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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Jan. 23, 



feet below the surface for a considerable distance from its outcrop*. 

 As in the counties in question the Upper Chalk is not far short of 300 

 feet in thickness, and sometimes quite that, it is clear that the top- 

 most beds of the Chalk do not generally occur either on or near the 

 escarpment, but that they have been denuded. The Chalk-rock will 

 serve as a datum for the measurement of the extent of this denu- 

 dation. 



There are two more points that may be noticed in connexion with 

 the Chalk-rock. To the north and north-west of Marlborough, the 

 Upper Chalk has an escarpment of its own, quite distinct and separate, 

 and often at a considerable distance, from that of the Lower Chalk, as 

 noticed by Mr. Avelinef. May not this be owing to the Chalk -rock 

 being there very thick, as there is every reason to suppose is the case ? 

 Further eastwards, where the Chalk-rock is thinner, the two escarp- 

 ments generally merge into one. 



In parts of the South of England there are flints from the top to 

 the bottom of the Chalk. I do not know whether it is supposed that 

 this flint-bearing Chalk represents both the Upper and Lower Chalk of 

 other parts, or the former only. The occurrence of the Chalk-rock 

 might set at rest the question whether the Upper Chalk only is there 

 present, or whether the Lower Chalk is flint-bearing. The former 

 case would imply an overlap between the Upper and Lower Chalk. 



Postscript. — Since this paper was read, Mr. Prestwich has told me 

 that he has seen the bed in question on the top of the chalk-hills 

 between Calne and Marlborough Downs ; that it was found in a well 

 at Harpenden, near St. Albans ; that it has been noticed by Mr. 

 Bensted and others on Kit's Cotty Hill, near Maidstone (where it is 

 but 2 or 3 feet thick) ; and that the nodules in it have been found to 

 contain ten per cent, of phosphates, 



A more detailed account of the sections of the Chalk-rock in 

 Oxfordshire and Berkshire will be given in a memoir (shortly to be 

 published) illustrating Sheet 13 of the Map of the Geological Survey; 

 and those in Buckinghamshire will be noticed in a memoir (now in 

 progress) to illustrate Sheet 7. 



* I find that Mr. Godwin-Austen has noticed the extent to which the chalk- 

 escarpment of Berkshire consists of Lower Chalk. He says, " the highest point, 

 Uifington Camp, appears to rise no higher in the series than the Chalk-without- 

 flints." (Quart. Journ. Geol. Soc. vol. vi. p. 461.) 



t See Memoir illustrating Sheet 34 of the Map of the Geological Survey of 

 Great Britain, p. 37. 



