part 2] CHALK maels in the somme vallet. 139 



and Pozieres. This line is the old axis of Artois, as shown on 

 Pi-of. L. Caj^eux's map.^ 



If the curves now plotted out are approximately correct, it will 

 be seen that the maximum crest-line does not swing round by 

 Bapaunie and Bihucourt to Sanity ; but rather that the whole 

 area between the valleys of the Tortille and Hirondelle on the east 

 and those of the Ancre and Sensee on the west is a region of low 

 altitude of the marls, where the main axis is lost in a slightly un- 

 dulating low-lying plain, and that the main crest-line on the west 

 of this area is on the line Bihucourt-Saulty : that is, about 7 miles 

 north of the position where a continuation of the Le Cateau-Nurlu 

 line would have been situated. 



When the Bihucourt-Saulty line is traced westwards, a repetition 

 of the same phenomena is observ^ed. Tlie anticlinal axis is lost in 

 the great synclinal depression Avhich runs from Hesdin to Arras. 

 The borings in this area are, however, not numerous, and the marls 

 are losing their extremely clayey nature, so that little information 

 is available from a study of the evidence afforded by them. The 

 map nevertheless clearly sliows how the Bihucourt-Saulty line is 

 lost in the west, and that the main crest-line is taken up in the 

 north by the Vimy-Notre Dame-de-Lorette lidges. 



M. H. Parent has studied the curves of the marls in the north 

 of Artois,^ and his map shows how the axes are not continuous, 

 but are separated b}^ north-and-south breaks, or what (from analogy 

 with the Bapaume area) might be explained by the disappearance 

 of an anticlinal ridge in an area of general low or high elevation, 

 and the replacement of the axis, not necessarily on the same line, 

 but more probably on a line situated somewhat en echelon to it. 

 This arrangement of maximum crest-lines of the marl-surface 

 is one that might be expected from general considerations of the 

 underl3dng tectonic structure of the region. 



Under the line of the Vini}^ and Notre Dame-de-Lorette ridges 

 the Devonian grits are packed on top of the Carboniferous deposits, 

 and in post-Cretaceous times folding and faulting has continued 

 along this old line, causing the maximum elevations of the marls in 

 this district. East of Arras, however, the great basin of Orchies 

 appears to have subsided under the lateral pressure of the post- 

 Cretaceous folding, and has thus given rise to the low area in the 

 marl-surface south of it. 



The manner in which the crest-line is formed by a series of 

 curved axes arranged en echelon would also be the natural result 

 of the folds accommodating themselves to the change in strike 

 which takes place on this north-and-south line of weakness. 



We ma}'- now turn to the question of the connexion between the 



^ ' Ondulations de la Craie de la Feuille de Cambrai, &c.' Ann. Soc. Geol. 

 Nord, vol. xvii (1890) pp. 71-90 & map. 



^ ' Notes Supplcmentaires sur les Plis du Nord de 1' Artois ' Ann. Soc. 

 Geol. Nord, vol. xxi (1893) pp. 93-104 & pi. v. 



Q. J. a. S. No. 306. M 



