328 NATURAL SCIENCE [November 



or on the upper part of the escarpment face. That the flints have 

 been derived from the chalk is unquestioned, and the process by 

 which they were removed from it need not be considered. During 

 the first stage of the life history of the independent flints they were 

 split into slabs or tablets, most of which have one flat side, and one 

 showing the original external surface. Some of the flint slabs were 

 flat on both sides, and they are often six or eight inches in length. 

 The formation of these flint slabs was probably due to extreme cold, 

 as many of the fractured surfaces resemble those of frost-flakes. 

 Then a siliceous encrustation was deposited over the flint. The 

 next process — leaving the chipping of the edges out of considera- 

 tion for the present — was the staining of the flints to a red or 

 reddish brown colour. The staining was no doubt due to the action 

 of ferruginous solutions. The iron may have been derived from 

 beds of iron sand in which the flints were once embedded, as grains 

 of dark ferruginous sand are found still adhering to the flints in the 

 hollows of chipped surfaces. Subsequently to the staining, the 

 flints were scratched by some glacial agent. The striae are very 

 abundant, and closely resemble those produced by ordinary glacial 

 action. They were no doubt caused by the movement of pointed 

 flints across the flat surfaces of other flints under considerable 

 pressure. The movement of frozen masses of gravel might produce 

 these scratches as well as the flow of dirt-laden ice, so that it is 

 perhaps unnecessary to assume the existence of true glaciers in Kent 

 on the evidence of these scratched flints alone. 



The next change in the flints was the deposition over them of a 

 thin layer of silex, which covers most of the chipped surfaces, and 

 often fills up the scratches. The silica occurs in two varieties, one 

 brown and often very fibrous, and the other white. They may have 

 been deposited at different dates. The material is a variety of 

 chalcedony, sometimes having the characteristic botryoidal form of 

 that mineral. 



The nature of this siliceous encrustation is not yet completely 

 understood, but it must have been deposited by some siliceous 

 solution similar to that which has often re-cemented shattered flints. 

 There is, for example, in the Devizes Museum a flint that has 

 been broken into countless fragments, many of which are as fine as 

 grains of sand ; but they are all united by a chalcedonic infiltration 

 into a mass sufficiently solid to bear a high polish. The occurrence 

 in situ of flints which have been similarly crushed and re-cemented 

 has been recorded by Englefield and Mantell. 



One feature that renders this siliceous encrustation the more 

 interesting is that it was sometimes deposited later than one set of 

 glacial scratches and earlier than another set. 



The last process which the plateau flints have undergone is a 



