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, 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!
12
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