142 MK. W. B. R. KTNa OlSr THE MIDDLE [vol. Ixxvii, 



One case, however, of a good supply from a boring on the top of 

 a hill occurred at Beaurains (south of Arras) ; but, as a general 

 rule, the local topography of the district was a much greater con- 

 trolling factor in determining the yield of a boring than the^ 

 general tectonic structure, provided that the surface of the water- 

 table was not less than about 50 feet above the surface of the 

 marls. 



In conclusion, I wish to acknowledge the liberal help and un- 

 failing kindness which I always received while in France from 

 Prof, (then Colonel) Sir T. W. Edgeworth David, K.B.E. 



EXPLANATION OF PLATE III. 



Sketch-map of tbe Somme Valley and neigiibonriBg districts of Northern 

 France, showing the contours of the surface of the marls of the Middle 

 Chalk at intervals of 25 metres (82 feet), on the approximate scale of 

 8 miles to the inch or 1 : 506,880. 



Discussiois-. 



Prof. W. W. Watts noted that the Author had observed a 

 definite relation between water-supply and surface-topography. 

 Water-engineers had the habit of sinking wells in valleys, and the 

 reason seemed to be that not onlj'^ did water travel in Chalk along 

 the fissures, but, moving in a definite direction towards any outlet, 

 it in the end produced by solution a definite system of converging 

 channels in that direction, which could be tapped by a well. The 

 necessary outlet had often been produced b}^ lowering of ground in 

 the formation of the valle}". 



Prof. P. P. Keistdall remarked that the Author's observations 

 were of great interest and value. If the same tj^e of structure 

 extended to the Chalk between the Boulonnais and Calais, it 

 would have to be taken carefully into account in laying out the 

 line of the Channel Tunnel, otherwise, in view of the amplitude of 

 the folds, it might be apprehended that the tunnel would need to 

 be deflected in order to avoid passing into heavilj^-watered divisions 

 of the Chalk. 



He asked the Author to explain the sense in which he used 

 the expression ' water coming out of the Chalk,' in contradistinc- 

 tion to that jdelded by fissures. Though the Chalk is perhaps the 

 most porous rock in the British sequence, having a porosity in 

 some examples as high as 46 per cent., the Avater contained in the 

 pores will not drain out to become available for the supply of wells 

 or boreholes. 



Dr. J. W. EvAis's suo-o-ested that the fact that the east-south- 

 east and west-north-west streams were usually a little to the 

 north-east of the parallel synclines might be accounted for by a 

 dip of the axial planes of the folds to the south-south-west, as ia 



