LITMIR.BIZ

       Mean - 201,468 109,650 1 : 2,731

      We see that the ratio of the weight of water to the

      [1] Russell, _River Development_ (John Murray, 1888).

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      weight of transported sediment in six out of the nine rivers does

       not vary widely. The mean is 2,730 to 1. But this is not the

       required average. The water-discharge of each river has to be

       taken into account. If we ascribe to the ratio given for each

       river the weight proper to the amount of water it discharges, the

       proportion of weight of water to weight of sediment, for the

       whole quantity of water involved, comes out as 2,520 to 1.

      Now if this proportion holds for all the rivers of the

       world—which collectively discharge about 27 x 1012 tonnes of

       water per annum—the river-born detritus is 1.07 x 1010 tonnes. To

       this an addition of 11 per cent. has to be made for silt pushed

       along the river-bed.[1] On these figures the minor limit to the

       age comes out as 47 millions of years, and the major limit as 188

       millions. We are here going on rather deficient estimates, the

       rivers involved representing only some 6 per cent. of the total

       river supply of water to the ocean. But the result is probably

       not very far out.

      We may arrive at a probable age lying between the major and minor

       limits. If, first, we take the arithmetic mean of these limits,

       we get 117 millions of years. Now this is almost certainly

       excessive, for we here assume that the rate of covering of the

       primary rocks by sediments was uniform. It would not be so,

       however, for the rate of supply of original sediment must have

       been continually diminishing

      [1] According to observations made on the Mississippi (Russell,

       _loc. cit._).

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      during geological time, and hence we may assume that the rate of

       advance of the sediments on the primary rocks has also been

       diminishing. Now we may probably take, as a fair assumption, that

       the sediment-covered area was at any instant increasing at a rate

       proportionate to the rate of supply of sediment; that is, to the

       area of primary rocks then exposed. On this assumption the age is

       found to be 87 millions of years.

      THE AGE BY THE SODIUM OF THE OCEAN

      I have next to lay before you a quite different method. I have

       already touched upon the chemistry of the ocean, and on the

       remarkable fact that the sodium contained in it has been

       preserved, practically, in its entirety from the beginning of

       geological time.

      That the sea is one of the most beautiful and magnificent sights

       in Nature, all admit. But, I think, to those who know its story

       its beauty and magnificence are ten-fold increased. Its saltness

       it due to no magic mill. It is the dissolved rocks of the Earth

       which give it at once its brine, its strength, and its buoyancy.

       The rivers which we say flow with "fresh" water to the sea

       nevertheless contain those traces of salt which, collected over

       the long ages, occasion the saltness of the ocean. Each gallon of

       river water contributes to the final result; and this has been

       going on since the beginning of our era. The mighty total of the

       rivers is 6,500 cubic miles of water in the year!

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      There is little doubt that the primeval ocean was in the

       condition of a fresh-water lake. It can be shown that a primitive

       and more rapid solution of the original crust of the Earth by the

       slowly cooling ocean would have given rise to relatively small

       salinity. The fact is, the quantity of salts in the ocean is

       enormous. We are only now concerned with the sodium; but if we

       could extract all the rock-salt (the chloride of sodium) from the

       ocean we should have enough to cover the entire dry land of the

       Earth to a depth of 400 feet. It is this gigantic quantity which

       is going to enter into our estimate of the Earth's age. The

       calculated mass of sodium

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