[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.  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 archaean and igneous rocks, the remainder being sediments.[2] On this estimate we may find valuable