136 ME. W. B. R. KIXG ON THE MIDDLE [vol. Ixxvii, 



clayey marls, practically impervious to water; while the Upper 

 Turonian {Kolaster-planus Zone) and the Senoniaii are made up 

 •of porous Hint-bearing chalk. 



In the lower beds, water only circulates in occasional joints, 

 while in the upper group the whole mass is extremel}^ porous, and 

 the water has a free circulation. If, however, the beds be traced 

 towards the west, the claye}^ nature of the Terehrafuliiut -gracilis 

 Beds is gradually lost, and the marls are confined to the Lower 

 Turonian and the Cenomanian ; but the T.-graciUs Beds remain 

 practically free from flints, and the HoJaster-pIaiiiis Zone still 

 •coincides more or less with the appearance of flint- bands and flint- 

 nodules in abundance. 



Since the marls of the Middle Chalk have a certain controlling 

 ■effect upon the behaviour of the underground water (and also on 

 the surface-water), it was important (1) to ascertain the amount 

 ■of this controlling effect, and (2) if that proved to be of sufficient 

 ■ consequence, to devise some plan whereby the information could 

 be put in a readily accessible form for the use of the water-supply 

 officers in the armies. 



To the first of these points I shall return later ; for the second, 

 a map of the contours of the summit of the marls (that is, the top 

 of the Terehratulina-gracilis. Zone) was constructed from the 

 evidence of the boreholes, which already existed — either French 

 (where records are available), or British Army borings up to date. 

 Where the marls cropped out at the surface, the altitude of the 

 outcrop of the highest bed was taken from the French geological 

 maps. 



A map, similar to those constructed by Prof. Gosselet (o^j>. jam 

 cit.), was thus plotted out. This enabled an estimate of the 

 depth to the marls at an}" point to be arrived at with ease, by 

 subtracting the figure obtained from this map from the surface- 

 altitude as shown by the contours of the topographical maps. 



As new records came to hand, the curves needed some modification, 

 in order that they might be brought into line with the new 

 information. PL III is taken froiu the final map, after records 

 of all the boreholes had been received and the necessary corrections 

 made. On this map was entered the position and altitude of the 

 marls in each borehole made by the six water-boring sections 

 which were part of the water-supply organization of the British 

 armies in France. 



Each of these water-boring sections was under the command of 

 an engineer officer possessed of the technical knowledge necessary 

 to direct the borino; work. One of his duties was to enter on a 

 form all details available regarding the strata through which the 

 bore passed, the yield of the bore, together with particulars of the 

 casing which had been installed b}^ the Boring Section, the location 

 of the site, and similar information. These forms were sent to the 

 water-supply officer, and a cop}" was forwarded by him, in due 

 course, to the Engineer-in-Chief's office at Gr.H.Q. Here the 

 information Avas condensed and entered on cards which were 



