40 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



east this coal-band is overlaid in the direction of the Valley of the 

 Rhine by Upper Tertiary sands, and is lost for an interval of 40 miles, 

 when it reappears in Westphalia, under exactly the circumstances of 

 position which it has in Belgium. On the south-west of Valenciennes 

 the coal is worked beneath White Chalk, and a subjacent conglo- 

 merate (Tourtia) of the age of the Gault. 



The national importance of the Valenciennes coal-field gave rise 

 from early times to numerous speculations as to its probable extension ; 

 trial-shafts were sunk at various times and at enormous cost over a 

 very wide area, which, though unremunerative so far as the adventurers 

 were concerned, may have served to indicate the line of the present 

 workings. The continuity of the coal-beds beneath the chalk of the 

 north of France may be considered to have been proved for 80 miles 

 west of Valenciennes, along a line passing by Douay and Bethune 

 and south of Lillers. 



For a long time the engineers of the School of Mines, guided by 

 theoretical considerations, directed their researches towards Arras ; 

 even now speculations based on the supposed parallelism of one part of 

 the coal-field with the " System of the Hundsriick," and of another 

 with that of the Vendee, of M. Elie de Beaumont's theory, are put for- 

 ward ; and in this way the line of the coal-measures is carried by 

 Ferques and Hardinghen into the Boulonnais, of which the several 

 small coal-basins are represented as the natural termination (Burat, 

 p. 361). 



M. Dumont has also apparently adopted this view, from the direc- 

 tion which he has conjecturally given as that of the coal beneath the 

 Chalk. 



These last speculations would not, perhaps, have been ventured 

 on had the real structure of the Boulonnais been better known. The 

 coal-series of this district is interposed between two great groups 

 of limestone, of which the upper, with Tereh7'atula elongata, repre- 

 sents the upper Mountain Limestone of Vise, which underlies the 

 productive coal-measures of the Liege basin. The Boulonnais coal 

 is most probably a great expansion (the result of more favourable 

 physical conditions) of M. Dumont' s middle division of his " Con- 

 drusian " group, consisting of " Psammite, Macigno, and Anthra- 

 cite." 



It may be as well to state here distinctly that the fragmentary cha- 

 racter of the Boulonnais coal- formation is not in any way dependent 

 on original conditions of formation, but of disturbances of long sub- 

 sequent date : viewed apart from these accidents, the palaeozoic group 

 dips away beneath the oolitic series in the south-west, and no doubt 

 whatever can be raised as to its continuity in that direction*. The 

 upper or true coal-measure series of the Franco-Belgian frontier is 

 not presented in the Boulonnais denudation ; but will probably be 

 found to occur beneath the oolitic rocks on the south of Marquise. 



When my communication on the Boulonnais f was read, the trial- 

 shaft at Guines had not been carried through. This was soon after 



* Quart. Journ. Geo!. Soc. vol. ix. p. 233. f Ibid. p. 231, &c. 



