668 Geological Periods of Mutation. 
has reduced JOLY'S result to fifty million years at the 
most. 1 
EUGENE DUBOIS has made use of the calcareous con- 
tents of rivers as the starting-point of his calculations. 2 
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 which 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 further methods 
of arriving at this result. HELMHOLTZ found that the 
1 For a further discussion of these calculations see E. DUBOIS, 
Kon. Akad. v. Wet. Amsterdam, Jan. 1902, p. 503. 
2 E. DUBOIS, ibid., p. 495, and also loc. tit., 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. 
