ca vi.] THE EVOLUTION OF TEE EARTH. 401 



Let us now proceed to note two or three conspicuous features 

 of geologic evolution, remembering that in so doing we are but 

 following out a portion of the phenomena of planetary evolu- 

 tion discussed in the preceding chapter. There is no demar- 

 cation in the series of phenomena, save that which we 

 arbitrarily introduce for convenience of study and exposition. 

 The process of integration of matter and dissipation of motion 

 which we have just witnessed in the solar nebula as a whole, 

 we have now to witness in that segregated portion of it which 

 we call our earth, and we have to observe how here also in- 

 determinate uniformity has been succeeded by determinate 

 multiformity. 



In the formation of a solid crust about the earth, there 

 appeared the first conspicuous geologic differentiation ; re- 

 sulting not only in increased heterogeneity, but in increased 

 definiteness, as the crust gradually solidified. For not only 

 did the planet thus acquire a more definite figure, but also a 

 more definite movement ; since the solidification of the crust 

 must have diminished the oblateness of the spheroid, thus 

 gradually reducing the disturbance known as precession. 

 Next with the deposit of water in the hollow places of this 

 crust, there came the differentiation between land, sea, and 

 atmosphere ; and this differentiation became more marked as 

 vast quantities of carbonic acid, precipitated in this primeval 

 rain, left the atmosphere purer, and purified also the ocean 

 by segregating its contained lime. At the same time that 

 this vast condensation of ocean-water from pre-existing steam 

 constituted a secondary integration attendant upon the earth's 

 loss of molecular motion, the further thickening of the solid 

 crust began to entail other more local integrations. As Mr. 

 Spencer points out, while the earth's crust was still very thin, 

 there could be neither deep oceans nor lofty mountains nor 

 extensive continents. Small islands, barren of life, washed 

 by shallow lakes, void of animate existence, and covered with 

 a dense atmosphere, loaded with carbonic acid and aqueous 



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