FUNDAMENTAL PROBLEMS OF GEOLOGY. 203 



Igneous processes on the earth give rise to niagmatic differentia- 

 tion resulting in a famihar series of minerals which make up large 

 portions of the crj-stalline rocks of the earth's surface ; so also 

 weathering and solution remove more of the basic than of the acidic 

 constituents of crystalline rock, and when the residue is metamor- 

 phosed a similar series of minerals arises. Among these are quartz, 

 orthoclase, the acid plagioclases, the micas, and the amphiboles — a 

 group absent from the meteorites. This absence suggests that in 

 the parent bod}' magmatic differentiation of this kind and selective 

 weathering did not take place. This, however, does not necessarily 

 exclude volcanic action, nor non-hydrous weathering, but merely 

 the dominant phases of weathering and magmatic differentiation 

 that prevail in the earth and probably in similar bodies havang 

 atmospheres and hydrospheres. 



The absence of water, of hydrates, and of free oxygen adds its 

 testimony against the derivation of the meteorites from the crusts of 

 all bodies like the earth. 



The high velocities and the diverse directions of the meteoritic 

 flights relative to the earth forbid assigning their origin, in general, 

 to volcanic action in the moon or in any of the planets. Sufficient 

 velocity might be given by a solar explosion, but the directions 

 would be radial and not promiscuous. Explosive action from the 

 members of the solar sj'stem may have made an occasional meteor- 

 oidal contribution, but scarcely more than that. 



Taken altogether, the combination of characteristics presented by 

 meteorites seems to fail of satisfactory- explanation in any hypothesis 

 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 terrestrial type. No more do they seem to find satisfactory 

 explanation in simple accretion in free space. 



It remained to inquire whether small atmosphereless bodies like 

 the asteroids and the satellites afford a more probable source. Fol- 

 lowing the doctrine of Stoney, 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 corol- 

 lary that they never have had permanent atmospheres and hydro- 

 spheres. They thus meet the criterion imposed by the absence of 

 oxygen and water. If built up by accretion, they should contain 

 the requisite variety of material, and if formed in some other way 



