DIASTROPHISM AND THE FORMATIVE PROCESSES 139 



Let it be noted at the outset that the material to be aggre- 

 gated was planetesimal in a very strict sense of that term. Each 

 integer, whether it be a molecule, a particle, or any such more 

 considerable aggregate as might be formed under the conditions of 

 the case, was pursuing a nearly circular orbit around the central con- 

 trolling body, the nebula at first, the sun later. This precisely 

 fits the definition of a planetesimal. This means that the parti- 

 cles were in a dynamic, not a static state; they were under con- 

 trol, not free. Whatever aggregation followed was therefore of the 

 planetesimal type, that is, particle joined particle in an individual 

 way as their orbits and orbital forces permitted. Their orbital 

 velocities hovered about that of the earth (18.6 miles per second) 

 let us say, as a mean, the inner faster, the outer slower, the orbits 

 of those equally distant from the center slightly inclined toward 

 one another. Beside these differences of velocity and inclination — • 

 that arose from the nature of the case — the planetesimals inherited 

 diverging courses from mutual collisions and rebounds as they 

 emerged from the gaseous into the orbital state. To overcome 

 these divergencies of orbit and these differences of speed and 

 develop aggregates of one kind or another in place of the molecules 

 inherited from the gaseous state, there were two classes of forces: 

 (i) the collective attraction of the whole ring or disk or some 

 bunched portion of it, and (2) the aggregating influences of indi- 

 vidual molecules upon one another. The first would tend to make 

 a single planet, if the whole were drawn together, or a few planet- 

 oids, if there was aggregation by bunching; the second would 

 make at first a multitude of minute particles which would grow to 

 larger sizes in proportion as the agencies of later aggregation 

 proved superior to the effects of fragmentation, exfoliation, tritura- 

 tion, and friction in other forms after the particles had grown large 

 enough to give these notable efi&ciency. Only the salient features 

 can be touched here. 



I. The ground of the first class of agencies has already been 

 covered. No general nucleus nor any effective bunching was in- 

 herited from the nebula; indeed concentration was definitely 

 inhibited up to the time of the withdrawal of the Roche limit. 

 There might be, to be sure, a certain kind of transient bunching of 

 the planetesimals in their orbits, such as affects the present planets. 



