While there is somewhat more than ten times as much lime sulphate as lime carbonate in the ocean (p. 324), the deposits of the carbonate (including the organic) have been very much greater than those of the sulphate. This is due partly to the fact that the sulphate is much more soluble in natural waters than the carbonate. Rivers bring much more carbonate than sulphate to the sea, so that the point of saturation for the sulphate would normally be reached much later than that of the carbonate. The more important fact, however, is that marine plants and animals use lime carbonate freely for skeletal and housing purposes. It is held by some that they get their lime from the sulphate, but if so they convert it into carbonate before it takes the form of shells, coral, etc., the sulphuric acid set free in the process reproducing, directly or indirectly, more sulphate. The secretion of lime carbonate by organisms is not dependent on the saturation of the water, but may be carried on when the amount in solution is very small. There can be little doubt that the chief deposits of lime carbonate have been and are being made through the agency of plants and animals in the form of shells, coral, bones, teeth, and other devices for supporting, stiffening, housing, protecting, and arming themselves; but while it is agreed that the larger part of the lime carbonate deposited in the open sea is of organic origin, it is equally clear that in closed seas subject to concentration from evaporation, simple precipitation takes place freely. There is some difference of opinion as to the importance of these two classes of deposits, past and present. The debated point is whether simple precipitation takes place in any appreciable degree under the usual oceanic conditions. There is much more evidence of solution by sea-water than of precipitation from it. The ocean appears to be under-saturated with lime carbonate on the whole, though it is still possible that deposition may take place in favorable situations, as, for example, where the very calcareous waters of rivers are spread out in thin sheets on the surface of the heavier salt water, and thus exposed to exceptional evaporation, or where there is very exceptional agitation and aëration.[175] Gypsum appears to be deposited in quantity only in the closed basins of arid regions where concentration reaches an advanced state. Since normal sea-water is far from saturation with common salt, the latter is precipitated only in lagoons, closed seas, or other situations favorable to great concentration. This is usually achieved only in notably arid regions, and in basins that receive little or no drainage from the land.