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 Archaean - - ? 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