ORIGIN OF THE EARTH 39 



The origin of the spiral is suggested as having been due to the 

 disrupting influence of the central body or sun by a passing star, 

 the disrupted particles or masses at first moving straight toward 

 the passing star, but, because of change in position of the passing 

 body, the disrupted masses gradually became pulled around and 

 their paths curved into spirals as shown in Fig. 20. In accord- 

 ance with the principle of the well-known tide producing force, 

 similar disrupted masses must also have shot out from the opposite 

 side of the sun or central body. Finally when the passing star had 

 so far gone by as to have largely lost its power of effectively attract- 

 ing the sun, the spiral orbits of the planetesimals gradually became 

 coiled into elliptical or nearly circular orbits around the sun. 



Briefly, according to this hypothesis, the earth was never a 

 highly heated gas and never necessarily more highly heated than 

 at present, hence sedimentary as well as igneous materials may well 

 be expected among the earliest formed rocks. Instead of a much 

 larger original earth, it increased in size by accretion of planetesi- 

 mals. With increase in size came increase in force of gravity, 

 causing compression of the earth's matter and generation of more 

 and more interior heat. Accompanying this increasing pressure 

 and heat, gases (including water vapor) were driven out to form an 

 atmosphere which gradually became larger and denser. When 

 the water vapor had sufficiently accumulated, precipitation resulted 

 to initiate the hydrosphere. 



Tabular Summary of Stages of the Earth's History l 



9. Cenozoic era ( 



8. Mesozoic era I Sedimentation predominant over vulcanism. 



7. Paleozoic era ] Higher forms of organisms. 



6. Proterozoic era ( 



5. Archeozoic era ( Vulcanism predominant over sedimentation. 



( Either initial or at least only very simple organisms. 



Nebular hypothesis Planetesimal hypothesis 



4. Hydrospheric (oceanic) stage. Initial hydrospheric (oceanic) stage. 



3. Lithic (congelation) stage. Initial volcanic stage. 



2. Molten stage. Initial atmospheric stage. 



1. Nebular (gaseous) stage. Nuclear (non-gaseous) stage. 



1 Modified after Chamberlin and Salisbury. 



