the beds formed of its insoluble 

 remainder is not a fair criterion ; for, in the case of the chalk, for 



* See Prestwick, Quart. Journ. Geol. Soc. vol. x. p. 82, footnote. 



t See Messrs. Foster and Topley, Quart. Journ. Greol. Soc. Tol. xxi. p, 443. 



