AUSTEN EXTENSION OF THE COAL-MEASURES. 61 



strata is only known from the trial-shafts, sinkings, and other works 

 which have been now carried as far as Therouanne, along the course 

 of the coal-trough, beyond which point the existence of productive 

 coal-measures has not been proved. Numerous sinkings have, how- 

 ever, been made over the intervening area, ending with the Artesian well 

 at Calais, where the coal-measures were again met with. An account 

 of many of these sinkings will be found in M. d'Archiac's memoir*, 

 from communications by M. Degousee. The results of these researches 

 will be found recorded on the accompanying Map, and it is evident 

 from these that the Mountain Limestone series westwards from 

 Tournay takes the same expansion that it does eastwards of that 

 place. At Houdain, on the S. side of the basin, older parts of the 

 series are in juxtaposition to the coal, whilst at other places the 

 limestone (either Carboniferous or Eifelian) intervenes, so that we 

 have quite sufficient to warrant the conclusion that the line of fault 

 and disturbance of the Meuse valley is carried on beneath the chalk 

 of Artois, and with like relations to the coal-band. By a careful 

 comparison of the rock-specimens obtained in the researches for coal 

 in the Department of the Pas de Calais, and which are preserved in 

 the offices of the several companies, the subcretaceous arrangement 

 of the Palaeozoic groups could be accurately laid down ; and such a 

 work would be a most important contribution to Economical Geology. 



The character of the disturbance which ranges along the lower side 

 of the Franco-Belgian coal-band is, that where, by means of a vertical 

 fault, the coal-series and some lower portions are brought to the 

 same level, the dimensions of the coal are greatest, and where the 

 older series rises into ridges and anticlinals, the coal-band narrows : 

 this is simply the result of that levelling of the surface before alluded 

 to, which is so remarkable a feature of this line of disturbance, and 

 by which portions have been planed off in exact proportion to the 

 elevation they received : in this way the contraction of the coal- band 

 towards Therouanne is the result of the widening of the anticlinal to 

 the S.W. of that place. Here, on the line of greatest elevations of 

 the axis of Artois, the thickness of the chalk is least ; and where the 

 line of abrupt fault is most marked, as from Houdain to Lievin, the 

 mass of overlying tertiary strata is the thickest. The whole line in its 

 geological features and its structure is in strict agreement with the 

 underlying older surface. 



Another feature which has influenced the contraction of the Car- 

 boniferous series in the direction of St. Omer is the rise and expan- 

 sion of the older Rhenane series south of and beneath the hill of 

 Cassel. 



From such considerations we may infer that on the N. limit of the 

 Boulonnais denudation there is a continuation of the Carboniferous 

 and Devonian groups, by means of a great linear disturbance, as they 

 are presented in immediate relation to the great coal-band from 

 Valenciennes by Namur to Liege. This is the old axis of Artois; 

 and the date to which the production of this line must be referred is 



* Mem. de la Soc. Geol. de France, 2 ser. torn. ii. p. 118 e# seq. 



