) 



68 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. | 



the filling-up of the basin of the Lake of Geneva by the detritus 

 brought down by the Rhone, that Hmited area would present accu- 

 mulations which in places would be nearly 1000 feet in thickness. 



It is very desirable with reference to the present inquiry that we 

 should know what next underlies the Wealden group at some point , 



near the Chalk-escarpment of Kent. Though the thickness of the « 



Wealden group has been estimated at 1000 feet, such dimensions a 



do not constitute any difficulty ; — the elevation of the beds and the 

 emergence of the lowest strata, together with the deep valley-sections 

 of the Wealden area, are conditions which offer many favourable 

 points for such researches. 



§ 4. Cretaceous Seines. 



On the eastern end of the axis of Artois, near Bellignies, the 

 Devonian or Eifelian series is overlaid directly by the Tourtia, an 

 accumulation of mid-Cretaceous age, and the fauna of which has been 

 fully described by M. d'Archiac^. This Tourtia everywhere under- 

 lies the Chalk along the axis of Artois, as may be seen at each of 

 the small denudations or fractures which the ridge presents ; it was 

 found near Arras, and occurs over the whole line of the coal-basin of 

 the Franco-Belgian frontier ; it is well seen near Tournay, and was 

 met with at Lille, as also N. of St. Omer. It is a conglomerate of 

 old crystalline and palaeozoic rocks, much water- worn, together with a 

 basis which is either argillaceous, sandy, or calcareous, and, as such, 

 is just such an accumulation as would be formed against a subsiding 

 ridge. The peculiar character of its fauna was the consequence of 

 such local conditions. The Neocomian group (Lower Greensand) 

 is altogether wanting over the district here described ; and this cir- 

 cumstance, taken in conjunction with the trend of the country towards 

 the Department of the Haute- Marne, before this lower portion is met 

 with, shows that it was an arrangement which was dependent on a 

 physical barrier. 



It is possible that some coarse quartzose sands, usually green, 

 which underlie the true fossiliferous Gault clay of the Boulonnais 

 may represent the Lower Greensand of our side of the Channel : the 

 evidence of any included fossils is wanting, and from certain appear- 

 ances of passage from one condition to the other, I at one time sup- 

 posed that they were sands of the Gault. These sands thin away 

 against the ridge of Palaeozoic rocks, so that they are wanting in 

 many sections, as at Blacourt, where the blue Gault clay fills the 

 inequalities of the limestone surface. A calcareous grit, composed of 

 the detritus of old rocks, underlies the Gault at Wissant j and 

 M. d'Archiac has called attention to the agreement between the 

 lowest Cretaceous beds of the Calais well with those of the Wissant 

 section. If these represent the Lower Greensand, it is here reduced 

 to a thickness of five yards. 



The limit of the area of the " Lower Greensand" or Neocomian 

 group is easily traced. It commences at Punfield on the W. in the 



'^ Mem. de la Soc, Geol. de France, t. ii. p. 291. 



