200 DE. E. L. SHERLOCK AND ME. A. H. NOBLE ON THE [June I912, 



of the Chalk -under atmospheric action. To the insoluble residue 

 left by solution acting during thousands of years would be added 

 clayey and loamy wash from the Tertiary lands, and remains of 

 Tertiary deposits left as pipes and hollows in the Chalk/ 



This view was opposed by Mr. Clement Eeid,^ who pointed out 

 that the amount of Chalk-with-Plints which would need to be 

 dissolved to produce enough insoluble residue was excessive; that 

 the proportions of clay to flints which would be produced by 

 solution are not those present in the deposit ; and that the bulk of 

 the material was derived from Tertiary rocks. 



Mr. Jukes-Browne ^ in 1906 published a paper in which he 

 maintained the essentially Eocene derivation of the deposit, and 

 gave the result of analyses of Chalk residues showing that the 

 bulk of the flints is disproportionately great as compared with that 

 of the clay. He also estimated that it would require the destruction 

 of 100 feet of the Micraster-corangiiinum Chalk to produce a residue 

 1-2 feet thick, together with a layer of flints 7 feet thick, so that 

 the product would be a bed of flint with about enough clay to fill 

 up the interstices. 



In South Buckinghamshire the thickness of the Clay-with-Flints 

 is known to be in places as much as 50 feet ; and, although this is 

 exceptional, and as a rule the thickness is unknown, it is probably 

 from 10 to 20 feet on the average. As the Reading Beds rest 

 upon the Micraster-coranguinum Chalk everywhere in South 

 Buckinghamshire except at Taplow, there is a maximum thickness 

 of 280 ^ feet of Upper Chalk available for solution. 



At Walter's Ash, near High Wycombe, there is about 70 feet of 

 Upper Chalk below the Clay-with-Plints, and so the most that we 

 could allow for denudation would be 210 feet, supposing that the 

 whole of the Micraster-coranguinum Chalk had been present : this 

 would yield 2-|- feet only of insoluble residue, different in compo- 

 sition from that of the actual deposit. Probably the amount of 

 Upper Chalk dissolved is much less than 210 feet: for the overlap 

 of the Eeading Beds is such that in the Colne Valley there is 150 

 feet, and in the Chess Yailey only 130 feet, of Upper Chalk under- 

 lying the Eocene. Allowing for this overlap, there would be not 

 more than 60 feet of Chalk- with-Flints dissolved away at Walter's 

 Ash : that is, sufficient for about 4 feet of flints and 9 inches of 

 residue to fill up more or less the interstices between them. 



The brickyard at Walter's Ash, 3| miles north-west of High 

 Wycombe, shows sections of a very remarkable character, which 

 have been previously described by the Eev. E. C. Spicer.^ At this 



1 ' Geology of London ' Mem. Geol. Surv. vol. i (1889) p. 282. 



2 ' Geology of the Country around Dorchester' Mem. Geol. Surv. 1899, p. 37 ; 

 ' Geology of the Country around Eingwood ' Ibid. 1902, p. 33 (and later 

 memoirs). 



3 Q.J. G.S. vol. Ixii (1906) pp. 132-62. 



* A. J. Jukes-Browne, 'The Cretaceous Rocks of Britain' Mem. Geok 

 Surv. voL iii (1904) p. 207. 



5 ' Sarsen-Stones in a Claypit ' Q. J. G. S. vol. Ixi (1905) pp. 39-41. 



