With the increase of population and the simultaneous pollution of rivers in this country, the question of water-supply for the purposes of human life becomes every year a more serious one. Among the many schemes which have been advanced for the supply of London with water, it has been proposed by a high authority to bring the Bagshot Sands of the London Basin under requisition. These strata, covering, as they do, a considerable area at no great distance from the metropolis, and being for the most part of a porous character, would appear to form a convenient reservoir of water, since they contain stored up within them a large portion of the rain-water which falls on portions of Berkshire, Surrey, and Hants. The water obtained from these sands, either from surfacesprings or deep wells, is free from calcareous hardness, but is highly charged with salts certain of iron, which, by oxidation on exposure to the air, yield an ochreous red precipitate, the nature of which seems generally to be but imperfectly understood. The inconvenience, however, which results, in the turbidity of the water after no great exposure to the air, and the rapid corrosion of iron pipes in which it is conveyed, are facts with which all inhabitants of the Bagshot country are only too familiar. The fact that this property of water drawn from the Bagshot Sands varies greatly according to the portions of those strata which furnish it, does not however appear to be so generally known. Recent difficulties in connection with the water-supply of our own particular district have forced the matter upon my attention; and as I have been able to arrive at some pretty definite results from the investigations in which I have been engaged for several months past, it has occurred to me that, as forming a subsidiary subject to geological science, they may be of interest to readers of the Geological Magazine.