212 Dowker — Junction of Chalk ivith Tertiary Beds. 



be accounted for by tbis means. The occurrence of a bed of semi- 

 tabular broken flints points to upheaval and fracture, and it is evident, 

 from the numerous small faults which appear in the Chalk, that 

 much disturbance has taken place. The re-cemented flints appear as 

 if fractured by some such means, and the surfaces presented to each 

 other being re-united, would imply some slow action, during which 

 the opposing surfaces have been re-combined. 



The green-coating of the flints and the Allophane, the result of a 

 chemical combination of Iron with Silica, and Silica with Alumina, 

 point to periods during which solution and re-combination have 

 taken place. It would seem that the junction of the Chalk with the 

 Tertiary sands favours the solution of the latter. Mr. Prestwich 

 mentions the occurrence of the Websterite and Hydrate of Alumina in 

 immediate contact with the Chalk, ^ and also the singular fact noticed 

 in the "Annales de Mines," that the mottled clays of the Argile 

 Plastique (of the Paris Basin), contain a very considerable portion 

 of gelatinous or soluble Silica. Should the Chalk, immediately below 

 the Thanet, have been subjected to subaerial action previous to 

 the deposition of the latter, organic matter would have beeii 

 deposited on its surface ; and, allowing vast periods of time, during 

 which solution of the Chalk had been brought about, the insoluble 

 flint alone would remain, together with the products of the decom- 

 position and re-combination of the soluble portion. This, I take it, 

 would efface, in great measure, the efi"ects of the subaeral action, 

 and would leave a tolerably uniform surface. The fractured flints, 

 by the presence of soluble Silica, might again be re-cemented. 



The faults in the Chalk have most of them taken place subse- 

 quently to the deposition of the Tertiaries. The bed of tabular 

 flint found in East Kent, immediately below the green-coated flints, 

 is the bed most subject to fracture ; and it is to be observed that 

 when these flints are taken from the Chalk they are particularly 

 brittle, and, for the most part, rejected by the road-makers on that 

 account. If it is possible, as stated in " Bischof 's Chemical Geo- 

 logy," that Carbonate of Lime may be entirely replaced by Silica, it 

 may be that the entire mass of this tabular-bed derives its origin 

 from the liberated Silica of the overlying Tertiaries. Whether this is 

 the case or not these facts are worthy the attentive study of the 

 chemist and geologist, and may tend to explain several of the 

 phenomena presented to the latter, such as the nature of the tabular 

 flint in the Chalk, and the absence of sand in thalf formation ; the 

 re-cementing of fractured flints I believe to"be going on at present. 



The great gap (if it be so large as generally represented) between 

 the Secondary and Tertiary strata may represent a period during 

 which the Chalk has been subjected to marine denudation and 

 subaerial influences, leaving the upper and re -stratified Chalk so 

 mixed with siliceous and organic matter that mutual decomposition 

 and re-combination of the substances therein contained may have been 

 brought about, the organic matter playing an important part in this 

 decomposition. The bed of green-coated flints with allophane and 

 1 Quarterly Journal of Geological Society, vol. x., p. 123. 



