Domker — Junction of Chalk with Tertiary Beds, 211 



hibits numerous fractures, that have been recemented by siliceous 

 matter ; these fractured surfaces are often shifted laterally and joined, 

 though the original opposed surfaces are not in contact. In many 

 cases there intervenes between the surfaces a layer of white-coated 

 flint, of a chalcedonic structure. Sometimes these masses of flint 

 appear to have been crushed and the fragments re-cemented, though 

 only partially, with siliceous matter.^ 



The more tabular flint at the top of the Chalk is not green-coated, 

 but the upper surface has often a ferruginous stain. 



All the green-coated flints, though not rounded, exhibit the 

 appearance of having undergone solution, and are not so large or 

 uneven as those found in the underlying Chalk. Professor Morris, 

 in a paper read before the Geological Society,^ drew attention to the 

 fact of the mineral Allophane occurring associated with the green- 

 coated flints, and in fissures of the Chalk immediately under them. 

 This Allophane is Hydro-silicate of Alumina ; the analysis, as given 

 by Mr. Dick, being, silica 18.89, alumina 32.52, water 42.73, 

 lime 1.67, carbonic acid 2.51, and a trace of organic matter. 



From the researches of eminent chemists it would appear, that 

 under certain conditions Silica becomes soluble, and capable not only 

 of making combinations with other substances forming silicates, but 

 also of replacing them. 



" It is very deserving of notice (observes Bisohof ) that Carbonate 

 of Lime may be displaced by almost all siliceous substances ; and 

 consequently it is possible that entire layers of Limestone may be 



displaced by Silica."^ "Organic matter seems to have a 



marked action on the silification of bodies."^ 



Nature accomplishes what the chemist in his laboratory is unable 

 to perform, because the conditions of time, space, and pressure are 

 not within the reach of the latter. The appearance of the green- 

 coated flints may be the result of the long-continued action of 

 natural causes. 



Mr. Whitaker has stated that in the eastern part of Kent 

 the Thanet beds rest conformably on the Chalk.* Should this 

 be the case, it would be difficult to account for the presence of 

 this bed of flint as the result of marine denudation. On the 

 other hand, what solvent process would remove such a mass 

 of Chalk as to remove the debris represented by these green- 

 coated flints, and still leave them conformable to the underlying 

 bed? It appears that there is a gi'eat break (in this country) 

 between the Secondary and Tertiary formations. It is natural to 

 suppose that marine denudation has been at work during the interval, 

 removing the upper part of the Chalk ; some such action might 

 account for a bed of worn flints ; but the angular flints could not 



' For a beautiful example of a fractured flint, re-cemented by infiltrated silica, see 

 the late Dr. S. P. Woodward's paper " On the Natui-e and Origin of Banded Flints,' ' 

 in Geol. Mag., Vol. I., p. 145, PI. VIII. Fig. 4.— Edit. 



* See Professor Morris's paper, Vol. xiii., p. 13, of Quarterly Journal of the 

 Geological Society. 



3 Chemical Geology, Vol. ii., p. 480, English Translation (Cavendish Society). 



* Op. cit. 



