Geological Society. 471 



these equations the first is equation (16) of my paper and 

 the second equation (15) ; I derived them as two of the con- 

 ditions that a principle of least action should be satisfied for 

 the path of a particle. To assume that the same equations 

 hold for a light wave is to say that the motion of a light 

 wave is the limit of that of a particle when the velocity at 

 a great distance from the Sun tends to that of light. My 

 suggestion was that this is a natural hypothesis to make, and 

 that Einstein's law is the only reasonable one that makes 

 it fit the observations. I do not think it is obviously true 

 a priori, but that it is highly probable. 



LV. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 200.] 



March 9th, 1921.— Mr. E. D. Oldham, E.R.S., President, 

 in the Chair. 



HPHE following- communications were read : — 

 X 1. 'The Surface of the Marls of the Middle Chalk in the 

 Somme Valley and the Neighbouring Districts, and the Effect 

 on the Hydrologv.' By William Bernard .Robinson King, O.B.E., 

 M.A., F.G.S. 



During the war numerous boreholes were made by the British 

 Armies in France. In the valley of the Somme and in neighbouring 

 districts of France, where the Chalk forms the main deposit of the 

 area, water was obtained for the troops, largely from boreholes. 

 These were made by the percussion method, so' that the passage 

 from the chalk of the Upper Chalk to the marls of the Middle 

 Chalk usually was only approximately determined ; the great 

 number of bores, however, enables one to construct a map of the 

 contours of the marl-surface with sufficient accuracy for it to be 

 of value. 



These curves bring out several points of interest, which may be 

 briefly tabulated as follows : — 



(1) The main anticlinal crest (axis of Artois) is not continuous, but consists 

 of a series of curved axes arranged en echelon, in such a way that the crest- 

 line is stepped gradually more and more to the north on going from east to 

 west. 



(2) The close relationship of the river -systems to the tectonic axes is 

 demonstrated ; but, in several cases, the physiographical linos, while being 

 parallel to, are often not coincident with, the tectonic axes (for instance, the 

 valley of the Somme lies several miles north of the synclinal axis of the 

 Somme). 



(3) The capacity of the Chalk to yield water for boreholes measuring about 

 6 inches in diameter is shown to depend more on the topography of the neigh- 

 bourhood than on the larger tectonic features, provided about 50 U\^{ c^i~ chalk 

 occurs between the marl surface and the surface of the water-table in the 

 Chalk. 



