402 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 21, 



2. Farther Documents relating to the Foemation of a New Island in 

 the neighbourhood of the Kaimeni Islands, By Commander Gr. 

 Teton. 



(Communicated by the Lords Commissioners of the Admiralty.) 



[An abstract of this communication was published in Quart. Journ. Geol. Soc. 

 No. 87, p. 319, by order of the Council.] 



3. Note on the Junction of the Thanet Sand and the Chalk, and of 

 the Sandgate Beds and Kentish Eag. By Thomas M*'Kenny 

 Hughes, B.A., F.Gr.S., of the Geological Survey of Great Britain. 



At the bottom of the Thanet Sand there is always a bed of green- 

 coated flints in a green and rust-brown clayey sand*. 



The following observations have led me to infer that this bed is 

 due to the decomposition of the top of the Chalk after the deposition 

 of the Thanet Sand : — 



1. The flints never show any traces of having been rolled or worn 

 by the action of water, or broken up and weathered by any subaerial 

 agency, but are, except in colour, exactly similar to those in place in 

 the Chalk. ' 



2. No fossils, except chalk fossils preserved in flint, have been 

 found in it. 



3. "Where a nearly continuous bed of flints, or a large tabular 

 mass of flint occurs, the base-bed of the Thanet Sand seems to be 

 arrested by it in a manner that would suggest rather the chemical 

 decomposition than the mechanical erosion of the surrounding chalk. 



4. Where masses of chalk are imbedded in the base of the Thanet 

 Sand they appear to be due to local undermining of the main mass 

 of the rock, and not to be transported fragments rearranged in a 

 hollow. 



Again, to look at the question from another point of view, it is 

 highly improbable that it could be otherwise. As water charged 

 with carbonic acid, soaking through the Thanet Sand, reaches the 

 chalk below, it must decompose the surface to a certain extent ; and 

 if the water can pass freely away so that new supplies, not saturated 

 with carbonate of lime, are brought to act upon it, that decomposi- 

 tion must go on ad infinitum. 



The only difl'erence, therefore, between this action extending over 

 the whole surface of the chalk where covered by Tertiary or later 

 deposits and that which forms pipes is, that in the case of the pipes 

 the water is collected at or near the surface into small streams; 

 whereas in the other case it permeates the whole of the deposit over- 

 lying the chalk, and acts more equally on its surface. 



There are cases where the surface of the chalk does not appear to 

 have suffered any great amount of decomposition — as, for instance, 

 north-west of Eainham, in Kent, where there is very little of this 



'"' For further description, see Prestwich, Quart. Journ. Geol. Soc. vol. viii. 

 p. 243. See also Wliitaker, mfra, p. 405. 



