THE ORIGIN OF THE EARTH. 3i 



have made an occasional meteoroidal contribution, but scarcely more 

 than that. 



Taken altogether, the combination of characteristics presented 

 by meteorites seems to fail of satisfactory explanation on any hypothe- 

 sis of their direct derivation from a sun or star, or from a planet sur- 

 rounded by a hydrosphere or an oxygen-bearing atmosphere, or from 

 any planetary body affected by mineralogic differentiations of the 



Fig. 9. — Section of the Franceville meteorite, showing the coarse crystallization of 

 the nickel-iron (Widmanstatten figures). Ward-Coonley Collection. (From H. A. 

 Ward.) 



terrestrial type. No more do they seem to find satisfactory explana- 

 tion in simple accretion in free space. 



Possible derivation from atmosphereless bodies. — It remains to 

 inquire whether small, atmosphereless bodies, like the asteroids and 

 the satellites, afford a more probable source, or whether new hypotheses 

 must be sought. Small celestial bodies are believed to be devoid of 

 atmospheres and hydrospheres because their gravity is too low to 

 overmatch the molecular velocities of the atmospheric gases and the 

 vapor of water. This interpretation carries the corollary that they 



