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 contained in this rock-salt is 14,130 million million tonnes.

If now we can determine the rate at which the rivers supply sodium to the ocean, we can determine the age.[1] As the result of many thousands of river analyses, the total amount of sodium annually discharged to the ocean