Deposits of salt usually, therefore, signify highly arid conditions, and where they occur over wide ranges in latitude and longitude, as in certain periods of the past, unusual aridity is inferred. Where confined to limited areas, their climatic significance is less, for topographic conditions may determine local aridity. The total area where salt is now being precipitated is small, though on the whole the present is probably to be regarded as a rather arid period of the earth’s history. On the other hand, ancient deposits of salt preserved in the sedimentary strata show that the area of salt deposition has been much more considerable than now at one time and another in the earth’s history. The salt and gypsum deposits of the past seem, therefore, to tell an interesting tale of the climates of the past. The magnesium salts are among the last to be thrown down as the sea-water is evaporated, and they most commonly take the form of sulphates and chlorides. They often form double salts with potassium, a relatively small and soluble constituent of sea-water. In the artificial evaporation of salt water to obtain common salt, the process is usually stopped before the saturation-point for the magnesium salts is reached, and the residue, the “mother-liquor,” or “bittern,” is drawn off to prevent these “bitter” salts from mixing with the common salt. The magnesium salts are among the last to be precipitated, not only because they are readily soluble, but because their quantity is small; yet in the original rock from which all the sea-salts came, there is at least as much magnesium as sodium, while in the sea there is about five times as much sodium as magnesium. Just what becomes of the remaining magnesium is not yet well understood. It has a notable disposition to form double salts with some other constituent, as noted above. In the earlier marine strata, dolomite, that is, limestone composed partly or wholly of the double carbonate of lime and magnesia, (CaMg)CO₃, abounds. This appears to have been formed by a gradual substitution of molecules of magnesium for those of calcium, but just how and when and why it was done has not been fully worked out. It appears to be a case where the saline matter of the sea made its contribution to the sedimentary deposits by chemical reaction upon them, rather than by precipitation because of saturation. The relatively small amount of potash in the sea-water is probably due to its disposition to remain united with the clays and earths of the mantle rock and of the shaley deposits. To some extent the salts in solution act directly on the earthy matter brought down into the sea by rivers, but where sedimentation is rapid, as it often is in shallow water, this action is limited and obscure. In the main, the ocean-waters protect the sediments from weathering and similar changes, except as organic matter buried with them induces change.