406 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mai. 21, 



although, it is the thinnest, being often but two or three feet 

 thick, and seldom more than five. For this reason, and from 

 the fact that the green-coated flints are unworn, the question 

 has occurred whether it may not have been formed, in some 

 measure, after the deposition of the beds above, by the slow ac- 

 tion of water flowing through the overlying sand, &c., to the Chalk, 

 dissolving away the latter and depositing clayey matter, salts of 

 iron (the green colouring-matter), and sometimes allophane. The 

 occurrence of the last mineral at the junction of the Thanet Beds 

 and the Chalk has been recorded by Prof. Morris*, who says that 

 '• it must have been deposited from a fluid or viscid state, not only 

 after the denudation of the Chalk and the deposit of the partially 

 abraded (?) flints, which are coated with it, and after the accumula- 

 tion of the Thanet Sand, but subsequently to the disturbance of the 

 whole series, whereby the fissures in the Chalk were formed, and in 

 which the allophane is now found." I have since found it at the 

 same junction at Chiselhurst and Paversham ; and I believe that it 

 occurs at the bottom of the Eeading Beds at Xorthaw, where they 

 rest on the Chalkf. 



Mr. Prestwich has noted '•' the constant occurrence " of the green- 

 coated flints at the bottom of the Thanet Beds, ''just as they occur 

 in the underlying Chalk, from which in fact they appear to have 

 (been) removed comparatively without wear or fracture," but has 

 inferred that a "powerful but transient action" must have been 

 needed "to uproot these flints from the Chalk "J. 



It is remarkable that, whatever rests on the Chalk, there should 

 nearly always be unworn flints at the junction : thus in the western 

 part of the London Basin there are flints at the bottom of the Read- 

 ing Beds ; andin tbe many places where the Chalk is hidden by the 

 irregular deposit of brick-earth so common in Wiltshire, Berkshire, 

 Oxfordshire, &c., the two are separated by the peculiar " clay with 

 flints," as I have called it§, the origin of which my Mend Mr. 

 Codrington, F.G.S., and myself have, independently, referred to the 

 slow dissolving away of the Chalk by water [j. I think it not un- 

 likelj' that the " argile a silex "^ of the French geologists (or some 

 part of it) is the same as our clay--with-flints, in which case, strange 

 to say, the same name will have been given to the deposit in two 

 countries and languages. Prof. Hebert's description of the former 

 in France ** would in great part serve equally well for the latter in 

 England. 



I have before noticed thp remarkable conformity between the 



-I ' 



.. .' i . . 



■^ Quart. Joui'ii. aeol. Soc. v . xiii. p. 13 (1857). 



t G-eol. Siu'vey Mem. on Sheet 7, p. 30 (1864). It was noticed before by Mr. 

 Prestwich as '• hydrate of alumina : " Quart. Journ. Geol. Soc. vol. x. p. 123 (1854). 



t Quart. Journ. Greol. Soc. vol. riii. pp. 243, 253. 



§ Greol. Smwey Mem. on Sheet 13, p. 54. 



i| Magazine of the Wilts. Ai-cha-ol. and Nat. Hist. Soc. vol. ix. p. 167 (1865), 

 and Geol. Survey Mem. on Sheet 7, p. 64 (1864). 



^ I have lately (July) seen some of the " argile a silex " on the coast of 

 Normandy, where it is exactly like the clay so common on our own Chalk. 



** BuU. Soc. Geol. France, 2^ serie, t. xix. p. 445 (1862), and t. xxi. p. 58 (1864). 



