AGE OF THE OCEANS According to Wegener the Atlantic is the youngest, the Indian older, and the Pacific the oldest of the oceans. Certainly the Atlantic, in the zone stretching from the West Indies to the Medi terranean, has experienced great changes since Tertiary times. Other investigators suggest that the Atlantic, with the exception of the West Indian region, is the oldest ocean, and that the Pacific is still, so to speak, in the making. As proofs of this are mentioned the many deep trenches or geosynclines around its borders and the numerous earthquakes which point to the continuation of the folding and subsidence. Some data now exist for the formation of estimates of the age of the oceans. The evidence comes partly from the study of radioactivity (q.v.) ; it is known that helium and lead are the end-products of radioactive disintegration of cer tain minerals, and from the ratios of these materials to their parent substances it is believed that a period of several thousands of millions of years must have elapsed since the first solid and cool earth-crust was formed. Some cosmographic investigations (due mainly to Jeans and Jeffreys) suggest that at least about 1,300, 000,00o years have elapsed since the first oceans were formed.
The elements in addition to oxygen which exist in largest amount in sea salt are chlorine, bromine, sulphur, potassium, sodium, calcium and magnesium. Since the earliest quantitative analyses of sea water were made by Lavoisier in 1772 the view has been held that the salts are present in sea water in the form in which they are deposited when the water is evaporated. The most numerous analyses have been carried out by Forchhammer, who dealt with iso samples, and Dittmar, who made complete analyses of 77 samples obtained on the "Challenger" expedition. Dittmar found the following average proportions of the salts in ocean water:— The variations in the proportion of individual salts to the total salts are very small; all analyses since Dittmar's have confirmed this result. Although the salts have been grouped in the above table it is not to be supposed that a dilute solution like sea water contains all the ingredients thus arbitrarily combined. There
must be considerable dissociation of molecules and as a first ap proximation it may be taken that of ten molecules of most of the components about nine (or in the case of magnesium sulphate five) have been separated into their ions, and that it is only dur ing slow concentration as in a natural saline that the ions combine to produce the various salts in the proportions set out in the above table. One can look on sea water as a mixture of very dilute solutions of particular salts, each one of which after the lapse of sufficient time fills the whole space as if the other constituents did not exist, and this inter-diffusion accounts easily for the uniformity of composition in the sea water throughout the whole ocean, the only appreciable difference from point to point being the salinity or degree of concentration of the mixed solutions.
The origin of the salt of the sea was attributed by some authorities entirely to the washing out of salts from the land by rain and rivers and the gradual concentration by evaporation in the oceans. This hypothesis, however, does not accord with the theory of the development of the earth from the state of a sphere of molten rock, surrounded by an atmosphere of gaseous metals, by which the first-formed clouds of aqueous vapour must have been absorbed. The great similarity between the salts of the ocean and the gaseous products of volcanic eruptions at the present time, rich in chlorides and sulphates of all kinds, is a strong argument for the ocean having been salt from the be ginning. Two other facts are totally opposed to the origin of all the salinity of the oceans from the concentration of the washings of the land. The proportions of the salts of river and sea water are quite different, as Julius Roth shows thus:— sample under examination with great exactness, as the volume of the areometer as well as the specific gravity of the water varies with temperature. All determinations must accordingly be re duced to a standard temperature for comparison. Following the practice of J. Y. Buchanan on the "Challenger," it has been usual for British investigators to calculate specific gravities for sea water at 6o° F compared with pure water at the maximum density point (39.2°) as unity. On the continent of Europe it has been more usual to take both at 17.5° C (63.5° F), which is expressed as A careful investigation of the relation between the amount of chlorine, the total salinity and the specific gravity of sea water of different strengths including an entirely new determination of the thermal expansion of sea water was made by M. Knudsen, 1899; the results are published in his Hydrographical Tables in a con venient form for use (1901).