THE BIRTH-TIME OF THE WORLD 41 



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 3 inches in a hundred years we get the time for all to 

 collect as 134 millions of years. If the rate be 4 inches, the 

 time is 100 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 by the Mass of the Sediments 



In the above method we obtain our result by the measure- 

 ment 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 : 



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 



1 Geikie, Text Book of Geology (Macmillan, 1903), vol. i. p. 73 et seq. Sollas, 

 loc. cit. Joly, Radioactivity and Geology (Constable, 1909), Phil. Mag. Sept, 

 .191 1. 



