“Taking the mean daily discharge of the Thames at Kingston at 1,250,000,000 gallons, and the salts in solution at nineteen grains per gallon, the mean quantity of dissolved mineral matter there carried down by the Thames every twenty-four hours is equal to 3,364,286 lbs., or 150 tons, which is equal to 548,230 tons in the year. Of this daily quantity about two-thirds, or say 1,000 tons, consist of carbonate of lime and 238 tons of sulphate of lime, while limited proportions of carbonate of magnesia, chlorides of sodium and potassium, sulphates of soda and potash, silica and traces of iron, alumina, and phosphates, constitute the rest. If we refer a small portion of the carbonates and the sulphates and chlorides chiefly to the impermeable argillaceous formations washed by the rain-water, we shall still have at least ten grains per gallon of carbonate of lime, due to the chalk, upper greensand, oolitic strata, and marlstone, the superficial area of which, in the Thames basin above Kingston, is estimated by Mr. Harrison at 2,072 square miles. Therefore the quantity of carbonate of lime carried away from this area by the Thames is equal to 797 tons daily, or 290,905 tons annually, which gives 140 tons removed yearly from each square mile; or, extending the calculation to a century, we have a total removal of 29,090,500 tons, or of 14,000 tons from each square mile of surface. Taking a ton of chalk, as a mean, as equal to fifteen cubic feet, this is equal to the removal of 210,000 cubic feet per century for each square mile, or of 9/100 of an inch from the whole surface in the course of a century, so that in the course of 13,200 years a quantity equal to a thickness of about one foot would be removed from our chalk and oolitic districts.” This destructive action, operating through long periods of time, destroys not merely the general surface of the limestone, but, where it is localized by the convergence of water, is capable of excavating the deepest gorges and the longest caves. The quantity of material carried away in solution is a measure of the power of carbonic acid in the general work of denudation. _The Circulation of Carbonate of Lime._