THE ORIGIN OF THE EARTH. 23 



merely the incidental products of such systems, and dependent on the pre- 

 existence of the systems. 



Theories of the origin of meteorites. — Meteorites have been regarded 

 (1) as matter projected from the earth by volcanoes and brought 

 back to it, (2) as matter discharged from the moon, (3) as matter 

 ejected from the sun or from stars, (4) as dispersed matter from exploded 

 stars, (5) or from exploded planets or satellites, (6) as the residue 

 of scattered comets, (7) as fragments of tidally disrupted atmosphere- 

 less bodies, such as asteroids and satellites, and (8) as accretions of 

 gas or fine particles of matter in open space. 1 All but the last pre- 

 suppose the existence of the present solar system. Some of them 

 further assume, by implication, that the meteorites constitute merely 

 minute detachments from the present system. A volcano cannot 

 shoot away any great part of the body of which it is itself, at best, but 

 a small dependency. Only those dispersive hypotheses of this group 

 that postulate the disruption of suns or stars carry any presumption 

 that suns or stars would again be formed from them, for lack of mass. 

 The last hypothesis alone assumes a primary organization. If meteor- 

 ites are collected in space, as rain-drops are gathered in the atmos- 

 phere, and if they then unite to form solar systems, either through 

 nebulae or more directly, they are the source of such systems in as true 

 and radical a sense as rains are the sources of lakes. The characters 

 of meteorites should decide between these alternative views. 



The characters of meteorites. — Among the distinctive and sig- 

 nificant characters of meteorites are: their fragmentary forms, their 

 brecciated structures in part, their occasional slickensided surfaces, 

 their veins, the glassy nature of a part of their material, the amor- 

 phous nature of another part, and the crystalline nature of still a third 

 and larger part, the variations in the coarseness of the crystallization, 

 the extraordinarily large crystals of the nickel-iron, the inclusion of 

 non-metallic crystals and nodules in the nickel-iron crystals, the scattered 

 condition of iron crystals or lumps among silicate crystals in many cases 

 (sporadosiderites), the large proportions of the nickel-iron and the 

 magnesia, the presence of peculiar spheroidal aggregations (chondri), 

 the fragmental nature of the chondri in many instances, the absence 

 of water and hydrates, the absence of free oxygen, the absence of a 



1 For an excellent summary of the structure, constituents, and theories of meteor- 

 ites see Farrington, Jour. Geol., Vol. IX, 1901, pp. 51, 174, 393, 522, and 623. 



