WATER 497 salt, the whole of the lime separates as gypsum or hydrated sulphate of lime, after which the sea salt is in great part deposited in a pure state, followed by a mixture of it with sulphate of magnesia, until there remains in the dense solution or bittern little more than chloride of magnesium. In addition to the above named compounds, the sea water contains in the form of bromides an amount of bromine equal to about yu.^j-g- of its weight, besides appreciable quantities of iodine, fluorine, phosphates, and borates. Moreover, it holds in solution a small and variable amount of carbonate of lime, some silica, and traces of various metals, including, besides iron and manganese, arsenic, copper, lead, silver, and gold. It may in fact be ex- pected that the sea will contain all the mineral elements which are capable of being held in solution in its waters, so that the progress of chemical investigation may greatly add to the preceding list. The waters of the ocean are subject to constant changes from several dif- ferent causes which have been active from re- mote ages, first among which may be placed the action of the alkaline waters, the origin of which has already been described, and which sooner or later find their way to the ocean. These, by the carbonate of soda which they contain, decompose the lime salts in the sea water, generating carbonate of lime and soda salts, of which the former remains in solution until it is taken up by growing plants and ani- mals, including the coccoliths and nullipores among the former, and the foraminifera, radi- ates, and mollusca among the latter, from the remains of all which organisms the limestones are at last built up. To this, however, the carbonate of lime directly brought down from the land contributes, and also the carbonate of magnesia, which reacts upon the lime salts in the sea water precisely like carbonate of soda, giving rise by double decomposition to carbon- ate of lime. The water of the ocean in early times contained a large proportion of chloride of calcium, which in the course of ages has been decomposed with the formation of car- bonate of lime and chloride of sodium, until at present the lime in the sea water is insufficient to form gypsum with more than a small pro- portion of the sulphate. The ultimate result of this process will be the elimination from the ocean's waters of the whole of its soluble lime salt, and its replacement by salts of soda and of magnesia. In these changes in the ocean waters it is evident that the intervention of vegetable and animal life in the formation of limestone is but incidental, since the insolu- bility of the chemically formed carbonate of lime would eventually lead to its separation in a solid form, independent of organic beings. But there are other changes in the composi- tion of the ocean's waters which are directly dependent upon the agency of life. The ash of marine plants, like that of those of the land, contains large quantities of potash salts and phosphates which have been abstracted from the waters of the sea, besides portions of iodine and of the rarer metals. These same elements are not confined to plants, but enter into the composition of the marine animals. The phosphate of lime of the sea weeds passes into the bones of fishes; copper is found in the fluids of certain mollusks and crustaceans, and iodine is concentrated in sponges as well as in sea weeds. Thus these various elements pass from the waters into animal and vege- table tissues, by the decay of which on land or in the ooze at the bottom of the sea they become fixed in insoluble forms, being thus removed from the oceanic circulation and re- stored to the solid earth. Through these re- actions the sea has doubtless suffered great modifications in composition, and but for them its waters would become charged with phos- phates and the various mineral matters men- tioned above. The composition of the waters of the sea in past ages has also been profound- ly modified by evaporation and by processes connected therewith. So long as the waters of the open ocean receive again the whole amount of water raised by evaporation, the only change which can result from this process is that already explained as effected by the soluble matters brought down from the land ; but the results are very different in the case of basins cut off from the ocean, like the Dead sea, which is a type of a vast number of much larger areas existing in former geological peri- ods under similar climatic conditions. These conditions are, an amount of evaporation ex- ceeding the rainfall of the enclosed sea and its geographical basin, from which results a grad- ual diminution of the volume of water and its consequent concentration, causing the precipi- tation from the water of beds of sulphate of lime or gypsum and of rock salt, often of great thickness and extent, and more rarely of solu- ble salts of potash and magnesia, the results of a still further evaporation, all of which are to- day found imbedded in the rocky strata of past geological ages, and represent large amounts of saline matter removed from the ocean's waters. Into these restricted and cut-off basins, more- over, the limited rainfall brings the soluble salts from the land, which by their reaction on the salts of the sea water effect, in addition to the changes already described, others pecu- liar and not less remarkable. These waters generally contain, as already explained, carbon- ates of lime, soda, and magnesia, of which the latter two decompose the chloride of calcium and the sulphate of lime present, with separa- tion of carbonate of lime, until at length the whole of these more soluble lime salts are con- verted into carbonate. Then begins a reaction between the carbonate of soda and the soluble magnesian salts of the sea water, resulting in the production of carbonate of magnesia. An- other and very different reaction gives rise to the same compound in waters which have lost their soluble lime salts either by the reaction just mentioned or by its separation in the form
