THE ORIGIN OF THE EARTH. 15 



stage of the earth is reached, it is not needful to follow it further here, 

 where the origin of the earth is the subject of quest. 



Is some other form of the meteoritic hypothesis tenable? — Is it pos- 

 sible to postulate, on probable grounds, some other phase of meteoritic 

 assemblage that will continue to be meteoritic throughout its evolution 

 down to the formation of the earth, and give distinctive geologic results? 

 Prolonged efforts on the part of one of the authors to frame such a 

 hypothesis, in definite terms and in a workable form, have been attended 

 by unsatisfactory results only. The difficulties are very grave. On 

 the one hand are dispersive agencies that tend to keep up the scattered 

 condition of the meteorites and to prevent assemblage; on the other 

 hand, if a tendency to assemblage be established, the growing swarm 

 seems sure to merge either into the gaseous or quasi-gaseous condition, 

 as above outlined, or else into the planetesimal system presently to be 

 considered. The true meteoritic condition appears to be an expression 

 of a dispersive function which tends to produce and to perpetuate 

 a scattered state. An adequate discussion of the possibilities and 

 limitations of meteoritic assemblage is beyond our limits, but there 

 are involved certain fundamental facts and principles of no little impor- 

 tance to the philosophic student of geology who wishes to probe, as 

 well as he may, the basal postulates of the earth's genesis, and these 

 merit attention. The subject, to be sure, is essentially astronomical, 

 but as geology is the domestic chapter of astronomy, the geological 

 student is entitled to go as far afield as his problem requires. 



The two general conceptions of the origin of the meteoritic state. — 

 Conceptions of the ulterior origin of the dispersed condition from 

 which evolution may be supposed to start, fall into two general classes: 

 (1) a primitive diffuse condition, without previous assemblage, a kind 

 of original chaos; or (2) a derived condition of dispersion arising from 

 the scattering of previous assemblages or the disruption of previous 

 bodies. If in the second case the dispersion be so great as to project 

 the disrupted material beyond the sphere of its own gravitative control, 

 its condition becomes dynamically the same as if it originated in such 

 a dispersed condition. It is therefore only necessary to consider those 

 cases of this class in which the scattered matter remains under its own 

 gravitative control and constitutes a diffuse flock or swarm of small 

 bodies. 



