.

       Total - - 335,000 feet.

      [1] Address to the Geol. Soc. of London, 1509.

      6

      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.

      7

      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.

      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]

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