The earliest recognised method of arriving at an estimate of the
Earth's geological age is based upon the measurement of the
collective sediments of geological periods. The method has
undergone much revision from time to time. Let us briefly review
it on the latest data.
The method consists in measuring the depths of all the successive
sedimentary deposits where these are best developed. 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]