116 GEOLOGY. 



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

 appear until after the composition of the atmosphere had been suitably 

 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 equality, 

 between the oxygen and the carbon dioxide in the air. At the same 

 time, the carbon dioxide was continually uniting with the rock-sub- 

 stance 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. 



The great factors now all at work. 



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

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

 the psychologic element remained to become a potential factor late 

 in the history. But though all were in function, their relative parts 

 were by no means equal, and the future stages were marked by the 

 successive dominance of one or another leading agent. The first of 

 these was the dominance of vulcanism. 



VII. The Climax of Volcanic Action. 



By hypothesis, volcanic action only began some time after the 

 beginning of the earth's growth, for it was delayed (1) 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 liquefy the more fusible con- 

 stituents. But once begun, it gradually 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 



