The Birth-Time of the World | Page 5

We go all over
the explored world, recognising the successive deposits by their fossils
and by their stratigraphical relations, measuring their thickness and
selecting as part of the data required those beds which we believe to
most completely represent each formation. The total of these
measurements would tell us the age of the Earth if their tale was indeed
complete, and if we knew the average rate at which they have been
deposited. We soon, however, find difficulties in arriving at the
quantities we require. Thus it is not easy to measure the real thickness
of a deposit. It may be folded back upon itself, and so we may measure

it twice over. We may exaggerate its thickness by measuring it not
quite straight across the bedding or by unwittingly including volcanic
materials. On the other hand, there
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may be deposits which are inaccessible to us; or, again, an entire
absence of deposits; either because not laid down in the areas we
examine, or, if laid down, again washed into the sea. These sources of
error in part neutralise one another. Some make our resulting age too
long, others make it out too short. But we do not know if a balance of
error does not still remain. Here, however, is a table of deposits which
summarises a great deal of our knowledge of the thickness of the
stratigraphical accumulations. It is due to Sollas.[1]
Feet.
Recent and Pleistocene - - 4,000 Pliocene - - 13,000 Miocene - - 14,000
Oligocene - - 2,000 Eocene - - 20,000 63,000
Upper Cretaceous - - 24,000 Lower Cretaceous - - 20,000 Jurassic - -
8,000 Trias - - 7,000 69,000
Permian - - 2,000 Carboniferous - - 29,000 Devonian - - 22,000 63,000
Silurian - - 15,000 Ordovician - - 17,000 Cambrian - - 6,000 58,000
Algonkian--Keeweenawan - - 50,000 Algonkian--Animikian - - 14,000
Algonkian--Huronian - - 18,000 82,000
Archæan - - ?
Total - - 335,000 feet.
[1] Address to the Geol. Soc. of London, 1509.
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In the next place we require to know the average rate at which these

rocks were laid down. This is really the weakest link in the chain. The
most diverse results have been arrived at, which space does not permit
us to consider. The value required is most difficult to determine, for it
is different for the different classes of material, and varies from river to
river according to the conditions of discharge to the sea. We may
probably take it as between two and six inches in a century.
Now the total depth of the sediments as we see is about 335,000 feet
(or 64 miles), and if we take the rate of collecting as three inches in a
hundred years we get the time for all to collect as 134 millions of years.
If the rate be four inches, the time is soo millions of years, which is the
figure Geikie favoured, although his result was based on somewhat
different data. Sollas most recently finds 80 millions of years.[1]
THE AGE AS INFERRED FROM THE MASS OF THE SEDIMENTS
In the above method we obtain our result by the measurement of the
linear dimensions of the sediments. These measurements, as we have
seen, are difficult to arrive at. We may, however, proceed by
measurements of the mass of the sediments, and then the method
becomes more definite. The new method is pursued as follows:
[1] Geikie, Text Book of Geology (Macmillan, 1903), vol. i., p. 73, _et
seq._ Sollas, _loc. cit._ Joly, Radioactivity and Geology (Constable,
1909), and Phil. Mag., Sept. 1911.
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The total mass of the sediments 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