The Birth-Time of the World | Page 7

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
[1] _Trans. R.D.S._, 1899. A paper by Edmund Halley, the astronomer,
in the Philosophical Transactions of the Royal Society for 1715,
contains a suggestion for finding the age of the world by the following

procedure. He proposes to make observations on the saltness of the seas
and ocean at intervals of one or more centuries, and from the increment
of saltness arrive at their age. The measurements, as a matter of fact,
are impracticable. The salinity would only gain (if all remained in
solution) one millionth part in Too years; and, of course, the continuous
rejection of salts by the ocean would invalidate the method. The last
objection also invalidates the calculation by T. Mellard Reade (_Proc.
Liverpool Geol. Soc._, 1876) of a minor limit to the age by the calcium
sulphate in the ocean. Both papers were quite unknown to me when
working out my method. Halley's paper was, I think, only brought to
light in 1908.
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by all the rivers of the world is found to be probably not far from 175
million tonnes.[1] Dividing this into the mass of oceanic sodium we get
the age as 80.7 millions of years. Certain corrections have to be applied
to this figure which result in raising it to a little over 90 millions of
years. Sollas, as the result of a careful review of the data, gets the age
as between 80 and 150 millions of years. My own result[2] was
between 80 and 90 millions of years; but I subsequently found that
upon certain extreme assumptions a maximum age might be arrived at
of 105 millions of years.[3] Clarke regards the 80.7 millions of years as
certainly a maximum in the light of certain calculations by Becker.[4]
The order of magnitude of these results cannot be shaken unless on the
assumption that there is something entirely misleading in the existing
rate of solvent denudation. On the strength of the results of another and
[1] F. W. Clarke, A Preliminary Study of Chemical Denudation
(Smithsonian Miscellaneous Collections, 1910).
[2] _Loc. cit._
[3] "The