668 Geological Periods of Mutation. 



has reduced Joly's result to fifty million years at the 

 most. ^ 



Eugene Dubois has made use of the calcareous con- 

 tents of rivers as the starting-point of his calculations.^ 

 He starts from the fact that carbonic acid is the source 

 of plant food, and that the process of assimilation is the 

 only one on this earth by which oxygen arises on a large 

 scale. His arguments led him to conclude that the total 

 amount of oxygen in the atmosphere has become free in 

 this way. Now, carbonic acid is contributed to the at- 

 mosphere by the action of volcanoes. Once arrived here, 

 it is partly decomposed by plants, and partly itself acts 

 on rocks, and especially in combination with lime and 

 magnesia forms salts which are washed out by the rain 

 and carried by the rivers to the sea. Here, however, 

 these salts are again laid down in coral banks, shells, 

 and so forth, and in this way arise the enormous calcare- 

 ous strata wdiich constitute so large a portion of the hard 

 crust of the earth. The volume of these layers can be 

 approximately calculated, and the figure thus obtained, 

 when divided by the annual contribution, gives some idea 

 of the duration of the whole process. In basing his cal- 

 culation on the chalk only, Dubois arrives at an estimate 

 of 45 million years; but if magnesia is included as well, 

 obviously a much smaller figure must be arrived at, viz., 

 36 million years. 



I have still to mention briefly two furthc^r methods 

 of arriving at this result. Helmholtz found that the 



^ For a further discussion of these calculations see E. Dubois, 

 Kon. Akad. v. Wet. Amsterdam, Jan. 1902, p. 503. 



^ E. Dubois, ibid., p. 495, and also loc. cit., June and August, 

 1900. The same author, Over den Kringloop der stof op aarde. Ley- 

 den. 1899; and Over den ouderdom der aarde. Kon. Ned. Aardryksk. 

 Genootsch., 1900. 



