1868.] TOPLEY BAS-BOTTLONNAIS. 481 



making of iron" in England and Wales 400 were in Surrey, Kent, 

 and Sussex*. The fuel was wood charcoal, which becoming scarce, 

 and at the same time pit-coal coming into use elsewhere, the trade 

 gradually declined ; only nineteen furnaces and forges are marked 

 in Bugden's map of Sussex (1724), whilst at the end of the last 

 century there were only two. The last furnace, at Ashburnham, 

 lingered on till the middle of the present century. 



6. Purheck. — Immediately overlying the Porcland beds on the cliffs 

 north of Boulogne are some thin beds which have been described as 

 Purbeck. I was not able to make out these clearly, and shall there- 

 fore only briefly refer to what others have written concerning them. 



Dr. Fitton first pointed them out, in 1826t — and again in 1836^, 

 when he described " a thin crust of Purbeck strata, resting upon 

 those of Portland, and consisting of slaty beds of limestone, which 

 contain freshwater shells, and include a bed of tough dark-coloured 

 clay, in which are numerous fragments of silicified coniferous trunks, 

 not distinguishable from those of the Dale of Portland." Fitton, in 

 1839 §, further refers to this bed, and mentions that the silicified 

 wood is not found in place, but lies loose on the surface. It is pro- 

 bably the same as that which Desmars, long before, described as " bois 

 fossiles " ||. Dr. Fitton says that Cypris, Cyclas, and Ampullaria (?) 

 occur in this bed. Rozet^ mentions a bituminous clay as underly- 

 ing the ferruginous sand in places. This may sometimes be the 

 base of the Wealden, but I have observed a very tough dark- coloured 

 clay in the Foret de Boulogne and south of Bernes : in both cases 

 Wealden sands rest on Oolitic limestone ; and I am inclined to think 

 the clay is only the result of carbonated water percolating through 

 the sands and dissolving the limestone. 



Further observations upon the supposed Pui'beck beds of the 

 coast will be found in late papers by Pellat and Hebert **. 



III. Conclusion. 

 A comparison of the Cretaceous beds of the Boulonnais with those 

 of Kent shows a striking diminution in thickness from west to east 

 (see fig. 5)tt. This alteration is seen in all the beds, but in some 

 more than others, while a few disappear altogether. It is seen very 

 markedly in the Lower Greensand ; the Folkestone beds are much re- 

 duced in thickness, whilst the Sandgate and Hythe beds are gone al- 

 together. According to some observers the Atherfield Clay is repre- 

 sented at Wissant ; but it is certainly not constant round the district. 



* Sturtevant's ' Metallica,' 4to, 1612, p. 5. (Reprinted at Wolverhampton, 

 1855.) 

 t Proc. Geol. Soc. vol. i. p. 9. | Trans. Geol. See. ser. 2. vol. iv. p. 326. 



§ Bull. Soc. Geol. de Prance, .ser. 1. vol. x. p. 440. 

 II De I'air, de la terre, et des eaux de Boulogne-sur-mer, ed. 1761, p. 7. 

 ^ Desc. Geognostique du Bas-Bo\ilonnais, 1828, p. 48. 

 ** Bull. Soc. Geol. ser. 2. vol. xxiii. 



tt As this paper refers only to the Bas-Boulonnais, I have made no remarks 

 upon the chalk of the bordering escarpment and the Haut-Boulonnais. Mr. 

 Phillips has shown that the various beds into which he divides the clialk of 

 south-east Kent are much reduced in thickness in the cliffs between Wissant 

 and Sangatte, (Trans. Geol. Soc. ser. 1. vol. v. p. 48.) 



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