424 GEOLOGY 



the planets, planetoids, and satellites, is assigned to the coming 

 together of these bodies as they pursued their slightly different 

 orbits, not as the result of falling directly together under the con- 

 trol of gravity. It is assumed that the planetesimals had rather 

 highly elliptical orbits arranged in disk-like form, and such orbits 

 would be favorable for the conjunction of the bodies following 

 them. It can be shown mathematically that under such conditions 

 the addition of planetesimals to the nuclei would give them more 

 and more circular orbits as the accessions took place, and it is 

 significant that the planetoids (asteroids), which presumably have 

 grown little, usually have the most eccentric orbits, that Mercury 

 and Mars, the smallest of the planets, have the next most eccentric 

 orbits, while the orbits of the larger planets approach circularity 

 most closely. The photographs of spiral nebulae show large knots 

 with small ones near them, which appear quite susceptible of evo- 

 lution into planets attended by satellites. They also show small 

 scattered knots susceptible of forming planetoids. The earth-moon 

 system is assumed to have been derived from companion nuclei of 

 very unequal sizes. 



The knots might have had a rotary motion at the outset, arising 

 from inequalities of projection at the time of their formation; but, 

 in the main, the rotations of the planets are assigned to the impacts 

 of the planetesimals as they joined the nuclei to form the planets, 

 and this would naturally give rise to notable inequalities. There 

 would be no fixed relation between the rotation of a planet and the 

 revolution of its satellites; the period of the latter might be either 

 longer or shorter than that of the former. Even if the revolution- 

 period of a satellite-nucleus was originally the same as the rotation- 

 period of the planetary-nucleus, the growth of the planet might 

 draw the satellite nearer to itself and shorten the time of its revo- 

 lution, and thus the difficulty of Phobos and of the innermost 

 particles of the ring of Saturn be obviated. The mode of accretion 

 thus assigned might give rise to forward rotation, or to retrograde 

 rotation of the planets and satellites; the forward rotation should 

 be the rule, and retrograde rotation the exception, as is actually 

 the case. In a spiral nebula, formed in the way assigned, the 

 outer parts of the arms should be composed of lighter materials 



