40 GEOLOGY. 



attraction, on the principles controlling a cooling gaseous body, but 

 would have been subject to accretion, one by one, in the modes presently 

 to be described. 



Planetesimal condition arising from the meteoritic. — Under certain 

 circumstances, meteorites might possibly be assembled in such a way 

 that they would come to revolve in concentric orbits about their com- 

 mon center of gravity, and thus assume a quasi-planetesimal condi- 

 tion, in contradistinction to that of a typical swarm of meteorites in 

 which each is habitually drawn toward the center, collides, and rebounds 

 after the fashion of gaseous molecules, as conceived by Lockyer and 

 Darwin. The meteoroids that are formed by the dispersion of a comet, 

 such as constitute the belts that give rise to the August and November 

 meteoric showers, are probably in the planetesimal rather than the 

 collision-rebound condition, and are becoming more and more scattered, 

 and individually independent, as time goes on. 



Planetesimal condition arising from original nebular dispersion. — 

 The type view however, assumes that the parent nebula had a planet- 

 esimal organization from the outset. The conception is a rather 

 radical departure from the gaseous conception of Laplace, and the 

 meteoritic conception of Lockyer, so far as fundamental dynamics 

 and mode of evolution are concerned. In the following sketch a rather 

 special phase of this view will be adopted, partly for the sake of definite- 

 ness and such clearness as the complexity of the subject may permit, 

 and partly for the following reasons : 



(1) It postulates that form of nebula which is by jar the most common, 

 the spiral; 



(2) This form of nebula gives a spectrum consistent with a planetesimal 

 constitution, if, indeed, it does not directly imply such a constitution; 



(3) This form of nebula seems to possess the requisite mass-distributions 

 and momenta; 



(4) The special view of the origin of this form of nebula assumes only 

 that which is liable to happen, is simple in its basal event, and is as mildly 

 catastrophic as the genesis of a system can well be assumed to be. 



While this will be followed as the type view, let it be distinctly 

 noted that the planetesimal doctrine of accretion does not stand or fall 

 with this particular conception. As a first step in its deployment, 

 let us turn to the heavens for such light as existing nebula? may shed 

 upon their own origin and constitution. 



