234< mr. J. w. j). rob ix son ok [vol. lxxvi, 



It has been assumed that the Devonian rocks thin out on the 

 western edge of the exposure ; but there is no evidence of this. 

 At La Malassise they are present in their normal thickness 

 (except the Fiennes Sandstone) ; but at Eslinghen, in the Blaeourt- 

 stream valley, the Fiennes Sandstone, the Fiennes Shales, the 

 Ferques Limestone have entirely disappeared, and the Carboniferous 

 touches the Beaulieu Shales. This is evidently due to a thrust- 

 fault, probably the one suspected at Eslinghen by J. Gosselet, 1 

 which also accounts for the occurrence of the Fiennes Sandstone 

 (reported in a well at this farm) that would be out of its proper 

 position. Traced eastwards, this thrust-fault fully explains the 

 outcrops of the Carboniferous dolomite in relation to the Devonian 

 beds ; finally, running down the lateral valley at Le Hure, it can 

 be seen in a new quarry, at the railway-crossing, as the junction 

 between the two systems. Farther eastwards it imiy bend slightly 

 to the south, or be shifted southwards by a dip-fault. 



Corroborative evidence of the existence of this thrust-fault is 

 afforded by the absence of the uppermost Devonian beds and the 

 beds equivalent to the Yoredale Shales, etc. The thickness of the 

 sandstones at Hydrequent and Sainte Godeleine, if not exaggerated 

 by faulting, shows that a much greater thickness of sandstone was 

 deposited towards the south. Borings made near Le Waast and at 

 Menneville, Bournonville, and Wirwignes, farther south, revealed 

 black shales (accompanied at Le Waast by red sandstones) which 

 are referred to the Upper Devonian, 2 but belong to a different 

 facies from those of this area, and may represent the missing beds. 



This thrust-fault is parallel to the one which bounds the inverted 

 Carboniferous beds, passing imirfediately south of Ferques, and also 

 to others which occur farther south, and appears to be of the same 

 age and due to the same thrust from the south, as that which is 

 known as the Hercynian movement. 



Minor thrust-faults may occur between the Devonian and the 

 Silurian ; but there is no direct evidence, except the reported 

 occurrence of Devonian rocks at Guines, possibly in an outlier 

 resting on the Silurian plateau (Brabant Tableland). 



The beds form the northern half of the JSTamur Basin, the Lower 

 Palaeozoic (probably Upper Silurian) to the north representing the 

 Brabant Tableland. The southern half of the basin is represented 

 by the Carboniferous Limestone of the Yallee Heureuse and the 

 Devonian of Hydrequent and Sainte Godeleine. The overthrusting 

 here is so severe that the beds are inverted. Farther south still, 

 the Condroz crest 3 has been identified by means of borings in the 

 Liane Valley (Chantraine). 



The Devonian rocks have remained practically unaltered ; the 

 shales are soft enough to be cut Avith a knife, and there is no trace 

 of slaty cleavage. 



1 ' Etude sur le Terrain Carbonifere du Boulonnais ' Mem. Soc. Sci. Agric. 

 & Arts de Lille, ser. 3, vol. xi (1873) p. 7. 



2 J. Gosselet, Ann. Soc. Geol. du Nord, vol. sxvii (1898) p. 141. 



3 Id. ibid. 



