Vol. 50.] AT NEW LODGE, NEAR WINDSOR FOREST. 153 



when the water gradually rose in the borehole, reaching the surface 

 and even rising 7 feet above it in a pipe. Of course, with so 

 small an orifice (1| inch) in the water-bearing beds a large supply 

 was not to be expected ; but, notwithstanding, the supply is suffi- 

 cient to give a continuous flow of 2 gallons per minute, equal to 

 2,880 gallons per day, and quite ample for the requirements of a 

 very large household. The water is soft, clear, and slightly 

 chalybeate. 



From the above account we may gather several interesting con- 

 clusions. Firstly, it will be evident that the hydrostatic pressure 

 is very great, being sufficient to cause the water to rise 7 feet above 

 the ground in a pipe ; and we may therefore suppose that with a 

 wider borehole a much larger supply could be obtained. We may 

 also infer that the surface gathering-ground of the Lower Greensand 

 is considerably more elevated than the site of the boring. The Lower 

 Greensand crops out at about an equal distance of 20 miles north and 

 20 miles south of the boring : the one outcrop being in Buckingham- 

 shire, and the other in Surrey, near Guildford. But the surface-area 

 in the former case is interrupted and never extensive, while in the 

 latter it is continuous, elevated, and extensive. To this latter source 

 we may therefore attribute the underground supply at Windsor 

 Forest; and we may affirm with considerable confidence that under 

 the "Windsor district large supplies of excellent water might be 

 obtained by borings carried out on a sufficient scale into the Lower 

 Greensand, should the supply from the Chalk fail. 



It is very probable that the Lower Greensand is altogether cut 

 out by the ridge of Mesozoic or Palaeozoic rocks, which may be 

 presumed to extend under the Valley of the Thames at Windsor ; 

 for it will be recollected that in the Bichmond boring there was a 

 thickness of only 10 feet of this formation. 1 



Taking the district from Dorking to Selborne, the Lower Green- 

 sand has a range of nearly 30 miles from east to west, with a general 

 dip towards the north and north-west, and an average exposed 

 surface of 5 to 7 miles wide (as, for example, at Haslemere), giving 

 an area of 150 to 180 square miles. As this formation is exceedingly 

 open and porous, and is destitute of superficial covering, except the 

 soil, the proportion of the rainfall that enters the rock and constitutes 

 a permanent source of supply must be very large indeed ; perhaps 

 not less than two-thirds of a rainfall of about 28 inches, or 18*6 

 inches, sinks down into the beds, the remainder being evaporated. 

 The ground occupied by the formation is very elevated, several points 



1 The magnetic survey of the British Isles, carried out by Profs. Riicker and 

 Thorpe, indicates that between Windsor and Reading there exists 'a high peak' 

 of old rocks rich in iron, and sending off ridges in the direction of Oxford 

 towards the north-west, and Cambridge towards the north-east. In this locality 

 the magnetic disturbance due to such concealed rocks is remarkable — causing 

 a variation of the needle, both as regards dip and horizontal force, altogether 

 unusual. This peak is overlain by the Chalk in the Valley of the Thames, but 

 is probably sufficiently high (so to speak) to dissever the Lower Cretaceous 

 beds of the south from those which reach the surface in Oxfordshire. See Phil. 

 Trans. Roy. Soc. vol. clxxxiii. a (1890) p. 283. 



Q.J.G.S. No.liJ8. m 



