The amount of sea-water is estimated by Murray at 323,722,150 cubic miles,[141] or about 15 times the volume of the land above sea-level. The volume and composition of the sea-water being known, the amount of mineral matter which it contains may be readily calculated. Assuming the average specific gravity of the mineral matter in solution to be 2.5, the 3.5% by weight becomes 1.4% by volume, and 1.4% of 323,722,150 cubic miles is 4,532,110 cubic miles. This then represents the aggregate volume of mineral matter in the sea if it were precipitated and compacted so as to have an average specific gravity of 2.5. Assuming the average depth of the sea to be 2076 fathoms (12,456 feet), as given by Murray, the mineral matter in solution, if precipitated, would cover the ocean bottom to a depth of about 175 feet. Assuming the area of the land to be to that of the sea as 28 to 72, this amount of mineral matter would make a layer about 450 feet deep over the land. Its amount is equal to about 20% of that of all lands above sea-level, and it falls but little short of that in all lands below 600 feet in altitude. If it were precipitated and concentrated in the shallow waters about the borders of the lands, it would fill the sea out to the depth of about 4000 feet, and would diminish its area by some 19,000,000 to 20,000,000 square miles, an area which is more than ⅓ of the present land surface. In other words, if the mineral matter in the sea-water were precipitated and concentrated in the shallow waters about the lands, it would restore the continental shelves to the land areas, and add an almost equal area beyond. These comparisons may perhaps help to give some idea of the amount of mineral matter in solution in the sea, but they give no more than a hint of the importance of the solvent power of water in the general processes of rock decay, for most of the substances carried to the sea in solution by rivers are extracted from the water about as rapidly as they are supplied. Thus calcium carbonate is about twenty times as abundant as sodium chloride in river-water,[142] but is only ¹⁄₁₂₅ as abundant in sea-water. The total river discharge into the sea is estimated at 6524 cubic miles of water per year.[143] This water is estimated to carry to the sea annually about half a cubic mile of mineral matter in solution. At this rate it would take about 9,000,000 years for the streams to bring to the sea an amount of mineral matter equal to that it now contains, but the proportions of the ingredients would be very different.