The Birth-Time of the World | Page 5

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
Arch?an - - ?
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 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.
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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.