THE EVOLUTION OF THE EARTH 



In the formation of a solid crust about the 

 earth, there appeared the first conspicuous geo- 

 logic differentiation ; resulting not only in in- 

 creased heterogeneity, but in increased definite- 

 ness, 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 dif- 

 ferentiation 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 preex- 

 isting 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 integra- 

 tions. 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. 1 Small islands, barren of 

 life, washed by shallow lakes void of animate 



1 [See Spencer's First Principles, Part II. chap. xiv. 

 § 109.] 



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