formed since denudation began may
be ascertained with comparative accuracy by a study of the
chemical composition of the waters of the ocean. The salts in the
ocean are undoubtedly derived from the rocks; increasing age by
age as the latter are degraded from their original character
under the action of the weather, etc., and converted to the
sedimentary form. By comparing the average chemical composition
of these two classes of material—the primary or igneous rocks and
the sedimentary—it is easy to arrive at a knowledge of how much
of this or that constituent was given to the ocean by each ton of
primary rock which was denuded to the sedimentary form. This,
however, will not assist us to our object unless the ocean has
retained the salts shed into it. It has not generally done so. In
the case of every substance but one the ocean continually gives
up again more or less of the salts supplied to it by the rivers.
The one exception is the element sodium. The great solubility of
its salts has protected it from abstraction, and it has gone on
collecting during geological time, practically in its entirety.
This gives us the clue to the denudative history of the
Earth.[1]

The process is now simple. We estimate by chemical examination of
igneous and sedimentary rocks the amount of sodium which has been
supplied to the ocean per ton of sediment produced by denudation.
We also calculate

[1] _Trans. R.D.S._, May, 1899.

8

the amount of sodium contained in the ocean. We divide the one
into the other (stated, of course, in the same units of mass),
and the quotient gives us the number of tons of sediment. The
most recent estimate of the sediments made in this manner affords
56 x 10¹⁶ tonnes.[1]

Now we are assured that all this sediment was transported by the
rivers to the sea during geological time. Thus it follows that,
if we can estimate the average annual rate of the river supply of
sediments to the ocean over the past, we can calculate the
required age. The land surface is at present largely covered with
the sedimentary rocks themselves. Sediment derived from these
rocks must be regarded as, for the most part, purely cyclical;
that is, circulating from the sea to the land and back again. It
does not go to increase the great body of detrital deposits. We
cannot, therefore, take the present river supply of sediment as
representing that obtaining over the long past. If the land was
all covered still with primary rocks we might do so. It has been
estimated that about 25 per cent. of the existing continental
area is covered with archæan and igneous rocks, the remainder
being sediments.[2] On this estimate we may find valuable

[1] Clarke, _A Preliminary Study of Chemical Denudation_
(Washington, 1910). My own estimate in 1899 (_loc. cit._) made as a
test of yet another method of finding the age, showed that the
sediments may be taken as sufficient to form a layer 1.1 mile
deep if spread uniformly over the continents; and would amount to
64 x 10¹⁸ tons.

[2] Van Tillo, _Comptes Rendues_ (Paris), vol. cxiv., 1892.

9

major and minor limits to the geological age. If we take 25 per
cent. only of the present river supply of sediment, we evidently
fix a major limit to the age, for it is certain that over the
past there must have been on the average a faster supply. If we
take the entire river supply, on similar reasoning we have what
is undoubtedly a minor limit to the age.

The river supply of detrital sediment has not been very
extensively investigated, although the quantities involved may be
found with comparative ease and accuracy. The following table
embodies the results obtained for some of the leading rivers.[1]

               Mean annual   Total annual   Ratio of
               discharge in  sediment in    sediment
               cubic feet    thousands      to water
               per second.   of tons.       by weight.

Potomac -        20,160          5,557        1 : 3.575
Mississippi -   610,000       406,250        1 : 1,500
Rio Grande -      1,700          3,830        1 : 291
Uruguay -       150,000         14,782       1 : 10,000
Rhone -          65,850         36,000       1 : 1,775
Po -             62,200         67,000       1 : 900
Danube -        315,200        108,000       1 : 2,880
Nile -          113,000         54,000       1 : 2,050
Irrawaddy -     475,000       291,430        1 : 1,610

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).

10

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._).

11

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,