The expressions “hard” and “soft” applied to water indicate that it has much or little mineral matter in suspension, as the case may be. When surface and spring waters are collected into streams and rivers they still contain in solution the greater part of the mineral matters which they at first carried. When well or spring waters have more than forty grains of mineral matter per gallon they are not suitable for drinking waters. Mineral waters, so called, are those which carry large quantities of mineral matter, or which contain certain comparatively rare mineral substances which are valued for their medicinal effects. The analysis of spring, well, or river waters will always give some indication of the character of the rocks over which they have passed.[29] The vast quantities of mineral matters carried into oceans and seas are gradually deposited as the water is evaporated. If, however, these matters be very soluble, such as common salt, sulfate of magnesia, etc., they become concentrated, as is seen with common salt in sea waters. In small bodies of waters, such as inland seas, which have no outlet, this concentration may proceed to a much greater extent than in the ocean. As an instance of this, it may be noted that the waters of the Dead Sea and Great Salt Lake are impregnated to a far greater degree with soluble salts than the water of the ocean. The solvent action of water on rocks is greatly increased by the traces of organic or carbonic acids which it may contain. When surface water comes in contact with vegetable matter it may become partially charged with the organic acids which the growing vegetable may contain or decaying vegetable matter produce. Such acids coming in contact with limestone under pressure will set free carbon dioxid. Water charged with carbon dioxid acts vigorously on limestone and other mineral aggregates. If such solutions penetrate deeply below the surface of the earth their activity as solvents may be greatly increased by the higher temperature to which they are subjected. Hence, all these forces combine to disintegrate the rocks, and through such agencies vast deposits of original and secondary rocks have been completely decomposed. The gradual passing of the firm rock into an arable soil is beautifully shown in Fig. 10, a print from a negative taken by Mr. Geo. P. Merrill, near Washington, D. C. [Illustration: FIGURE 10. View on the Broad Branch of Rock Creek, Washington, D. C. The fresh but badly decomposed granitic rock is shown passing upward into material more and more decomposed until it becomes sufficiently pulverulent and soluble to support plant life. The roots showing in the upper part of the picture formerly penetrated the decomposed rock, but have been exposed through quarrying operations. Photograph by George P. Merrill, 1891. ]