part 3] the deyojNta> t or fekques. 235 



The Devonian period in this area appears to have opened with 

 shore-deposits of conglomerate, which would overlap each other as 

 the basin sank. In the next stage sandy deposits were laid down, 

 with remains of plants. Even the limestones were formed in a 

 shallow sea, in places being coral-reefs ; while the typical black or 

 grey colour is due to organic remains. The two series of dolomite 

 were probably the result of periods in which conditions especially 

 favoured reef -formation, though they do not appear to be coral-reefs, 

 but formed from the accumulation of calcareous debris. The period 

 is closed bj deposition of fine-grained sands, indicating a shallower 

 and less tranquil sea : in this last stage the amount of iron-oxide 

 is decidedly less, as indicated by the lighter coloration, than in the 

 earlier stages ; this may be due to some comparatively sudden 

 change in the area draining into the sea, either by alteration of the 

 river-courses or of the currents. 



The land continued to sink during Carboniferous times, but in 

 the great Hercynian movement a mountainous mass of Carbon- 

 iferous and Devonian strata was piled up. The thrust from the 

 south which caused this movement was resisted by the Brabant 

 Tableland, but the coast-line of the seas in which the Devonian 

 and Carboniferous deposits had been laid down was slightly oblique 

 to the direction of the thrust ; the Devonian rocks, therefore, 

 moved slightly on the Silurian beds, and relieved the stresses set 

 up (except that of compression) by fracturing along the planes 

 of the dip-faults, and moving in blocks progressively more to the 

 north from east to west. 



In the district between the Devonian exposures and the coast, 

 borings have revealed similar beds at depths of 574 feet (Wissant) 

 and 597 feet (Hervelinghen) below sea-level. There is, consequently, 

 a difference in the level of the Devonian surface at La Capelle and 

 Hervelinghen of 909 feet. Borings have also revealed the presence 

 •of Carboniferous rocks farther north at Strouanne, etc., and the 

 Kent Coalfield lies still farther in that direction. G-osselet 1 sug- 

 gested the presence of a great fault west of the Calais- Boulogne 

 road, to explain the distribution of the beds as revealed by the 

 borings ; but there seems to be no other evidence of it. 



The presence of a great thickness of Jurassic deposits indicates 

 that the spot at which they were found was remote from the edge 

 ©f the depression in which they were laid down. The Mesozoic 

 coast-line must, therefore, have taken a decided northerly trend 

 west of the Calais-Boulogne road. The basin in which the Jurassic 

 deposits were laid down and the Devonian -Carboniferous basin 

 correspond in a general manner ; the old Silurian coast-line, there- 

 fore, also probably took a northerly trend at this point, and passed 

 to the north of Strouanne and the Kent Coalfield. Consequently, 

 at Hervelinghen pressure would be applied at a much greater 

 distance from the supporting Silurian massif, and the resultant 

 structure would differ from that of the exposed Devonian, becoming 

 more complex with overfolding as well as faulting. 



1 Ann. Soc. Geol. du Nord, vol. xxvii (1898) pp. 143-44. 



