68 UEPOR'r--1875. 



These two folds are separated by a very distinct syucliual depression, which is 

 anterior to the Bel. nmcrvnata chalk, which at the north, near Moreuil and Hardi- 

 villiers, haye only been deposited in the depression and not on the rise. 



A third parallel fold extends from Mentheville, near Fecamp, to Tr(;port ; to 

 the north-east it passes near Lillers, where the Devonian crops out ; at Dieppe 

 this fold has caused a fault of about 250 feet. All the Boulounais is to the N.W. 

 of this line, and the beds dip to the sea. The Gault descends more than 330 feet 

 from Fiennes to Wissant. 



But this movement of depression does not continue, and the strike of the beds, 

 which can be perfectly followed at low water, indicates a gradual rise. This strike 

 makes an angle of 38° to 40*^ with the coast-line (Chelloneix). The beds get further 

 off to the north ; but as we find them again exactly the same at Dover, they must 

 necessarily curve round towards the west ; and this can only be by the eifect of an 

 undulation similar to the last. 



It is a legitimate inference to draw that the lie of the beds is the same under the 

 channel as on the coast; and direct observation brings us to the same conclusion, 

 viz. that the Cretaceous beds are raised in the form of an anticlinal axis in the 

 middle of the Straits. Most certainly, starting from any point of the coast between 

 Blanc Nez and Calais, so as to proceed to Dover in a straight line, it will be 

 impossible to keep in the same horizon of Chalk Marl, or Chalk without flints, 

 even if this bed were 400 feet thick. An undulation or boss some 400 to 5C0 feet 

 must be expected. 



We should therefore have here, in consequence of the preceding facts and argu- 

 ments, a fourth axis (S.W. and N.E), Faxe de la Manr.he. 



The existence of this axis was announced in 1846 by D'Archiac, who reasoned 

 from very diil'erent data. Every thing concurs to prove its reality. 



It will be noticed that the extent of these undulations, which always bring the 

 Upper Greeusand to the surface in the north of France, increases constantly on 

 approaching the sea. 



At Fecamp the Upper Greensand, which is here quite sandy, crops out at more 

 than 270 feet ; at Aumale it reaches 400 feet ; there is therefore serious cause for 

 thinking that a tunnel carried in a straight line would go through not only the 

 Upper Greensand, but also beds of older age. 



The two systems of folds which I have described both bring about important 

 faults, from 260 to 400 feet in throw. The first parallel to the axis of the Boulou- 

 nais and Weald is not likely, in my opinion, to have produced any great fault in 

 its line of trend in the beds to be cut through by the tunnel. As to the second, 

 there would be nothing surprising if it should have caused a line of fracture parallel 

 to the Channel axis ; but hitherto no evidence has been forthcoming on this point. 



It follows from what I have said that if the tunnel is bored in the Chalk Marl or 

 the ChaUi ivitlioid Flints, or even proximately above this layer, it will leave not 

 only this bed, but also the Chalk Marl and G}-ee9i Chalk, and meet Upper Green- 

 sand, the sandy base of which, lying on the clay of Gault, may, or rather must, 

 contain a sheet of water. 



A means of avoiding the Upper Greensand would be to take a northerly direction 

 towards Calais, and to enter first the Chalk with Flints and then to go down into 

 the Chalk Marl. But this Chalk with Flints contains permeable beds; so that the 

 boring made at Calais in 1844 tapped at 5S0 feet a feecier of brackish water. 



In my opinion these permeable bands are beds of hard limestone pierced by holes 

 such as I have ii'equently observed in the Chalk. It is to one of these bands that 

 Mr. Whitaker has given the name of Chalk Hock. I do not say that these beds 

 are always permeable, but that they may be so. I believe that it is they which feed 

 the Artesian wells of Artois. 



Now there is one of these beds at the top of the Chalk with Amntonites ManteUi 

 and Ammonites rotomagensis (Grey Chalk) ; and there is usually one, sometimes 

 two or three, at the top of the Chalk with few flints. Above that horizon come 

 the Holaster j)lanus Chalk-beds of Dover, then those with Mic. cor-tcstudinarivm of 

 the base of the southern cliff" of St. Margaret ; all these beds are generally haid, 

 fissured ; yet they will occasion less dangers than the base of the Upper Greensand, 

 and their permeableness may be but accidental. 



