66 



GEOLOGY. 



longer axis of the smaller orbit is equal to the shorter axis of the larger 

 orbit, and the planes of the orbits are properly related, collision is 

 rendered contingent. Actual collision is dependent, of course, upon 

 the bodies reaching the crossing of their paths at the same time. The 

 planes of the planetary orbits now lie near to one another and are 

 presumed always to have done so. Thsse planes, though not necessarily 



Fig. 24. — Diagram of a smaller orbit, B, lying within and parallel to a larger orbit, 

 A, both having a common focus, S; the absence of the conditions of conjunction 

 is obvious so long as they retain this relation even approximately, but when they 

 shift differentially to the positions A' and B', conjunction is made possible, if the 

 planes are properly related. If the orbit A were to shift the more rapidly, the 

 effect would be the same. 



the orbits, intersect one another, and the lines of intersection are shift- 

 ing so that in time all assignable intersections are realized. Under 

 these conditions, the mechanics of such a system furnish the requisite 

 contingencies for collisions between the planetesimals and the nuclei, 

 if sufficient time be granted. 



