252 CARNEGIE INSTITUTION OF WASHINGTON. 



atmosphere of to-day might be changed to oxygen in about 300,000 

 years by an equally active vegetation. The early plant action may 

 have been much less efficient than that of to-day, and the requisite 

 period might be correspondingly lengthened, but it might still be 

 geologically short. Besides, the early atmosphere, by hypothesis, 

 was much less abundant than the present one and probably much 

 more active in the carbonation of rocks. 



It is assumed that life requiring a high content of oxygen did not 

 appear until after the composition of the atmosphere had been suit- 

 ably changed in this way. After oxygen-consuming, carbon-dioxide- 

 freeing organisms came into existence the reciprocal action of the 

 two classes of life tended to maintain an equilibrium, though not an 

 equalit}^, between the oxygen and the carbon dioxide in the air. At 

 the same time the carbon dioxide was continually uniting with the 

 rock substance of the outer part of the earth, as it does now, and 

 was thus being removed from the atmosphere. The same is true of 

 the oxygen ; but probably then, as now, oxidation was less active 

 and prevalent than carbonation, and so the combined result of plant 

 life and of inorganic action was to bring down the content of carbon 

 dioxide to a subordinate place. The nitrogen, being relatively inert, 

 gradually accumulated, and has now become much the most abundant 

 constituent. 



So soon as plants and animals had come into action, all the great 

 factors potential in the earth's physical evolution were in play. 



By hypothesis, volcanic action only began some time after the 

 beginning of the earth's growth, for it was delayed (i) by the lack 

 of sufficient compression in the central parts to give the requisite 

 heat, and (2) by the time required for this central heat to move 

 out to zones of less pressure, where it would suffice to melt the more 

 fusible constituents. But, once begun, it is supposed to have grad- 

 ually increased in actual and in relative importance until it reached 

 its climax. This obviously came much later than the climax of 

 growth, for it was dependent on the growth to give the increased 

 compression from which arose the central heat on which the vulcanism 

 depended. And so, owing to the .sources of delay just cited, the 

 maximum of volcanic action must have lagged much behind the 

 accession of the material which remotely actuated it. It is there- 

 fore inferred that vulcanism continued to increase in activity long 

 after growth had entered on its decline, and that there was an 

 important period in which the dominant activity was volcanic. 



It is conceived that in the late stages of the earth's growth the 



