CHAPTER XIV 

 THE EARTH BEFORE THE CAMBRIAN 



Speculations and theories as to the origin of the earth are as old 

 as the human race, but of the many that have been offered only two 

 are based upon physical laws, and these only demand our attention. 



Theories of the Earth's Origi 



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Nebular Hypothesis. — The theory of a molten earth is generally 

 associated with the Laplacian or nebular hypothesis of the earth's 

 origin, although in a modified form the hypothesis holds that the 

 earth has never been in a molten condition. This hypothesis, in 

 brief, holds that the material of which the sun, earth, and other plan- 

 ets are composed was once in the form of a vapor (nebula) diffused 

 throughout the space now occupied by the solar system and extending 

 some distance beyond the orbit of the outermost planet (Neptune). 

 This great mass of vapor began to rotate because of contraction and 

 formed a much-flattened spheroid. As contraction continued, a time 

 came when the centrifugal force of the particles near the equator of 

 the mass was equal to the pulling force of gravity, and they were left 

 as a ring, while the remainder of the mass continued to contract. 

 This process being repeated successive rings were abandoned. Each 

 of the rings thus formed probably revolved for a time as a whole, 

 but finally broke up, the material of each ring being concentrated 

 into a single planet with its satellites, the satellites being formed from 

 rings abandoned by the planets as they contracted, just as the planets 

 were formed from the parent nebula. In this way, according to the 

 nebular hypothesis, the planets were originated, the sun remaining 

 as the central and uncooled portion of the nebula. 



According to this hypothesis, therefore, the earth was first a globe 

 of highly heated vapor which was later condensed to a liquid and was 

 finally cooled sufficiently to permit of the formation of a crust over 

 the surface, while the interior was still a liquid. In this early stage 

 the atmosphere was very heavy and hot, and contained not only all 

 the water now on the earth, but many of the gases that are now united 



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